Equations

RPM/Tire Size/Gear Ratio

MPH = (rpm/60)*(pi*d/12)*(60/88)/(gear*od)

Where rpm is what your engine is turning, pi is the constant 3.14159..., d is the actual mounted diameter of your tire in inches, gear is your rear end ratio, and od is your transmission ratio (or overdrive ratio).

An example using my Cummins 4BT with 36inch Michelin 900r16 XZL tires with an Advance Adapters Ranger OD paired with the original transmission gives us:
(2500/60)*(pi*36/12)*(60/88)/(4.89*0.73)=74.97MPH

1990 Cummins 4BT is rated at 2500rpm

Ranger OD is 0.73:1 ratio (same as a NV4500 overdrive ratio)

Michelin XZL is 36.4" in diameter according to the company's literature (bias ply 900x16 tires tend to be closer to 34.5-35")

Truck has 4.89 gears installed in the differentials (stock truck has 5.83 gears)

Tire Size

For a metric sized tire: the first number is the width in mm, the second is the ratio of width to sidewall height, the third number is the rim diameter in inches.

So for a 315 85 16:
315mm x 85% = 267.75mm x 2 sidewalls = 535.50mm
535.50mm x .03937 (conversion factor) = 21.08 inches
21.08 inches + 16 (rim diameter) = 37"