That's a good point about the tire diameter and it's not quite as simple as just the rear-end ratio. What really matters is the overall ratio which is a combo of tranny ratio, rear-end ratio and tire diameter, especially when you are dealing with other than the 1:1 tranny ratio. It's made even more complicated if you are running an auto tranny with a racing converter as the converter slip affects the overall ratio. A converter slipping at 10% (assuming it is unlocked) is going to make a 3.90 act more like 4.29. Combine that with a 26" tire and an engine strong enough to trap 135mph and the engine is buzzing at 7600 thru the traps. Drop it down to a 3.27 and it traps at 6400 and if you don't want to go over 6000, you have to run a 3.08. Not a problem for a turbo car as the taller gear puts the engine under more load making more power/torque.
A manual tranny has no slip so a 3.90 is still a 3.90. However, combined with the tranny's 1st gear ratio and a 24.5" tire, the engine revs too quick in 1st and does not put the engine under load and most likely the turbo will not make max boost in 1st. It is sometimes hard to remember, but you cannot apply the n/a gearing mindset to a F.I. car.