Which gear 3:90, 3:73 or 3:55
Moderators: 93Cobra#2771, SPEEDFREEK, 1993cobra
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Guest
Which gear 3:90, 3:73 or 3:55
I want to change my rear end gear for more performance. Is 3:90 to big for street? I know alot of people say 3:73 is the one to get. But just curious.
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black93svt
- Posts: 728
- Joined: Mon Jun 20, 2005 8:04 pm
- Location: Colorado
Which gear?
Gas mileage will suffer as a consequence of changing to a numerically higher (lower) rear axle ratio. Top end speed will decrease significantly. However, off the line acceleration will be greater. This greater acceleration has a price -- added wheelspin, which means an upgrade to stickier tires is needed to really appreciate the difference.
Pros:
1) initial off the line acceleration with increased wheelspin over the 3.08:1 gears
Cons:
1) gas mileage will suffer
2) top end speed decreases significantly
3) the purchase of a speedometer driven gear to make the speedometer accurate
4) add the expense of sticky tires into the equation to appreciate the gear swap
Pros:
1) initial off the line acceleration with increased wheelspin over the 3.08:1 gears
Cons:
1) gas mileage will suffer
2) top end speed decreases significantly
3) the purchase of a speedometer driven gear to make the speedometer accurate
4) add the expense of sticky tires into the equation to appreciate the gear swap
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Guest
Gears will 'wake' your car up big time. You're car will feel like it has 50 more ft/lbs of torque. It seems to always be in the powerband and makes second gear more explosive and third gear more usable. I went from 2.73's to 3.55's in my 89 GT and the difference was huge. City gas mileage stayed the same and highway dropped significantly from 25-27 to 22ish but I've heard of people having a less dramatic drop. I'd go with 3.73's in a Cobra. 5.0 Mustangs should have come with 3.27's across the board from the factory. 2.73's are just plain ridiculous- cruising at 100 mph at only 2500 rpm- a little overdrive overkill if you ask me.
Oh, get your traction lock diff rebuild at the same time. This is the best $500ish you can spend on a 5.0 Mustang in my opinion. Add a shifter and subframe connectors and your car is how it should have been build right from the factory.
Oh, get your traction lock diff rebuild at the same time. This is the best $500ish you can spend on a 5.0 Mustang in my opinion. Add a shifter and subframe connectors and your car is how it should have been build right from the factory.
'93 Cobra #220- Black over Gray Cloth- 4.10's, Long Tubes, 57K miles- SOLD 8/8
'06 GT- Windveil Blue over Light Graphite Leather- 5spd
'93 Notch- 144k miles
'06 GT- Windveil Blue over Light Graphite Leather- 5spd
'93 Notch- 144k miles
Re: Which gear?
I disagree with your cons. Maybe my car was the exception.Ford wrote:Gas mileage will suffer as a consequence of changing to a numerically higher (lower) rear axle ratio. Top end speed will decrease significantly. However, off the line acceleration will be greater. This greater acceleration has a price -- added wheelspin, which means an upgrade to stickier tires is needed to really appreciate the difference.
Pros:
1) initial off the line acceleration with increased wheelspin over the 3.08:1 gears
Cons:
1) gas mileage will suffer
2) top end speed decreases significantly
3) the purchase of a speedometer driven gear to make the speedometer accurate
4) add the expense of sticky tires into the equation to appreciate the gear swap
1) my car's economy improved by 3 mpg with 3.73's installed
2) 5th gear becomes a little more useable instead of running out of steam around 3800 rpm with the 3.08's
3) chump change
4) 3.08's need sticky tires to
Re: Which gear?
[quote="QWKSNKE"][quote="Ford"]Gas mileage will suffer as a consequence of changing to a numerically higher (lower) rear axle ratio. Top end speed will decrease significantly. However, off the line acceleration will be greater. This greater acceleration has a price -- added wheelspin, which means an upgrade to stickier tires is needed to really appreciate the difference.
Pros:
1) initial off the line acceleration with increased wheelspin over the 3.08:1 gears
Cons:
1) gas mileage will suffer
2) top end speed decreases significantly
3) the purchase of a speedometer driven gear to make the speedometer accurate
4) add the expense of sticky tires into the equation to appreciate the gear swap[/quote]
I disagree with your cons. Maybe my car was the exception.
1) my car's economy improved by 3 mpg with 3.73's installed
2) 5th gear becomes a little more useable instead of running out of steam around 3800 rpm with the 3.08's
3) chump change
4) 3.08's need sticky tires to :D[/quote]
1) my car's economy improved by 3 mpg with 3.73's installed --
Sure it did. With 4.10:1 rear axle gears the car would probably get 410 miles to a 15.4 gallon tank of gas. <wink> <wink>
Wait a minute -- 4.30:1 rear axle gears --hmmm--the car would probably get 430 miles to a tank.
<cough>
With the correct speedometer driven gear to correctly mesh with the rear axle gear and tire combination/combinations how many miles does the car get to a tank of gas on average?
Pros:
1) initial off the line acceleration with increased wheelspin over the 3.08:1 gears
Cons:
1) gas mileage will suffer
2) top end speed decreases significantly
3) the purchase of a speedometer driven gear to make the speedometer accurate
4) add the expense of sticky tires into the equation to appreciate the gear swap[/quote]
I disagree with your cons. Maybe my car was the exception.
1) my car's economy improved by 3 mpg with 3.73's installed
2) 5th gear becomes a little more useable instead of running out of steam around 3800 rpm with the 3.08's
3) chump change
4) 3.08's need sticky tires to :D[/quote]
1) my car's economy improved by 3 mpg with 3.73's installed --
Sure it did. With 4.10:1 rear axle gears the car would probably get 410 miles to a 15.4 gallon tank of gas. <wink> <wink>
Wait a minute -- 4.30:1 rear axle gears --hmmm--the car would probably get 430 miles to a tank.
<cough>
With the correct speedometer driven gear to correctly mesh with the rear axle gear and tire combination/combinations how many miles does the car get to a tank of gas on average?
been awhile since I have checked an entire tank of gas but...
**Stock my car got 21.x mpg hwy driving with the a/c going.
**3.73's, MSD 6a. took it up to hair over 24 mpg on the highway
** additon of a b cam, AFR 165 heads it hit 25 mpg
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg highway That should go back up a little when I finalize my street tune in the next week or so (finally got a wideband 02)
Oh just for the smart ass comment,
yes the speedometer gear is correct and has been verified with radar
**Stock my car got 21.x mpg hwy driving with the a/c going.
**3.73's, MSD 6a. took it up to hair over 24 mpg on the highway
** additon of a b cam, AFR 165 heads it hit 25 mpg
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg highway
That should go back up a little when I finalize my street tune in the next week or so (finally got a wideband 02)Oh just for the smart ass comment,
yes the speedometer gear is correct and has been verified with radar
21 mpg on the highway with stock gears seems very low. I hit 30 mpg one tank on a long trip with the 2.73's cruising at 75. The other tanks were 27-28. With the 3.55's, I never hit 24 on the best tank.
'93 Cobra #220- Black over Gray Cloth- 4.10's, Long Tubes, 57K miles- SOLD 8/8
'06 GT- Windveil Blue over Light Graphite Leather- 5spd
'93 Notch- 144k miles
'06 GT- Windveil Blue over Light Graphite Leather- 5spd
'93 Notch- 144k miles
Interesting. Out of all the mustangs I have ever owned, none of them hit over 22-23mpg in stock configuration.phorty wrote:21 mpg on the highway with stock gears seems very low. I hit 30 mpg one tank on a long trip with the 2.73's cruising at 75. The other tanks were 27-28. With the 3.55's, I never hit 24 on the best tank.
Must be my driving
[quote="QWKSNKE"][quote="phorty"]21 mpg on the highway with stock gears seems very low. I hit 30 mpg one tank on a long trip with the 2.73's cruising at 75. The other tanks were 27-28. With the 3.55's, I never hit 24 on the best tank.[/quote]
Interesting. Out of all the mustangs I have ever owned, none of them hit over 22-23mpg in stock configuration.
Must be my driving :D[/quote]
Sure, sure, it is the driving. So how many miles to a 15.4 gallon tank with all those modifications?
Interesting. Out of all the mustangs I have ever owned, none of them hit over 22-23mpg in stock configuration.
Must be my driving :D[/quote]
Sure, sure, it is the driving. So how many miles to a 15.4 gallon tank with all those modifications?
[quote="QWKSNKE"]been awhile since I have checked an entire tank of gas but...
**Stock my car got 21.x mpg hwy driving with the a/c going.
**3.73's, MSD 6a. took it up to hair over 24 mpg on the highway
** additon of a b cam, AFR 165 heads it hit 25 mpg
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg highway :D That should go back up a little when I finalize my street tune in the next week or so (finally got a wideband 02)
Oh just for the smart ass comment,
yes the speedometer gear is correct and has been verified with radar :D[/quote]
** additon of a b cam, AFR 165 heads it hit 25 mpg
This statement produces 385 miles to a 15.4 gallon tank of gas.
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg
This statement produces 308 miles to a 15.4 gallon tank of gas.
**Stock my car got 21.x mpg hwy driving with the a/c going.
This statement produces 323.4 miles to a 15.4 gallon tank of gas.
Sure, sure, it is the driving!!!!
**Stock my car got 21.x mpg hwy driving with the a/c going.
**3.73's, MSD 6a. took it up to hair over 24 mpg on the highway
** additon of a b cam, AFR 165 heads it hit 25 mpg
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg highway :D That should go back up a little when I finalize my street tune in the next week or so (finally got a wideband 02)
Oh just for the smart ass comment,
yes the speedometer gear is correct and has been verified with radar :D[/quote]
** additon of a b cam, AFR 165 heads it hit 25 mpg
This statement produces 385 miles to a 15.4 gallon tank of gas.
**Additon of Procharger D-1 at 14 psi and it went down to 20mpg
This statement produces 308 miles to a 15.4 gallon tank of gas.
**Stock my car got 21.x mpg hwy driving with the a/c going.
This statement produces 323.4 miles to a 15.4 gallon tank of gas.
Sure, sure, it is the driving!!!!
Gas Mileage '93 Cobra with 3.08 gears
Hi Folks -
Except for synthetic fluids (engine, transmission, rear and front wheel bearings), Motorsport Shorty headers, removal of the intake silencer, and Pro 5.0 shifter, my car is bone stock. As I mentioned in other posts, I'm also careful to always keep my tire pressures at 33 psi cold.
I keep diligent records every time I fill the gas tank. Therefore I have an mpg for every tankload I've put ever in. At 197,000 miles, the overall average is 23.9 mpg. In strictly highway driving at 70 mph with relatively few warm-up cycles, it hits 27-28 mpg, and in the winter (more warm-up cycles) with mostly short trips it dips down into the 18-20 range. I've exceeded 400 miles on a tankload a couple of times, but that's running it pretty close to empty. Phorty's 30 mpg with 2.73's in a stock 5.0l Mustang sounds very plausible to me.
I don't beat on my cars a lot, but they have seen their share of high speed / WOT operation. With diligence, I think my results are similar to what other could reasonably expect to acheive from a stock '93 Cobra. And these results are in line with those I've acheived with other performance cars over the years.....my overall average is typically right around the EPA highway estimate. (including an overall average of 25-26 mpg for 50,000 miles in my 'old' 2000 Z/28.)
I do not believe that changing from 3.08's to 3.73 could possibly improve mpg. Indicated mpg might go up, but that's an issue with odometer calibration as already mentioned. The other source of error in measuring mpg is determining the amount of fuel put in. My '93 Cobra's tank is not easy to fill to the same level each time, because of the air space in the tank to allow for expansion and how level the car is. If the tank's not filled to the same level each time that'll throw off the mpg reading significantly. I've found you can add another 1-2 gallons to fill it right to the mouth of the filler after the automatic feed kicks off, but the feed rate must be verrrrry slow. I only do this if I'm going to immediately drive some distance before parking it as I did during the 3 years I was driving 85 miles one way to work on the interstate (required filling up every other day). I also try parking at the pump with the right rear slightly higher if possible. It's also more accurate to average several consecutive tankloads filled to the same level to measure mpg than to just use a single tankload.
I also have significant experience with two nearly identical 327/350 midyear Corvettes: one with 3.36's and the other with 3.70's, and can say from that 'real world' experience that the 3.70's hurt fuel economy compared to the 3.36's. The whole reason the carmakers have gone to numerically lower final drive ratios over the years is to reduce friction and improve fuel economy.
Changin' from 3.08's to 3.73's to increase mpg.....it just ain't gonna happen!
Best Regards,
Bob S.
P. S. In case you haven't guessed, I'm an automotive engineer with 25+ years / about 50,000 hours of dyno test supervision experience while dealing with numerous household name automakers on 3 continents.
Except for synthetic fluids (engine, transmission, rear and front wheel bearings), Motorsport Shorty headers, removal of the intake silencer, and Pro 5.0 shifter, my car is bone stock. As I mentioned in other posts, I'm also careful to always keep my tire pressures at 33 psi cold.
I keep diligent records every time I fill the gas tank. Therefore I have an mpg for every tankload I've put ever in. At 197,000 miles, the overall average is 23.9 mpg. In strictly highway driving at 70 mph with relatively few warm-up cycles, it hits 27-28 mpg, and in the winter (more warm-up cycles) with mostly short trips it dips down into the 18-20 range. I've exceeded 400 miles on a tankload a couple of times, but that's running it pretty close to empty. Phorty's 30 mpg with 2.73's in a stock 5.0l Mustang sounds very plausible to me.
I don't beat on my cars a lot, but they have seen their share of high speed / WOT operation. With diligence, I think my results are similar to what other could reasonably expect to acheive from a stock '93 Cobra. And these results are in line with those I've acheived with other performance cars over the years.....my overall average is typically right around the EPA highway estimate. (including an overall average of 25-26 mpg for 50,000 miles in my 'old' 2000 Z/28.)
I do not believe that changing from 3.08's to 3.73 could possibly improve mpg. Indicated mpg might go up, but that's an issue with odometer calibration as already mentioned. The other source of error in measuring mpg is determining the amount of fuel put in. My '93 Cobra's tank is not easy to fill to the same level each time, because of the air space in the tank to allow for expansion and how level the car is. If the tank's not filled to the same level each time that'll throw off the mpg reading significantly. I've found you can add another 1-2 gallons to fill it right to the mouth of the filler after the automatic feed kicks off, but the feed rate must be verrrrry slow. I only do this if I'm going to immediately drive some distance before parking it as I did during the 3 years I was driving 85 miles one way to work on the interstate (required filling up every other day). I also try parking at the pump with the right rear slightly higher if possible. It's also more accurate to average several consecutive tankloads filled to the same level to measure mpg than to just use a single tankload.
I also have significant experience with two nearly identical 327/350 midyear Corvettes: one with 3.36's and the other with 3.70's, and can say from that 'real world' experience that the 3.70's hurt fuel economy compared to the 3.36's. The whole reason the carmakers have gone to numerically lower final drive ratios over the years is to reduce friction and improve fuel economy.
Changin' from 3.08's to 3.73's to increase mpg.....it just ain't gonna happen!
Best Regards,
Bob S.
P. S. In case you haven't guessed, I'm an automotive engineer with 25+ years / about 50,000 hours of dyno test supervision experience while dealing with numerous household name automakers on 3 continents.
Ok. I didn't change gear ratios to increase mpg (as no one does) I did it for performance. If I was worried about mpg, I wouldn't own 4 of the 5 cars in my driveway. But in my application it happened. Just as it seems to help mod motor cobras when they go through their gear changes.
Without getting into rear end gear ratios.
Because of the different power curves my car has gone through over the last 4-5 years, it seems to get better economy when cruising in the 2300-2500 rpm range. This seems to be where my engine is happy at and is not 'lugging' to keep a 3500# car moving down the road.
I performed this test due to noticing that some of the smaller engine cars we had owned in the past that during normal highway cruising they were at (or over) 3000 rpm because of the 1:1 gear ratio in top gear. So I started expermenting with my cobra and found my economy to go up when moving my cruising range up to around 2400 or so rpm instead of 1800-2000 rpm. Anything above 2700 rpm and the economy starts dropping pretty quickly.
I would like to add that I am not arguing with anyone here. Just giving what I have witnessed with my car
Without getting into rear end gear ratios.
Because of the different power curves my car has gone through over the last 4-5 years, it seems to get better economy when cruising in the 2300-2500 rpm range. This seems to be where my engine is happy at and is not 'lugging' to keep a 3500# car moving down the road.
I performed this test due to noticing that some of the smaller engine cars we had owned in the past that during normal highway cruising they were at (or over) 3000 rpm because of the 1:1 gear ratio in top gear. So I started expermenting with my cobra and found my economy to go up when moving my cruising range up to around 2400 or so rpm instead of 1800-2000 rpm. Anything above 2700 rpm and the economy starts dropping pretty quickly.
I would like to add that I am not arguing with anyone here. Just giving what I have witnessed with my car
Hats off for the gas mileage/rear axle gears debate!!!
To further subjectively expound from real world experience:
Bone stock 92 GT bought new (2.73:1 rear axle gears):
interstate trips - 375 miles
city/combination - 285 miles
5 speed car
Modified 88 coupe:
4.10:1 rear axle gears
Massaged E7TE heads
Cobra intake
75 mm TB
Exhaust
interstate trips - 200 miles (with an annoying engine drone at 50mph and just getting worse toward the speed limit.) Furthermore, couldn't even stomach the speed limit. Most times a cruise speed of 60mph.
city/combination - almost laughable - 150 to 170 miles
more times than not a start off in 2nd gear and a shift to 4th gear happened.
Modified 88 GT:
3.73:1 rear axle gears
Massaged E7TE heads
Cobra intake
75 mm TB
1 5/8" headers
2.5" exhaust
interstate - 250 miles, paying close attention to the speed limit, most often travelling around 65mph.
city/combination - 180 miles
5 speed car
Two 89 LX 5.0 convertibles, both AOD, both 2.73:1
350 - 375 miles
82 coupe 5.0 with 4 speed(SROD) and factory 2bbl carb with 3.08:1 rear axle gears. Bone stock. 275 miles highway, 225 city/combination.
85 GT with T-tops, 3.73:1 rear axle gears, intake, 4bbl carb
laughable mileage of 175, 200 if lucky.
5 speed car
91 GT with 3.08:1 rear axle gears, bone stock
275 city/combination
350 highway
5 speed car
93 LX 5.0 with 2.73:1, bone stock, purchased new
225 city/combination
275 highway
5 speed car
It should be noted this car was a dog from the factory
In summary:
Very doubtful 4.10:1 rear axle gears will go into a daily driver of mine. If a 4 cylinder is in the garage beside it, then just maybe.
3.73:1 appear to be much more streetable than 4.10:1.
2.73:1 and 3.08:1 rear axle gears do get better gas mileage.
Cruise speeds on the 2.73:1 and 3.08:1 rear axle geared cars was anywhere from 10 to 20 to 25mph faster and they still got better gas mileage.
It would be nice to contribute more real world experience to this thread....ho-hum...
To further subjectively expound from real world experience:
Bone stock 92 GT bought new (2.73:1 rear axle gears):
interstate trips - 375 miles
city/combination - 285 miles
5 speed car
Modified 88 coupe:
4.10:1 rear axle gears
Massaged E7TE heads
Cobra intake
75 mm TB
Exhaust
interstate trips - 200 miles (with an annoying engine drone at 50mph and just getting worse toward the speed limit.) Furthermore, couldn't even stomach the speed limit. Most times a cruise speed of 60mph.
city/combination - almost laughable - 150 to 170 miles
more times than not a start off in 2nd gear and a shift to 4th gear happened.
Modified 88 GT:
3.73:1 rear axle gears
Massaged E7TE heads
Cobra intake
75 mm TB
1 5/8" headers
2.5" exhaust
interstate - 250 miles, paying close attention to the speed limit, most often travelling around 65mph.
city/combination - 180 miles
5 speed car
Two 89 LX 5.0 convertibles, both AOD, both 2.73:1
350 - 375 miles
82 coupe 5.0 with 4 speed(SROD) and factory 2bbl carb with 3.08:1 rear axle gears. Bone stock. 275 miles highway, 225 city/combination.
85 GT with T-tops, 3.73:1 rear axle gears, intake, 4bbl carb
laughable mileage of 175, 200 if lucky.
5 speed car
91 GT with 3.08:1 rear axle gears, bone stock
275 city/combination
350 highway
5 speed car
93 LX 5.0 with 2.73:1, bone stock, purchased new
225 city/combination
275 highway
5 speed car
It should be noted this car was a dog from the factory
In summary:
Very doubtful 4.10:1 rear axle gears will go into a daily driver of mine. If a 4 cylinder is in the garage beside it, then just maybe.
3.73:1 appear to be much more streetable than 4.10:1.
2.73:1 and 3.08:1 rear axle gears do get better gas mileage.
Cruise speeds on the 2.73:1 and 3.08:1 rear axle geared cars was anywhere from 10 to 20 to 25mph faster and they still got better gas mileage.
It would be nice to contribute more real world experience to this thread....ho-hum...