Smaller Tires - Rear Drive Ratio

[crouse]

I bought a set of 2007-2009 Shelby GT500 wheels. The rear tires are Goodyear Eagle F1 285/40-18. Diameter is 27". I have 4.10's now. Is there a way to determine the new rear drive ratio? The speedometer is now about 5 mph slow. My Marauder wheels have the Nitto NT555 265/50-18 which has a tire height of 28.4"

[Turbov6Bryan]

Wallace racing has every calculator you would ever want

[gdmjoe]

The difference in wheel size is 5%. The RAR is going to be affected according.
.

[03Merc]

Adjust the tire revs on the tuner, use tire size calculator.com


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[justbob]

4.31


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[crouse]

Thanks guys.

I'm just going to run these for Auto Cross events, so I'm not worried about the speedo.

[cmalo]

Calculate the circumference (distance around the tire) of the new tires/wheels by multiplying
Pi X the diameter - shown below.

> 3.1416 X the diameter in inches
divide this by 12 to get feet

> This gives us the distance traveled for each tire rotation

> Figure out how fast the tires rotate in 4th (high) gear at 2000 RPM (the torque converter slips a little unless the clutch is locked - so be sure it locked)

> Divide that by 2 (2000 RPM)

> Now we have feet traveled per 1000 RPM

> There are 5,280 feet per mile


Chas358

[justbob]

Thanks guys.

I'm just going to run these for Auto Cross events, so I'm not worried about the speedo.

You’ll still need to adjust the revolutions per mile to keep your shift points in check.


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