<Gearing>

[tb8125]

I have a 1981 trans am with a LS1 - 4l60e and a procharger.... I want this to have guts off the line but still be ok on the highway... It has 3:08 now but I was thinking about my be 3:73 just not sure about the highway use with these gears.... Can I get away with 3:73 or should I get 3:42.....?????
Tire size is 275/35 20 in the rear....

[Simmo]

The last LS1 vehicle I had was running 3:73. Ended up going 3:9 as it was at about 1600rpm at 65mph. Was running a T56 though, but this should help you out:-

http://api.viglink.com/api/click?for...13158895913033

It's not the one I used but if it's no good there's more plenty on the web.

[79-TA]

This kind of thing is so much simpler to figure out with a lockup converter. Consider yourself lucky.

Simmo, that excel file is confusing. It says 4L80, but I only see 3 gears, which happen to have the same ratios as a TH350.
**EDIT** Simmo's Excel file has different tabs on the bottom, one of which is for the 4L60. Just get your tire diameter, rear gear ratio, and converter loss plugged in and you're good to go.

Anyway, here's the straight math.

Assumed: top gear is an overdriven gear with a ratio of 0.7 and the transmission has a lockup converter.

First, let's estimate tire diameter:
275 mm /(25.4001mm/1 inch)*2*0.35+20 = 27.5787 inches. 4 decimal places is overkill here, given that it's only an estimate and tires can vary a bit from manufacturer to manufacturer, but I never liked rounding much.

Let's focus on the cruising RPM at 70 mph

70mph*(5280ft/1 mile)*(12in/1 ft)*(1 hour/60 minutes)*(1 tire revolution/27.5787*pi)*(3.73 driveshaft rev/1 tire rev)*(0.7 engine rev/driveshaft rev)=2228 rpm at 70 mph

With 3.42's, that would be 2041 rpm at 70 mph.


I'd say you should be just fine with either combination, assuming your transmission locks up in high gear (that is, the converter doesn't continue to slip.)

[astroracer]

I am running 3.73's in my NA '02 CETA and am very happy with them. I'd think a Procharged LS1 would have NO problem with guts off the line. Going to a taller gear may help you hook it up. Like Brett said either way would be fine. Just depends on how you want it to hook.
Mark

[Project69]

Have a 3.42 in my LS1 and 3.73 in my lt1 both are A4 cars, im changing out the lt1 to 4.10 because 3.73 doesnt seem like enough gear for me.

[Nemesis RR]

3.43 will give you plenty of take off without the added revs on the highway. Only go with a 3.73 if you have a very sticky tire and a great suspension set up for the takeoff. If not all the 3.73 will do is add wheelspin and more problems adding throttle on the turns. I have an LS2, A4, suspension work, 3.42, and sticky summer tires. I cannot take off the line to save my life. At 35 to 40 if I smash the throttle it will spin for 3 to 4 car lengths. 3.73 would make my car slower, add wear, and increase gas consumption.