3.42 vs 3.73 rear gear ratio's question

[nailheadpowered]

Wanted to see how much of a difference there is going from the 3.42 rear gear to a 3.73 in regards to performance and fuel economy. I still have a lockup converter as i do use the car for around town etc. but wondered if the taller gear would make a big difference in gas mileage and or would give it better times at the track. thanks for any input.

 

[RobsIron]

Do a search on this and you'll get a ton of info.

I do think that the overall opinion in the TR community is to stick with the 3.42s.

 

[Googs]

My '87 GN has 3.73's in it when I bought it. When it comes time to rebuild the rear end, I will be going back to the stock 3.42 ratio. I'm loosing approx. 2-3 mpg, if not more under normal driving, and the motor turns 500 rpm more at freeway speeds. The only real gain that I see is,if it can be considered a positive, the shorter gear makes it easier to brake the rear tires loose.

 

[FlyinGN]

I have 73's out back and like em. My gas mileage is OUTSTANDING. Its more about tune and chip parameters when it comes to gas mileage anyway. I love the way my car come out the hole

 

[G McCall]

Are you concerned about mileage or performance ?
I gained 60fts by going to 3.90s from 3.42s with a mid to high 10 second car.
For the record I wanted to run 28" tall tires on the street, I had a big roller rocker cam, lifters and a converter that would tighten up nicely
Top end really depends on the converter and cam etc. anyway.
Eventually I went back to 3.42s as I was "running out of gear" at the end of a quarter mile. When I did this the 60fts seemed to be off a few hundreths but the RPMs at the finish were back within reason.

IMO, If you are changing nothing but gear with your combo you should gain on the track IF your engine and torque converter combo can pull at the higher RPMs, and of course if you can hold traction.
Mileage will suffer slightly. Back to can't have both.

 

[RobsIron]

Anybody know how TRs would run with 3:55s? Just Curious.

 

[Roc87]

Had 3.73's up until a month ago. Ran the 11.87 with them, almost 1/2 way thru O/D! Haven't been to the track with the 3.42's yet. Can't really tell a difference in mileage around in town. Hoping I don't have to shift into O/D at all or not as soon, plus I'm hoping traction is better! I would leave on 5# of boost and just spin the 10.5" MT slicks!

 

[VadersV6]

First of all, if you're in overdive in the 1/4, you're losing alot of torque multiplication. It actually becomes torque reduction. Anything you can do to prevent that would be a good thing.
Some jackass put 4.56's in a 7.5 rear, in my regal before I bought it. It was the best gear for my cobra, but flat out stupid for the TR.
Increasing torque multiplication in a car that has a peaky torque curve is the way to go, but the TR's have such a fat, flat curve, that it makes no sense. With the TR, you're in the powerband, right here, right now. No need to gear it. Plus, a turbo engine needs to see a load for it to even produce boost. The higher 3.42 gear gives it this load, and it also allows you to build more boost off the line.
My godfather bought an 86 TType new. He eventually had a 4k stall, GN1 heads, cam, custom headers, DP, full exhaust, and then put in 3.90 gears. The best he could ever muster was a 12.12 with slicks. It was those gears killing his E.T. It forced him into OD and killed torque multiplication throughout most of the second half of the track.

 

[Turbo1dr]

Done the swap years ago with my GN. Lost about 1.5 tenths and about 1 to 2 mph in the 8th mile. Swapped back and it picked back up again. Unless you're going with a taller tire than a 28"... stay with the 3.42's.

 

[FlyinGN]

would you please explain to me HOW you could build more boost 'off the line' with 42's as oppossed to 73's??

First of all, if you're in overdive in the 1/4, you're losing alot of torque multiplication. It actually becomes torque reduction. Anything you can do to prevent that would be a good thing.
Some jackass put 4.56's in a 7.5 rear, in my regal before I bought it. It was the best gear for my cobra, but flat out stupid for the TR.
Increasing torque multiplication in a car that has a peaky torque curve is the way to go, but the TR's have such a fat, flat curve, that it makes no sense. With the TR, you're in the powerband, right here, right now. No need to gear it. Plus, a turbo engine needs to see a load for it to even produce boost. The higher 3.42 gear gives it this load, and it also allows you to build more boost off the line.
My godfather bought an 86 TType new. He eventually had a 4k stall, GN1 heads, cam, custom headers, DP, full exhaust, and then put in 3.90 gears. The best he could ever muster was a 12.12 with slicks. It was those gears killing his E.T. It forced him into OD and killed torque multiplication throughout most of the second half of the track.

 

[VadersV6]

Its going to be hard to explain that, if you dont get it. Its pretty fundamental. I'll do my best. With a mountain bike or a 10 speed, the taller, high speed gears are harder to get going from a dead stop. You throw it down into a lower gear, so you get gear multiplication, so you can take off with very little force from your legs. Imagine someone is standing in front of you, holding the bike from moving, while you're trying to pedal forward. The force you're putting on the pedals, is the torque of the motor. In a low gear, it would require little force to overcome the person trying to hold you back. That = low boost, low load, low torque. In high gear, it would be much harder to move forward and overcome the person stopping the bike. It would require much more force. This = high boost, high torque, high load. The higher the load, the higher the boost. Without a load, a turbo would never build boost. The 3.42 is a higher gear than the others. There is more resistance to begin moving than a lower gear would give you. You're probably thinking..."You're on the brakes. Why does it matter what gear you have?" Because a lower gear ratio, through all that torque multiplication, will overcome the brakes much easier. That equals less boost. Get it?

 

[Cablebandit]

Here is my experience with 3.23 to 3.73 gears in a f-body (00 T/A)
I have a turbo car and it is heavy...3820# raceweight. i thought the 3.73 gears would pick up my 60'. They actually did help SLIGHTLY in the 1/8....we are talking .05-.10 but lost that same time from the 1/8 to the 1/4 because i was way over my powerband with that gear. My best with the 3.73 gear was 10.16 @ 136. I went 2 weeks later after going BACK to the 3.23 and ran 10.12 @ 137. Not only was it a tick faster but way safer because I didnt have to turn near as many rpms. Everyone says turbos love load but I had to see it for MYSELF. They were right! Also...my mileage was the same. I do 50/50 driving highway/city. The 3.73 tightened my converter...better gas mileage around town. 3.23 better on highway so my mileage stayed the same with either gear. Stay with the 3.42 gears.