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Thread: Smaller Tires - Rear Drive Ratio

  1. #1
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    Smaller Tires - Rear Drive Ratio

    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"

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  3. #3
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    The difference in wheel size is 5%. The RAR is going to be affected according.
    .

  4. #4
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    Adjust the tire revs on the tuner, use tire size calculator.com


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  5. #5
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    Smaller Tires - Rear Drive Ratio

    4.31


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    Cobra short block / ProCharger F1-A / Cog drive upgrade / Manley valves / Brian Tooley Racing valve springs / 80# inj. / -8 fuel line / -6 return / Aeromotive 340 pumps and regulator / 2012 GT500 TR6060 six speed / twin disc clutch / Driveshaft Shop GT500 upgraded CV joint shaft / BA5000 blow thru MAF/ 4.10's / Ford Racing 31 spline set up and Girdle / Metco D.S. safety loop / Stainless Works headers, cats, and everything else out back / ClassGlass hood / Bob's hood strut mod / Hurst line lock / Metco control arms / Addco front and rear sway bars / Monroe severe duty shocks all around / Race concepts rotors / Badgeless grille / 35% tint / FBM's interior LED kit and ambient lighting kit / Pioneer FB700 deck / kenwood speaker 4 Ch. / Rockford Fosgate 10" sub and amp / Kicker 6 X 8's all around / back up cam / bluetooth / Pioneer Xm / 704 RWHP 603 RWTQ tuned by Mo's Speedshop.

  6. #6
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    Thanks guys.

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

  7. #7
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    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


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  8. #8
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    Quote Originally Posted by crouse View Post
    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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    Cobra short block / ProCharger F1-A / Cog drive upgrade / Manley valves / Brian Tooley Racing valve springs / 80# inj. / -8 fuel line / -6 return / Aeromotive 340 pumps and regulator / 2012 GT500 TR6060 six speed / twin disc clutch / Driveshaft Shop GT500 upgraded CV joint shaft / BA5000 blow thru MAF/ 4.10's / Ford Racing 31 spline set up and Girdle / Metco D.S. safety loop / Stainless Works headers, cats, and everything else out back / ClassGlass hood / Bob's hood strut mod / Hurst line lock / Metco control arms / Addco front and rear sway bars / Monroe severe duty shocks all around / Race concepts rotors / Badgeless grille / 35% tint / FBM's interior LED kit and ambient lighting kit / Pioneer FB700 deck / kenwood speaker 4 Ch. / Rockford Fosgate 10" sub and amp / Kicker 6 X 8's all around / back up cam / bluetooth / Pioneer Xm / 704 RWHP 603 RWTQ tuned by Mo's Speedshop.

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