Speed and tire size and speedometer gear ratios questions

[BabyBronco89]

so first question is what size tires did the bronco II Have when it rolled off the production line?

i know mine has bigger than stock tires but not by much ill look tom

anyways well driving behind my brother on the highway, he has a 04 mazda 6 stationwagon, i was talking to him on the phone trying to calcuate my speed
at apox 70 mph it says 65mph on my gauge and at 73-74mph it was 80mph on my gauge and by the time it reached 90(my guage only goes to 85)
my brother informed me we were going 105!!!!!! lol that 4.0 will really move a b2 and it was still gaining speed lol

so basically my gear in the trans case needs to be adjusted acordingly
i want to get a nice set of 30's with my tax money what size gear would i need with these tires?

whats the formula to figure this out?

i have 3.73 gears in the 7.5 rear, (no front because the uvjoints are shot)
but i have a 91 explorer lined up for 200$ with a unkown mileage of 100 0r 200 thou on her and i do not know what size gears are in that.
but it has a working five speed man trans and 4x4 so hopefully i can put the d35 up front to replace the shot d28 and also replace my rear.

so which of the transfer cases is more desirable the ex or b2 both are electric are they the same?

does rear end ratio and tire size both affect speedo gearing?

thanks for the help!

 

[PetesPonies]

There are some formulas, but its easier than that with the info you have. Its completely linear. In other words, if you increase the tire height by 10%, your speedometer will be off by 10%. It also means a change of speedometer gear teeth of 10% will put it back correct.Figuring tire height is easy, but do you know how?? If not, you can just measure the height. What size do you now have? Your numbers don't make sense as you wrote it. You have your speedo being behind in speed once and ahead at another speed. Thats impossible. The % it is off stays the same, there is nothing variable here, its not a curve either, its linear math. If you give me accurate info, I can tell you an exact answer.