Okay, the formula first. I don't know if there is a simpler formula, but this one works.
Take RPM Divided by the transmission Gear ratio of whatever gear you are in)
Divide this by your gear ratio in your differential(i.e. 3.73,3.27,etc.)
Multiply that by the circumference of your tires
To find the circumference measure the diameter of the tire and multiply it by pie(3.141592653) Measure the diameter in inches
Divide the result by 12, and then multiply by 60, and finally divide by 5280 and if everything is entered correct it will give you your MPH.
I can't remember the ratios of the T-5 offhand except for overdrive and first which are 0.68:1 and 3.35:1.
Go to
www.mustanggt.org/gttranny.htm for exact ratios for your year car.
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1991 GT, AOD, Moroso Cold Air, 3.73s, Pulleys, 3-chamber(the left one fell off-NO time+No Money=College), FMS c-springs, and KYBs