Can anyone tell me the best way to calibrate speedometer in my 94 3rd gen. Or let me know how fast you speedometer says when you are at 1000 RPM

 

at 60mph in 5th you will be at 2500rpm with stock wheel and tire sizes. that's the easiest way to do it.

 

^ Is this calculation accurate? Does anyone know what the formula might be with 18" rims?

 

wheel size does not make a difference, only outside tire diameter.

 

When you take your cluster apart, there are printed equalities between MPH and RPM right on the guage face. But, as stated earlier, this is with stock sized tires.

 

OK....well, I have 18" rims with 35's in the rear & 40's in the front. Is it based on the front or rears?

Also, the printed formula on the speedo says: 60 MPH= 1024 RPM. The question I have is at what gear? And how do I adjust for my rim & tire sizes?

Thanks,

Ian

 

You also need to give the section width. The 35 and 40 numbers have to be used as a percentage and multiplied by the section width to determine sidewall height.... In other words, if you are running 265mm section width tires in the rear, then the sidewall height is 0.35 x 265mm = 92.75mm = 3.65in

So....(18in wheels + (3.65in x 2)) x pi = 79.48in rolling circumference.
Which I know isn't your question exactly, but I'm trying to show how the numbers on the sides of your tire actually work.

-James

 

You can use this calculator to compare tire sizes and how they affect the speedo
http://www.miata.net/garage/tirecalc.html

 

If you're comparing the speedometer to the tach you'll get the same speed vs. rpm no matter what size tire is on the car. The speedometer has no idea how fast you're actually going, it only knows how fast the driveshaft is turning.