What transmission does your car have? Here is a good calculator. West Coast Classic Cougar Speedometer Gear Calculator
Just put in 3.50 (your rear ratio), 26.1 (your tire's outer diameter) and find out how many teeth the drive gear (the gear mounted on the tailshaft) has. I put in 8 (that was the standard for the C4) automatic, and it said you should use a 21 tooth gear (that's the closest to what you would need to be 100% accurate). With that gear your speedometer would read 1mph fast. But that's if you have a C4 with 8 teeth on the drive gear.
Drive gear=gear mounted on tailshaft, Driven gear=gear mounted on speedometer cable