Speedometer 10% off?

[Low_HP]

After comparing my speedometer to two different GPS units, it seems its reading about 10% too fast. 1988 Tercel coupe, auto trans, 155/80 13" tires. Any ideas? Thanks!

[bkftl]

hmm, ur speedomoter is conencted with thick wire that spins, if it works it must be connected right.

I think theres something wrong with ur transmissioin might be but check ur tire size, it might be off by a little because of tire size. although i think ur wheels must be ok since there 13? mine are R13 too

might be the speedometer it self, im not sure, mine doesnt work

[noahthefrog]

it's probably because you are using 155-80-R13 instead of 155-70-R13 that the car came with. Your tires are a little bit taller than stock, therefore take more distance to complete a revolution, making the speedo wrong.

Tire size calculator says your speedo should be off by 5.7% when I plug in the numbers:

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

Incidentally, this also alters your gearing since the tire is bigger around, so it makes the car ever so slightly less spunky.

EDIT: Correction, your speedo should be reading that you are going slower than you are not faster... Not sure why that would be. Maybe the GPS just isn't compensating for your altitude? If you can find a speed trap that has the display setup to tell you how fast you are going, that may shed some light on the actual speed of your car

[Low_HP]

GPS is at sea level, so I think that may not be the issue. I'll double check the tire size and make sure I've got that part of the equation right. Thanks for the input so far!

ETA: tires are indeed 155/80R13's....wonder if I can 're-gear' the cluster to read correctly?

[noahthefrog]

There's no re-gearing you can do. It is all electronic on the late '80's Tercels... the needle is the only moving part in the dash. Maybe the computer can be adjusted, but I'm not sure.

EDIT: My mistake. The Haynes manual says there is a speedo cable going from the cluster to the transmission, which presumably. I remember reading somewhere else that it was electronic, but that must have been for a different car. Perhaps it can be re-geared...

BTW... GPS is not going the most accurate thing to use to gauge your true speed in any event... it's readings are subject to be off by 3ft or more for location, so if it's using location to calculate speed... it won't be exact.

[WilliamRobert]

If your speedo is off, that means that your odometer should be off as well. We have a couple of odometer test sections on the highway here , just outside of Edmonton. However mile markers should provide the same trick. keep the speed constant and zero the odometer at a mile marker and do 5 miles. If the odometer reads a little off from the markers, then you can do some basic math and figure out how far off your odometer and speedo are.


I assume that there are mile markers everywhere in the usa.



BillyBob

[Low_HP]

Thanks for the tips...I'll try the mile marker suggestion. Worse case, I guess I could upgrade the tires and/or wheels (not a bad idea anyhow) and compensate for the error. Is there a formula to play around with? Off to Google!

[Tercel GTS]

Look up tire calculator on the miata forums.

[nighthawk8]

I have a 1997 toyota tercel 2d auto. and mine seems to be off as well. my gps says its off by 5 km's and i do have the right tire size 155 70 r 13.

[scys]

My 95 tercel with auto/overdrive with 155/80/13, reads 10% to high as well. I have an extra set of tires and can tell you that if you put 195/60/15 it will read 4% to slow and I had very slight rub one way. It handled much better though.
Cy