How fast am I going?

Physics? Just a little math�

Tire height is approx: 15" + (345 * 40% * 2)/25.4 = 25.87" (25.4 is mm per inch)
So circumference is approx: 3.14 * 25.87" = 81.23" (3.14 is Pi)

So speed in MPH = RPM * 81.23" / gear ratio / final ratio * 60 / 63,360

Where:
60 is minutes/hour
63,360 is inches/mile

So, if 5th gear is 0.67 (I am making this up) and final ratio is 4.22 (I think that is right) and we use 2,500 RPM we get�

2,500 RPM * 81.23" / 0.67 / 4.22 * 60 / 63,360 = 68 MPH

Seem about right? Hope that helps.

I've found that unless you run a 26-1/2" to 26-3/4" dia tire, the speedo will always be optimistic. If you measure your particular tire diameter, then it's a simple ratio of 26-1/2 divided by whatever the skin is on your car, to find what speed it should be showing. Note- a 345-40 Goodyear will almost never not be the same dia or width as a 345-40 BFG, MIchelin or whatever; you MUST measure what you have, on your wheel. A type 1 gearbox originally had a very low ratio ring & pinion; most were recalled by Ford. Some have since been modified. Type 1's have a bellhousing with two small access ports centered in the top edge, the type 2 has a single larger port in the left side. The gearboxes & bellhousings do NOT interchange- its a package deal.