Cheat Sheet for TH400 Drive Gears?

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Feb 15, 2012 | 05:35 PM

y2k_ta

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From: Central Indiana Honors: 4th grade spelling bee contestant

Cheat Sheet for TH400 Drive Gears?

I have a TH400 out of an '89 G20 Chevy van that had the 5.7L engine.

I've seen all the calculators online (messed with TCI's already). My tire height is 28" & the gear ratio is 3.42. I'd like to get a driven gear so my speedometer will be correct, but I have no idea what drive gear I have. The transmission is already in the car, so I was hoping for a "cheat sheet" somewhere that might list the drive gears used in a particular vehicle. Does one exist or do I need to crawl under the car?

Thanks for any help!

Scott

Feb 16, 2012 | 04:01 PM

Yank

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From: Alma, Ar.

go to your local GM dealer they have the tire size speedo ratio charts

Feb 19, 2012 | 06:12 PM

chris718

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The formula is the axle ratio(3.42) x the number of teeth on the drive gear(15 is common) divided by the tire diameter(28")x 20.2.So 3.42 x15 divided by 28x20.2 +# of tooth on the driven gear.