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Old 04/06/2011, 08:30 PM   #7
Mosesatm
 
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Location: Spokane, Washington
Joined: Jan 05
Posts: 7,351

Using the correct diameter rear tires these should be your estimated RPMs at 60mph


2.79 = 2150
3.00 = 2320
3.25 = 2510
3.55 = 2740

Here's a formula for calculating the numbers.
RPM = MPH (Transmission Final Drive)(336)(Rear Ratio) / (Tire Diameter)

For example, on my car I'd get:
RPM = (60mph)(1)(3.00)(336) / (26.1") = 2371

There are also calculators on the web
http://www.csgnetwork.com/multirpmcalc.html
http://www.ringpinion.com/Calc_RPM.aspx
http://www.angelfire.com/space/slr/RPMCalculator.htm

I won't swear to this but I think the "5" transmission code on the data plate was for the wide-ratio toploader, and that was the only one offered on '68 Mustangs.

Last edited by Mosesatm; 04/06/2011 at 08:56 PM..
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