Speed Calculator
OK, so you got the old Truck running, but you find it tops out at 42 mph. Hmmm.. geared too low? Tires too small? Use this calculator to determine your speed at any given RPM. Play around with various differential ratios and tire sizes to find the set-up that works for your driving habits. I created an Excel spreadsheet that will also do these same calculations, but will let you print and compare a whole series of different info. Please email me if you would like a copy.
Notes - Tire diameter is the the height of your tire (i.e. 24", 26" 28"...etc, not the rim size (i.e. 15", 16" etc). Most manual transmissions are 1:1 ratio in high gear (i.e. 4th on a 4-speed, 3rd on a three-speed). If you leave the Transmission and Transfer case fields blank, it will use 1:1 and will not affect the computed MPH.
If you do not know your axle's ratio, it is probably stamped somewhere on most old Dodge truck differential casings. If you can't find it, you can also raise one wheel, carefully count the number of turns at the wheel for each turn of the drive shaft, and then multiply times two.
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