Please Clear Up Odometer Error Question

Paulporsche · 08-19-2010, 06:16 AM

I know that running a smaller diameter than stock tire will make my speedo read higher than actual speed. I've searched this, but not found a definitive answer: will the same thing happen to the odometer, namely, will the odometer read more miles traveled than actual?

It seems as though it would, but somewhere, in the recesses of the mind, I seem to recall that someone wrote that it did not, and why.

ossiblue · 08-19-2010, 06:29 AM

The only input to the speedo/odo is through the connection to the tranny, be it cable or wiring. AFIK, there is no "magical" way the tranny can determine distances covered other than by the revolution of the wheels through the axles. That being said, smaller tires=more revolutions of the axles=higher recorded "speeds"=more miles "covered". It is possible to recalibrate a speedometer/odometer to compensate for smaller tires, however.

Paulporsche · 08-19-2010, 06:49 AM

I might have been confusing it with engine rpms.

Did I read that engine speed (observed on gauge v actual) is not dependant on tire diameter?

Bob Kontak · Bob Kontak's Garage

I think he said the trans is blind to tire diameter and it is dependent - just reports to the speedo the number of pulses it experiences.

Edit - I also read on a speedo repair post that the odo and speedo are not driven by the same thing. But I don't know if this is true so this does not really answer your odo question. I would like to know as well.

I have a 'fast" speedo and odo - about 10% faster from GPS on speed and mileage markers on highway on odo. They are consistent in their error. Need to check this (source feed) before performing the potentiometer adjustment referenced in other threads - would be a bummer to fix speedo but leave odo in error.

Bob Kontak · Bob Kontak's Garage

I just called North Hollywood Speedometer and talked to Kevin. He said the speedo and odo feeds both come from the transmission but the speedo is adjustable with a potentiometer and the odo is not as there is an integrated circuit in the mix for the odo.

He said changing a gear in the speedo unit can change odo readings - but to a very large degree (20%) or so. I would imagine the gear is for driving the actual numbers on the dial.

So as far as I can see you (we) are stuck with odo readings - regardless of tire diameter or speedo potentiometer adjustments.

He also said they adjust the speedo based on the tire size you are running on you car.

Paulporsche · 08-19-2010, 09:58 AM

Thanks, LJ and Bob. That's my understanding too. It must have been the tach reading (engine speed) that has no relation to tire size.

I realize of course that engine speeds per se can be affected by tire speed. I'm referring to the tach "reading".

JJ 911SC · JJ 911SC's Garage

Quote:

Originally Posted by Paulporsche

Thanks, LJ and Bob. That's my understanding too. It must have been the tach reading (engine speed) that has no relation to tire size.

I realize of course that engine speeds per se can be affected by tire speed. I'm referring to the tach "reading".

This "cryptic" thread has been forward to the RCMP for possible decoding of the timing and delivery point of the importing/exporting illegal substance... Thank God for legal booze