Danny Thomas

Doing and LS conversion in my 81 TA that has a 308 gear in it now. What would be a taller gear that would be all round and maintain good gas mileage. I was think 342 but someone else suggested 373 which I had in my 73 Z28 when I was in college. Car will see some auto crossing but be mostly a cruiser. Would like to keep mileage in low 20's to very high teens. I'm using a 4L60E trans and the car is close to 3000 lbs.

Exidous

iTrader: (11)

How heavy is the car? If it's at all close to 3k I'd go to a 3.55. I have had 4.10 and 3.90 in my rx7 and they are WAY too much. A 3.73 I think would still be too much. I am going to a 3.55

krochus

iTrader: (1)

What trans you have matters a great deal as to what would be appropriate.

Overdrive =3.73-4.10

No overdrive 3.23-3.42

1989GTA

What size tires will you be running?

locrzn

iTrader: (9)

The post says its close to 3000k and 4l60e overdrive and will have a .75 or so final gear ratio. If your looking for mileage I'd keep it about 3.42. I'm running a 3.27 gear with 26.6 tall tire and my rpms are in the upper 2200's. If you run a simular tire height with a 3.42 you'll be close to 2350. Still should get reasonable mileage unless your running a modded motor.

i r teh noobz

iTrader: (4)

I have a 3.7 rear with a .81 overdrive (manual trans). 225/50/16 tires. It turns close to 3k at highway speeds. Mileage for a 45 minute highway run, 8 passes at the track, and 45 minutes home was about 18 mpg. Car weighs around 3200 pounds. Engine is an H/C/I 5.3 with a DIY wideband tune.

3.42s should break you into the 20s, especially with a stock engine and some tuning.

1981TA

iTrader: (7)

I have a Ls1/T56 combo and started with stock 3.23s. I blew the posi unit up in short order and then lucked into a relatively cheap parts car with a rebuilt 3.73 diff. It ran beautifully and would have lasted quite a while had the original owner not been a cheap SOB and skimped on replacing axle bearings. 500 miles later, oil runs low enough to roach the posi clutches . Now, I have a 3.42 rear end with a TruTrac diff and Moser Ford-style bolt in axles.

What I can say is this:

with the 3.73s, I hit 2000 RPM in 3rd at 20MPH, 4th(1:1) at 40 MPH, 5th(1:.83) at 60 MPH and 6th(1:.50) at 80MPH

I don't bother with 6th gear anymore with the 3.42s.

If you're in the South Suburban Chicago area, I can give you a killer deal on a 8.5" 10-bolt axle with 28-spline Summit axles, 3.73 Richmond gears and a rebuildable posi diff. I also have a NOS GM posi made for 30 spline axles. The stuff is collecting dust now that I've moved on to the new rear end.