Does anyone know how to measure tachometer accuracy?

[Joester]

the theoretical engine RPM @ 70mph with the numbers given would be 3083rpm.

I'm not missing a reduction am I? I have the transmission, the diff, and the wheel. There's no primary reduction in the transmission or anything is there?

If not, then you should be cruising somewhere closer to 3k rpm at 70mph.

[hrach1]

the theoretical engine RPM @ 70mph with the numbers given would be 3083rpm.

I'm not missing a reduction am I? I have the transmission, the diff, and the wheel. There's no primary reduction in the transmission or anything is there?

If not, then you should be cruising somewhere closer to 3k rpm at 70mph.

I am not sure what you mean about missing a reduction. I am clueless when it comes to calculating the rpm so I take whatever you say. Will take the car for a drive tonight and are what is the rpm I am cruising on. Also, I know my speed is not accurate either so I will use my iPhone and GPS app to measure my speed correctly. I would rather cruise at 2500 RPM or below and I think the 2.93 diff would do it.
Would you reply with the calculation formula also please?

Thanks again

[Joester]

engine RPM/gear ratio/diff ratio/13.55 = mph - if you have a known RPM and you want speed

OR

mph * 13.55 * diff ratio * gear ratio = rpm - if you have a known speed and want RPM




the 13.55 is just a conversion factor from rear tire RPM to car speed, in case you were curious.

NOTE TO ANYONE WHO STUMBLES UPON THIS POST LATER: 13.55 can only be used if your tire diameter is 24.8"!!!!! I don't want anyone getting wrong info from this post if they stumble upon it in a few months, or years. You can get your own conversion factor by taking 1056/(tire CIRCUMFERENCE in inches). 1056 is the conversion from in/min to mph.

so if you have a tire 80 inches in circumference, your conversion factor would be 1056/80=13.2, and your equation that would work for your car would be

engine RPM/gear ratio/diff ratio/13.2 = mph

[hrach1]

engine RPM/gear ratio/diff ratio/13.55 = mph - if you have a known RPM and you want speed

OR

mph * 13.55 * diff ratio * gear ratio = rpm - if you have a known speed and want RPM




the 13.55 is just a conversion factor from rear tire RPM to car speed, in case you were curious.

NOTE TO ANYONE WHO STUMBLES UPON THIS POST LATER: 13.55 can only be used if your tire diameter is 24.8"!!!!! I don't want anyone getting wrong info from this post if they stumble upon it in a few months, or years. You can get your own conversion factor by taking 1056/(tire CIRCUMFERENCE in inches). 1056 is the conversion from in/min to mph.

so if you have a tire 80 inches in circumference, your conversion factor would be 1056/80=13.2, and your equation that would work for your car would be

engine RPM/gear ratio/diff ratio/13.2 = mph

Thank you very much Joester
So, to find the rpm I have to so the following, right?

70 ( for 70 MPH) x 3.55 x 3.25 (diff ratio) x 1.1 (for ZF tranny gear ratio)?

That gives me 888.3875

What does that mean?

[Joester]

70 ( for 70 MPH) x 3.55 x 3.25 (diff ratio) x 1.1 (for ZF tranny gear ratio)?

13.55, not 3.55

70 * 13.55 * 3.25 * 1 (i am pretty sure 5th gear ratio is 1) = 3082.625 engine rpm.

[hrach1]

13.55, not 3.55

70 * 13.55 * 3.25 * 1 (i am pretty sure 5th gear ratio is 1) = 3082.625 engine rpm.

Got it! thanks

[Joester]

no problem. That could be very slightly off because of odd tire deformation issues, but its not going to be 1000rpm off. I'd measure your tires just to be sure that 24.8" is a legit measurement.

[hrach1]

no problem. That could be very slightly off because of odd tire deformation issues, but its not going to be 1000rpm off. I'd measure your tires just to be sure that 24.8" is a legit measurement.

Well, I took the measurement from the Khumo's website and my tires are brand new. Shouldn't trust manufacturer's measurements?;)

[hrach1]

no problem. That could be very slightly off because of odd tire deformation issues, but its not going to be 1000rpm off. I'd measure your tires just to be sure that 24.8" is a legit measurement.

Ok, I drove the car at 70 MPH and my RPM reading was 3600. I also tried it at 80 MPH and the rpm reading was 4100. So, it looks like my rpm is showing 600+. Anyway, now that I know what my rpm is, I will get the 2.93 diff.

[Joester]

70mph on a GPS, right?

[nando]

you know you could fix this whole issue with the right coding plug, right? E28 M5 AFAIK.

a regular 6 cylinder coding plug doesn't work because the scale of the tach face is different on the M3.

[hrach1]

70mph on a GPS, right?

Yes, I measured it by two different GPS programs. My speedometer is off by almost 3 miles. Not bad.

[hrach1]

you know you could fix this whole issue with the right coding plug, right? E28 M5 AFAIK.

a regular 6 cylinder coding plug doesn't work because the scale of the tach face is different on the M3.

Really? If so I will get it right away. what is AFAIK?

[nando]

as far as I know

[hrach1]

as far as I know

Lol!