Gear Ratio
Burning Brakes
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From: Arlington TX
Last edited by Strokemyaxe; Oct 1, 2018 at 01:49 AM.
Race Director
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From: Virginia
Do you trust your tach? The best way to check is to jack both wheels off the ground, mark the driveshaft with tape, and have a helper count the driveshaft rotations as you spin a wheel 360 degrees. I did this, and got 3 and a smidge, so 3.08, which matches what came with my car.
Counting rotations, opening the diff, or nuking it from orbit are the only ways to be sure.
Counting rotations, opening the diff, or nuking it from orbit are the only ways to be sure.
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Intermediate
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Burning Brakes
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From: Arlington TX
Le Mans Master
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From: Lake Tahoe, CA
Le Mans Master
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From: Lake Tahoe, CA
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Race Director
Joined: Mar 2016
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From: Napa Valley California
Do you trust your tach? The best way to check is to jack both wheels off the ground, mark the driveshaft with tape, and have a helper count the driveshaft rotations as you spin a wheel 360 degrees. I did this, and got 3 and a smidge, so 3.08, which matches what came with my car.
Counting rotations, opening the diff, or nuking it from orbit are the only ways to be sure.
Counting rotations, opening the diff, or nuking it from orbit are the only ways to be sure.
Do this^^^^^^^^^^^^^^
It is the best and most accurate way.
It doesn't matter if your tach or speedo are off, you don't need a GPS and it works every time.
Just remember to block the front tires and place the trans in neutral.
Melting Slicks
Joined: Aug 2016
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From: CLEVELAND STUCK IN OHIO
2024 C3 of the Year Finalist- Modified
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Intermediate
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https://www.corvsport.com/1975-c3-corvette/
Last edited by sixx27am; Oct 2, 2018 at 07:24 AM.
Parts Sherpa
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From: ICE boneyard, Pa
Race Director
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From: Long Island
RPM vs. Gear Ratio and Tire Height
This chart shows you the cruise rpm at 60 miles per hour based on gear ratio and tire height. For maximum performance with less converter slip at the lockup point, your converter's stall speed should stall close to these figures. If the gear ratio and tire size you want to use isn't on the chart, use the following formula to calculate cruise rpm:
MPH x Axle Ratio x 336
Tire Diameter
Rear Axle
GearRatio Tire
MPH x Axle Ratio x 336
Tire Diameter
Rear Axle
GearRatio Tire
......….24" ..26" ..28" ...30" ...32"... 34" ...36" ...38" ...40"... 42".. 44"
3.23 2713 2504 2326 2170 2035 1915 1809 1714 1628 1550 1480
3.50 2940 2714 2520 2352 2205 2075 1960 1857 1764 1680 1604
3.73 3133 2892 2686 2507 2349 2212 2089 1979 1880 1790 1709
3.90 3276 3024 2808 2621 2457 2312 2184 2069 1966 1872 1787
4.10 3444 3179 2952 2755 2583 2431 2296 2175 2066 1968 1879
4.56 3830 3536 3283 3064 2873 2704 2554 2419 2298 2189 2089
3.23 2713 2504 2326 2170 2035 1915 1809 1714 1628 1550 1480
3.50 2940 2714 2520 2352 2205 2075 1960 1857 1764 1680 1604
3.73 3133 2892 2686 2507 2349 2212 2089 1979 1880 1790 1709
3.90 3276 3024 2808 2621 2457 2312 2184 2069 1966 1872 1787
4.10 3444 3179 2952 2755 2583 2431 2296 2175 2066 1968 1879
4.56 3830 3536 3283 3064 2873 2704 2554 2419 2298 2189 2089
Last edited by BLUE1972; Oct 1, 2018 at 10:36 PM.
Race Director
Joined: Mar 2016
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From: Napa Valley California
The gear calculator I use says that if he is running 255-60-15 tires, with a 1 to 1 gear in the trans, at 60 mph and 3,000 rpm he should have a 4.11 rear gear ratio.
That is providing his tachometer and speedometer are accurate.
That is providing his tachometer and speedometer are accurate.
Drifting
Joined: Jul 2015
Posts: 1,765
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From: Catlett VA
Why not just get the rear axle code off the differential? This will tell what the gear ratio was that came from the factory. The various calculations here for tire size don't help much. The speedometer displays speed by how many revolutions the transmission makes- it does not care what the tire size is. BTW- 3000 rpm on my 68 convert reads 60 mph on the speedometer. Actual speed due to my oversize tires on 20 inch rims is 68 mph by GPS. I have a 3.36 posi rear axle.
1968 axle codes: CODE RATIO AK 3.36:1Standard (327/350) AL 3.08:1Positraction (327/350) AM 3.36:1Positraction (327/350) AN 3.55:1Positraction (327/350
1975 axle ratios:Standard rear axle ratio3.36:1 Optional rear axle ratio3.70:1, 3.08:1, 3.36:1, 3.55:1
1968 axle codes: CODE RATIO AK 3.36:1Standard (327/350) AL 3.08:1Positraction (327/350) AM 3.36:1Positraction (327/350) AN 3.55:1Positraction (327/350
1975 axle ratios:Standard rear axle ratio3.36:1 Optional rear axle ratio3.70:1, 3.08:1, 3.36:1, 3.55:1
Last edited by ronarndt; Oct 2, 2018 at 12:21 AM. Reason: typo
Safety Car
Joined: Oct 2004
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From: Youngstown Ohio
That is correct. I have 255-60-15 tires and a 4.11 rear gear. My speedo reads 60 mph at 3100 rpm, auto trans.
Team Owner
Joined: Jun 2001
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From: Virginia USA
Team Owner
Joined: Jun 2001
Posts: 23,375
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From: Virginia USA