I have a 65 Ranchero, 289, C4, stock rear end (2.80?), 205/70 14's. Of course the speedometer is off, but I'm confused as to what gear I need to change to. Can anyone shed any light on this for me? Thanks!
This might help: https://lmr.com/products/Mustang-Speedometer-Gear-Calculator But if you're bad at math like me, just buy a few and try them out- they're cheap.
I plugged my numbers into the calculator and it comes up with 19.5 teeth (rounded up to 20), but I know a 21 tooth is accurate for both speed and odometer. This calc uses the unloaded rear tire radius based on tire size. My tires are 255/60/15 and has an unloaded radius of 13.5". The loaded radius is the correct value to use for this sort of calculation. My 255/60/15's measure 12.7". I was curious so I checked what gear would be calculated for a 255/50/15. The calc came up with a 21 tooth. The unloaded radius for the 50 series tire is 12.5". If you decide to buy more than one gear, I'd go with more teeth, not less, based on the above. I can help you dial in which gear you need, but you'll have to take a some measurements. The first is an accurate measure of the rear tire loaded radius. The second is the speedometer reading when you're going 60 mph using a GPS. You can get a phone app that has GPS speed. Speedo value when GPS reads 60 is needed. It would also be helpful to know how many teeth your current speedo gear has. All 3 of the above will assure an accurate recommendation. If you want to do it yourself, here's the formula: # Speedo Teeth = ((5280/(2*LoadedTireRadius*3.14)/12)*((RearGearRatio*DriveGearTeeth)/1000)) The number of teeth on the drive gear may be hard to come by unless you remove the tailshaft housing and count them.