Gear ratio question - Lateral-g Forums

RDuke · #1 01-07-2021, 04:20 PM

Hi guys,

Quick question for those of you who have/drive high HP cars on the street. I'm going back and forth between 3:25 and 3:50 rear gear ratio's on my current project I'm building. I'm playing with ratio calculators online and I'm leaning towards 3:25, but wanted to know any other options.

Combo:
Wanger built LS supercharged (1K hp, 850tq, with lots of low down torque).
4L80e trans (final 4th gear ratio I believe is .75)
28" tall tire (27.9 actually, 335/30/R20's)
Street car only, mostly around town with some highway/longer trips.

Am I right to think that the 3:25 is better since it has such high torque at a lower RPM? My thoughts, similar to a turbo car, it would use more of the torque/boost through the RPM range.

I recall Charley on his Rivi swapped out low 3 something gears for high 2 something after about 5K miles and he's about the same HP/Trans combo. is 3:25 too high, just right?

Zspoiler · #2 01-07-2021, 09:44 PM

If it was a 4 speed car like I have A 1979 Z-28 with a 377 500 HP small block Chevy with a Super T-10 /3:42 gears .But with an overdrive trans you can get away with a higher gear ratio if you wish

**camcojb** · #3 01-07-2021, 10:43 PM

Malitude was pretty close to yours power-wise and the same 4L80E. Shorter tire, but the tall gears (3.0) worked great. No issues with low end power or acceleration and the taller gear made it much nicer on the freeway for longer drives since the 4L80E doesn't have as tall of an overdrive as compared to the 5-6 speeds.

**will69camaro** · #4 01-08-2021, 01:26 AM

I did 3.50 in mine. Considered going to 3.25 but I geared it for 4th gear In the quarter. Should be able to do around 160.

See where you fall for cruising. 4l80e gearing isn’t super great so some gear advantage will be nice.

572Camaro · #5 01-08-2021, 11:03 AM

I did 3.55 in mine.

572ci. 68 Camaro with a 4L80E.
Rear tire is the 335. 18” wheels

I like a little steeper gear because I run street driving with occasional interstate but mostly two lane highways in central Missouri

**will69camaro** · #6 01-08-2021, 12:24 PM

Just running some quick checks:

3.25 Rear Gear
Cruising @70 is 2000-2100rpm
Cruising @80 is 2300-2400rpm
Assuming you rev to ~6800rpm; it'll do 175mph in 3rd (1:1 gear)

3.50 Rear Gear
Cruising @70 is 2200-2300rpm
Cruising @80 is 2500rpm
Assuming you rev to ~6800rpm; it'll do 162mph in 3rd (1:1 gear)

Not TOO different between the two gears, only 100ish rpm at cruising speed for either condition.

You make a good point on big torque and power, but unlike a turbo car, you dont need to load up the powertrain to make boost, take advantage of the mechanical advantage of gearing if possible. Can you live with the higher RPM highway speed knowing the difference isn't, to me, that significant.

RDuke · #7 01-08-2021, 05:31 PM

Thanks guys,

The input is very much appreciated. I currently have it as 3:25 as of this morning when I put the order for the chassis in with the roadster shop. I have some time to change it if need be.

Motor is going to be one of the Wegner street engines 376 LS3's with a Whipple.

Top speed isn't really that important, as it's a 56 Bel Air hard top. I'm a bit concerned that with 3:50's it will be completely unusable even with really stick tires on it. Maybe I'm wrong though and it would actually hook under normal street acceleration with the 3:50's....

**WSSix** · #8 01-09-2021, 09:43 PM

The difference between the 3.25 and 3.50 won't be much with that much power. I think you'll be fine with the 3.25's. You're going to have traction problems even with the 3.25's.

Blown353 · #9 01-09-2021, 11:50 PM

If the cam will tolerate it and be smooth at the lower cruise RPM, I might even consider a 3.00 rear gear especially if you do a lot of freeway driving given the .75:1 OD ratio of the 4L80E.

Just make sure the cam will run cleanly at your typical freeway cruising speed in OD with the converter locked, if your cruise RPM is too low for the cam to clean up the resulting engine surge and driveline "chuggle" at your typical freeway speeds is very annoying.

You're going to have traction issues regardless of gear ratio if you don't have right foot discipline. A friend's 70 Chevelle has a 900hp or so F1A procharged 410, 4L80E with an ATI "tight" 2800 stall triple disc converter, 3.42 rear gears, and 305 wide Bridgestone RE-71Rs... WOT is pretty much useless under about 70 on a decent road surface, you have to be smooth and pedal it. If you're going 75-80 in overdrive and stand on it, it will leave stripes after the downshift until you back out of the throttle a bit. Fun? Yes, but you had better be ready for it...