My '89 LX 5.0 (96k. miles with an AOD) is completely stock except for 40 series Flowmasters and I eliminated my catalytic converters. My transmission guy has heard weird sounds coming from the rearend and he suggests that it needs to be rebuilt. I want to put a high performance rearend but I can't decide between the 3.73 and the 4.10s. I know that the 4.10s will provide better acceleration but I am concerned about gas mileage due to the fact that I drive about 90 miles everyday on the highway. Any educated suggestions will be greatly appreciated. Also, do I need to buy an installation kit with the ring and pinion set?
Tame '89 needs robust rear end.
- Thread starter jtstang89
- Start date
TrophyHead
15 Year Member
Yeah you need the installation kit for the gears.
This is what I've heard.....4.10's in a auto act like 3.73's in a 5 speed.
I'd still get the 4.10's yeah gas will be a little bit worse but it's all about keeping your foot of the gas pedal.
This is what I've heard.....4.10's in a auto act like 3.73's in a 5 speed.
I'd still get the 4.10's yeah gas will be a little bit worse but it's all about keeping your foot of the gas pedal.
From what I've heard around town, 4.10s are really awesome, but they say that there is a big difference in gas mileage. Do you know what RPM i'll be runnin at 70mph? Also, I do have a bit of a lead foot. Ok, you have to think that if you are adding gas mileage into the equation, then its really important to you. Don't go too steep with gears and then end up having poor driving conditions and bad gas mileage. Dont forget that they also make a 3.90 gear which is a great compromise. Either way, if your rear gears are stock then anything will beat that. I would suggest 3.73's or 3.90's. The 4.10's will kill top speed and gas mileage, and shouldnt be used unless you plan on utiziling them at the strip a lot. Of course they will be more fun, but that isnt the only think you are concerned with since you mentioned highway speeds and mileage. Besides, you cant take full advantage of 4.10's unless you have a converter and slicks, and since you dont sound like you have the funds for that, I would definitely suggest 3.73 or 3.90.
Highway mileage is not significantly effected, mainly because the engine will still be at the lower end of it's power band...at 70 mph roughly 2500 RPM with the 4.10's, a stock 26" tire, and 5-speed. Around town, is were mileage tends to suffer with the 5-speed cars. Auto's not so much, simply because the shift points are, in part, based upon RPM and not actual speed. All you end up doing is shifting to the 4th gear (auto) or 5th gear (manual) a lot sooner. Of course, if you have a heavy foot, then all bets are off as far as mileage is concerned.
The other reason why mileage may not necessarily suffer on the highway is because of torque multiplication. As a numerically steeper gear is placed in the rearend, significantly more torque is reaching the wheels. When this happens, the engine doesn't have to work as hard to accellerate at any given rate. As you know, the computer will advance timing and richen the air/fuel mixture as the engine is placed under load. With less load on the engine, it will have less tendency to do this under routine driving conditions. Thereby producing better gas mileage. Of course, this effect is somewhat offset by the fact that the engine may be operating at a higher RPM.
I know a few people that have replaced their stock 3.08's with 3.55's or 3.73's and actually achieved better gas mileage.
The other reason why mileage may not necessarily suffer on the highway is because of torque multiplication. As a numerically steeper gear is placed in the rearend, significantly more torque is reaching the wheels. When this happens, the engine doesn't have to work as hard to accellerate at any given rate. As you know, the computer will advance timing and richen the air/fuel mixture as the engine is placed under load. With less load on the engine, it will have less tendency to do this under routine driving conditions. Thereby producing better gas mileage. Of course, this effect is somewhat offset by the fact that the engine may be operating at a higher RPM.
I know a few people that have replaced their stock 3.08's with 3.55's or 3.73's and actually achieved better gas mileage.
Michael Yount
Mustang Master
Calculating the impact on rpm is easy - it's just simple ratios. If you have 3.08's now, and you put in 4.10's, the rpm at a particular speed will be increased by the ratio of 4.10/3.08. Plug in other numbers depending on the gear you have now and the ones you want to know the impact of.
My experience on fuel mileage has been the opposite of autoXr's. I ran some extensive tests in my wife's BMW with her trip computer on the impact of running in 4th gear vs. 5th gear on the highway (in her car, the difference is like the difference between 3.08's and 3.73's). At 60 mph there was a 4% reduction in fuel mileage; at 65 mph it was 7%; at 70 mph it was 10%. I believe it works out such that you may find that your around town mileage improves - with more gear it'll take less throttle opening to get the car moving which may result in slightly better around town mileage. But that's the beauty of the forums --- you'll get different theories from different folks.
My experience on fuel mileage has been the opposite of autoXr's. I ran some extensive tests in my wife's BMW with her trip computer on the impact of running in 4th gear vs. 5th gear on the highway (in her car, the difference is like the difference between 3.08's and 3.73's). At 60 mph there was a 4% reduction in fuel mileage; at 65 mph it was 7%; at 70 mph it was 10%. I believe it works out such that you may find that your around town mileage improves - with more gear it'll take less throttle opening to get the car moving which may result in slightly better around town mileage. But that's the beauty of the forums --- you'll get different theories from different folks.
OK, well I've pretty much dropped the idea of installing the 4.10s mainly because of the mileage factor and the fact that my stang is an everyday driver. It's between the 3.73 and 3.90. I'll leave it in the hands of you guys to convince me which is better. Thanks again. 
Ok, I'm sorry but the idea of installing 4.10's to get better gas mileage is the stupidest thing Ive ever heard. I dont care what anybody says, if you want good drivability and gas mileage and top speed, you DO NOT install 4.10's. And dont forget he doesnt have a Tremec or something with the .63 overdrive. I know everbody on here is going to tell you to install 4.10's, but they simply are not the gear you are looking for.
"Ok, I'm sorry but the idea of installing 4.10's to get better gas mileage is the stupidest thing Ive ever heard. " 
If you had actually read what was posted, nobody said that.
3.55's especially, and even 3.73's to a lesser extent...yes. Proven, several times over. Talk to any professional tuner or anyone who has actually done it with their daily driver.
As RPM goes up, frictional forces do play into it and may offset the benefits of torque multiplication...which was said...and was the experience of Michael Yount.
With anything, if you go too far with it, then you start offsetting the benefits with other factors.
If you had actually read what was posted, nobody said that.
3.55's especially, and even 3.73's to a lesser extent...yes. Proven, several times over. Talk to any professional tuner or anyone who has actually done it with their daily driver.
As RPM goes up, frictional forces do play into it and may offset the benefits of torque multiplication...which was said...and was the experience of Michael Yount.
With anything, if you go too far with it, then you start offsetting the benefits with other factors.
Michael Yount
Mustang Master
Settle down there Smock! 
Nobody said you up the gear ratio in order to TRY and get better fuel mileage. What we said was the mileage impact shouldn't be signficant, and you MIGHT actually get slightly better mileage under certain circumstances. And, the T-5's didn't all come with the .63 top gear ratio; the majority came with a .675 5th gear, which is almost identical to the top gear ratio in the AOD. Top gear in the AOD's is EXTREMELY long - just like in the T-5's. Also, in your previous post, you mention an impact on top speed. Well there is an impact - but not like you've suggested. Most stangs would see an INCREASE in top speed by switching from gears like 2.73's or 3.08's up to 3.73's. The reason is that LONG top gear ratio you pointed out. It keeps the engine revving so low that you can't get up into the peak power range of the engine because of aero drag. With 3.08's and a 26" tire, at 140 mph the car's only turning over about 3800 rpm. And it would take a REALLY good stock Stang to hit that speed, most are slower; the manuals mostly had 2.73's which made the problem even worse. The bigger gear allows the engine to turn up into it's power peak range - which is gonna allow for more top end, not less. It's the same dynamic that allows Vipers, Vettes and F-bodies with the six speeds to hit their top speeds in 5th gear, not 6th. A change to a shorter overall ratio is only top-speed limiting if the car is rpm limited at it's top speed. The Stang's aren't - they're aero limited; meaning they run out of steam/HP long before they run out of rev. And it's because of that long top gear. For example, the opposite philosophy occurs at BMW - they believe a car should hit its top speed at the redline in top gear, and in general that's how they build their cars. My wife's car hits 128 mph in 5th gear (which is a 1:1 ratio, not overdriven) at 6200 rpm. That's also where the computer limits it because of the speed rating on the tires. Put a shorter rear gear in her car, and it will limit top speed - because she's redline limited at the aero limit.
jtstang - do the math. There's very little difference between 3.73's, 3.90's and 4.10's. If you're overly concerned about mileage - go with the 3.73's. However, my car runs shorter tires than the Stang - so my 3.73's act like 3.90's in a Stang. My aero is much worse than a Stang (I drive a brick) - and I get 16-19 mpg around town, and 23-25 on the highway - as long as I keep my foot out of it. My engine was built to be about like a stock Cobra engine. The bigger issue that people have with mileage is that they modify the engine so that it makes more power on top, but it loses significant torque production below 2500 rpm - which is where most street driving/highway cruising occurs - even with bigger gears. Without big torque down low, the engine becomes less efficient in that range - and it shows up in fuel mileage. You won't have that problem as your engine is stock - it makes great torque down low. I think you'll find that you have more options for suppliers with the 3.73's than the 3.90's - that may make the decision for you. Don't forget to change the speedo cable gear to adjust your speedometer.
Nobody said you up the gear ratio in order to TRY and get better fuel mileage. What we said was the mileage impact shouldn't be signficant, and you MIGHT actually get slightly better mileage under certain circumstances. And, the T-5's didn't all come with the .63 top gear ratio; the majority came with a .675 5th gear, which is almost identical to the top gear ratio in the AOD. Top gear in the AOD's is EXTREMELY long - just like in the T-5's. Also, in your previous post, you mention an impact on top speed. Well there is an impact - but not like you've suggested. Most stangs would see an INCREASE in top speed by switching from gears like 2.73's or 3.08's up to 3.73's. The reason is that LONG top gear ratio you pointed out. It keeps the engine revving so low that you can't get up into the peak power range of the engine because of aero drag. With 3.08's and a 26" tire, at 140 mph the car's only turning over about 3800 rpm. And it would take a REALLY good stock Stang to hit that speed, most are slower; the manuals mostly had 2.73's which made the problem even worse. The bigger gear allows the engine to turn up into it's power peak range - which is gonna allow for more top end, not less. It's the same dynamic that allows Vipers, Vettes and F-bodies with the six speeds to hit their top speeds in 5th gear, not 6th. A change to a shorter overall ratio is only top-speed limiting if the car is rpm limited at it's top speed. The Stang's aren't - they're aero limited; meaning they run out of steam/HP long before they run out of rev. And it's because of that long top gear. For example, the opposite philosophy occurs at BMW - they believe a car should hit its top speed at the redline in top gear, and in general that's how they build their cars. My wife's car hits 128 mph in 5th gear (which is a 1:1 ratio, not overdriven) at 6200 rpm. That's also where the computer limits it because of the speed rating on the tires. Put a shorter rear gear in her car, and it will limit top speed - because she's redline limited at the aero limit. jtstang - do the math. There's very little difference between 3.73's, 3.90's and 4.10's. If you're overly concerned about mileage - go with the 3.73's. However, my car runs shorter tires than the Stang - so my 3.73's act like 3.90's in a Stang. My aero is much worse than a Stang (I drive a brick) - and I get 16-19 mpg around town, and 23-25 on the highway - as long as I keep my foot out of it. My engine was built to be about like a stock Cobra engine. The bigger issue that people have with mileage is that they modify the engine so that it makes more power on top, but it loses significant torque production below 2500 rpm - which is where most street driving/highway cruising occurs - even with bigger gears. Without big torque down low, the engine becomes less efficient in that range - and it shows up in fuel mileage. You won't have that problem as your engine is stock - it makes great torque down low. I think you'll find that you have more options for suppliers with the 3.73's than the 3.90's - that may make the decision for you. Don't forget to change the speedo cable gear to adjust your speedometer.
OK, I've almost made my decision. I'm leaning heavily towards the 3.73s. I've got just a couple of quick questions though(hope they don't sound stupid because I'm not mush of a stat guru or mechanic for the most part):
1. What is my the stock ratio for an '89 with an AOD?(I've heard guys mention 2.73 and 3.08)
2. Will the 3.73 impress me? My car is completely stock except for the exhaust and body kit. With the AOD I can't even peel out without power stalling or slamming it into gear! I've heard that 3.73s will slam my head back and leave nice little black tread marks just by slamming on the gas.
If this is true, then I'm sold!
Thanks for the input so far guys.
Keep it comin!
1. What is my the stock ratio for an '89 with an AOD?(I've heard guys mention 2.73 and 3.08)
2. Will the 3.73 impress me? My car is completely stock except for the exhaust and body kit. With the AOD I can't even peel out without power stalling or slamming it into gear! I've heard that 3.73s will slam my head back and leave nice little black tread marks just by slamming on the gas.
If this is true, then I'm sold! Thanks for the input so far guys.
Keep it comin!
Michael Yount
Mustang Master
No way to know for sure what ratio is in the car other than jacking it up, mark the tire and the driveshaft, and rotate the tire 1 complete rotation (be exact) while counting the rotations of the driveshaft (be exact). If the driveshaft rotates just a hair less than 2 3/4 times, you've got 2.73's. If it rotates a bit more than 3 you've got 3.08's. Unless you know the complete history of the car and know for sure that the gears haven't been changed, or that a different rear end hasn't been swapped in, trying to read the tag on the rear end or analyzing a VIN number simply isn't certain.
Will the 3.73 impress me? My car is completely stock except for the exhaust and body kit. With the AOD I can't even peel out without power stalling or slamming it into gear! I've heard that 3.73s will slam my head back and leave nice little black tread marks just by slamming on the gas. My current setup (stock) just seems very week and I'm looking for something I can feel. Plus I'm tired of my friend's 2001 Camaro V6 beating me off the line. 
Will I really feel a big change by switching to the 3.73s? Any advice from someone who has made the swap will be appreciated. Thanks.
Will I really feel a big change by switching to the 3.73s? Any advice from someone who has made the swap will be appreciated. Thanks.
Michael Yount
Mustang Master
OK -you gotta stop screwin' us around. You want gas mileage or you want to be impressed? Make a choice for cryin' out loud. Put the 4.10's in and leave us alone.
wishihadastang125
Founding Member
Michael Yount said:OK -you gotta stop screwin' us around. You want gas mileage or you want to be impressed? Make a choice for cryin' out loud. Put the 4.10's in and leave us alone.
I'm sorry for confusing you guys, I just want to make sure that I will actually FEEL some power after I spend the money. I was told by an outside source that freeing up my exhaust some would impress me, but it didn't. I like the way it sounds, but the power increase felt next to nothing. I know that I've been bit#$$@ about gas mileage (that's why I'm gonna most likely get the 3.73s), but I wanna make sure that I'm not wasting my money. If I'm not gonna feel a difference then I'm not gonna waste my money. I'm not trying to build a race car, just something to be proud of and something I can enjoy on the weekends. I just want to know if there will be a significant difference between my stock setup and the setup with a 3.73 rearend? Plus it would be cool for my "Honda Loving" friends to acknowledge my Stang as "fast". Any input will be appreciated. Thanks
TrophyHead
15 Year Member
Yeah 3.73's are gonna be a huge improvement over stock. You can always go 3.90's it's like the best of both worlds.
Michael Yount
Mustang Master
Someone mislead you if you thought the exhaust mods on an otherwise stock engine were gonna make much power. At low power levels, the stock system isn't much of a bottleneck. According to the dyno, replacing the stock exhaust manifolds with shorty headers on my low power set up caused me to lose 1 to 2HP across the boards - withing the statiscal range of "no change" on the dyno. The gears however, you will definitely feel.
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