Checking Gearing calculator
 

https://waltermotorsports.com/pages/...ing-calculator

Could someone confirm this gearing calculator is correct? I bought a 3.6 diff a while back and ran the numbers and noticed that my cars rpm/mph matches up to a 3.9, not a 3.6. Im using a gps to test speed.
Are there any other factors I should be considering before i go complain to the seller?
Im running a 195/50r15 tire on a 7" wide rim.
Thanks!

I don't have that setup, but people regularly reference KMiata's calculator, so I assume that's accurate.
I just checked that against the calculator you linked and the numbers are the same across the board. I'd expect Walter's to be good considering they're known for transmission work.

cool, thanks for the feedback.

This is driving me nuts. I just got back from a drive, using my phone as the gps and ecu output for tach. 3,500 rmp in 6th was 75mph and 4000rpm in 6th was 85mph. That seems to work out to 3.73 gearing, which makes no sense.

Go on tirerack, and look at the specs for all 195/50r15 tires, you'll find a range of numbers for revs per mile, many in the 915 to 916 range. Do the math.

3.73 is fairly close. I'd assume with inaccuracies your within margin. If you still aren't convinced, you can jack the car up and spin the driveshaft. It should spin 3.63 times for 1 turn of the wheels. ~3.5 rotations. If it's closer to ~4 then you got sold a 3.9

Biggest variable in those gearing calculators is usually the tire circumference. They're calculated off the tire size, which are hugely variable between manufacturers. Best bet is to mark your tire and the ground, roll the car one rotation to get the most accurate circumference, and calculate from there.

I like this calculator.
https://www.tremec.com/calculadora.php

Useful miata specific ratios.
https://www.miata.net/garage/KnowYourCar/S8_Gears.html