Pop quiz hot shot… [automatic speedometer in a manual]
 

I have an 86 NA with an S5 Automatic gauge cluster. Also my tires are 245/45/R17.

How far off will my speedometer be?

I can figure out the difference due to the tires; but what about the auto gauge cluster? Do I only need to account for the rear end gear ratio or is the speed-o gearing in the tranny and gauge different on the auto?

Thanks,
Jason

All the speedo's read the same, regardless of series or auto/manual. The differences are in the transmission speedometer output gear, the thing that the other end of the speedo plugs into. For instance...the auto car comes with the 3.9 rearend, and the auto trans has the speedo gear for the 3.9 (orange). The 5sp cars come with the 4.1 rearend, and the trans has the speedo gear for the 4.1 (gray). The GTUs has the 4.3, and the trans in it has the gear for the 4.3 (forget what color).

SO, if you take an auto car, and put in a trans from a 5sp, that trans has the gear for the 4.1 in it. IF you kept the 3.9 rearend, your readings would be way off. IN this case, you'd need to get the old gear out of your auto trans and swap into the 5sp...now it would read right (not accounting for funky tire sizes). OR, where my 'vert was originally 5sp, and I swapped in the 4.3 GTUs rear, I had to also swap the GTUs speedo gear for it to read right.

As far as differences with your tires, check here:

http://www.miata.net/garage/tirecalc.html

BTW, the speedo gear is held in place by a single 10mm bolt, and is pretty easy to get it out. Expect fluid to spill everywhere, so drain it ahead of time, or be very fast with putting the replacement gear back in the hole. Sometimes you have to take pliers or the like and wiggle the gear assy. around to get it to slide out, they fit tightly to prevent fluid leakage.

Thanks!