Incorrect spedometer

Topic: Incorrect spedometer (Read 896 times) previous topic - next topic

Incorrect spedometer

May 23, 2009, 04:41:50 PM

well today i borrowed my dad's gps and found out that my cougar's spedometer is 5mph fast. I was wondering if there was anything i could do without having to take it to a mechanic. I'm not very mechanically inclined but i could follow simple steps if possible.

Incorrect spedometer

Reply #1 – May 23, 2009, 04:51:34 PM

also i realize that this is in the wrong forum, i apologize for that.

Incorrect spedometer

Reply #2 – May 23, 2009, 06:23:41 PM

What year car?
What kind of car?
What size tires?

In short -- we need a bit more info. Welcome aboard BTW.

Incorrect spedometer

Reply #3 – May 23, 2009, 06:28:44 PM

96 cougar xr7, 3.8L v6, I pretty sure it's got the right size tires, i know my spedometer was correct when i first started driving it, and i haven't changed my tires since then so idk what's wrong.

Incorrect spedometer

Reply #4 – May 23, 2009, 07:43:20 PM

Unless the speedo is defective(rarely and still operate), tires or the speedo drive gear in the transmission is going to be the problem... Now, it's not uncommon for a speedo to be off by 10%, which is going to be 6mph at 60... I'd be willing to bet it's been wrong all along...

According to a speedo calibration shop owner I've talked with, each tooth added to the drive gear count will slow the reading approx 3mph(at best that's a rough est, there are variables)... That meaning if yours has a 19 tooth gear, replacing it with a 20 will put it within 2mph... Rarely is any older vehicle closer than that, even when new...

Incorrect spedometer

Reply #5 – May 23, 2009, 08:15:10 PM

With my old gaerset and 235/60-15's my indicated speed was less than actual.

Now I'm 3MPH off throughout the entire range. Indicated 70 Actual 67

It's 3 MPH off at speeds as low as 14 MPH..... The new tires I have in mind should correct that.

Incorrect spedometer

Reply #6 – May 24, 2009, 12:04:17 PM

Go here. http://www.miata.net/garage/tirecalc.html play around a bit and you should be able to locate the tire size you need to make it more accurate. I would start with the stock size and work off that. Also not every brand is the same so even if that tells you are dead on it will all depend on the brand and such.

Stuckman