Any auto mechanics here?

[Kurt]

I am trying to calibrate my speedometer in my 1991 Cadillac DeVille, and I was curious if anyone in here knows how to do it.

Thanks
Kurt

[planedick]

Calibration...

Most all speedos, auto and MC, require that 2000 RPM input equals 60 MPH.

[81xsproject]

That doesn't sound right. At 60mph my yota is at around 2500rpm and my xs is at over 4000rpm.

[DiverRay]

The 2000 RPM is the WHEEL speed, not engine speed. I don't remembere the exact figure, but I think it is 1875 revs per mile.
An easy way to check is with the milage posts that are on most major streets. Starting at one, with the trip odo zeroed, drive two miles, and check the odo. The amount it is off should tell you how far off the speedo is. BTW, most US speedo's are optomistic, and read about 5% off.
Ray

[TopCatGr58]

To add to this mile marker check. Use a stop watch, hopefully with a passenger. At 60 mph, it should take you exactly 1 minute to travel from 1 marker to the next, 2 for 2, etc. So, if speedo says 60MPH, and you hit the 2nd marker before the 2 minutes lapses, then you are actually going faster. If it takes longer, then you are going slower than the displayed 60 mph!
T.C.

[planedick]

Not wheel speed...

The 2000 RPM is not the wheel speed, it's the speed of the speedo cable at the speedometer back. Calibration is done by attaching a dpeedo drive that is rotating at exactly 2000 RPM and the speedo should read 60 MPH

[desert1]

GPS

I checked my Honda civic speedo using a hanheld Garman GPS and it was bang on at 60MPH .....
Bruce

[planedick]

Radar

Around here there is many radar signs to tell you if you're going too fast. Especially around schools. Just go too fast and they light up and tell you how fast you're going, then tell you to slow down. That's how I found the Yamy speedo to be 5 MPH fast.

[mn2346]

One possibility, if you need a paper copy, is that you drive around and find a black and white car with colourful buckets upside down on the roof.

Drive fast either passing or crossing near this vehicle, if there is no immediate response repeat couple of times (they might have another customer) but keep the speed and try to remember that for a while.

The customer service rep. will be stop you when a solid reading has been made and come to bring you a personal certificate of your speed with a bill. Check the speed and match it to your own number.

That's it, you're done!

[Kurt]

That's it, you're done!

That's funny. I'll try it and send you a bill.

I know it's off by at least 10-15mph from all of your suggestions. The real problem is that it is very inconsistant. It will read very high at low speeds, and within 5mph at high speeds. I have no idea about how to fix the problem. Id rather not tear apart everything trying to find out how to adjust it.

Would the problem be at the wheel, in the dash, or ?

Thanks for all the help.

[Snow]

I'm not sure what speedo drive mechanism a 91 caddy has, but I'd think it's probably electronic from the transmission and that is probably where the source of a problem might be if it's intermittent...just a guess

[Kurt]

I thought that might be it too, but I couldn't find it. I think that the previous owner did something to it to accomidate his Bling Bling 18" rims that were on it.

I didn't notice the speedo until I put the stock rims back on it.

[DiverRay]

Kurt,
The adjustment can be made by a dealer. The computer needs to be "readjusted" to the stock wheels, if the PO did what I think he did. You can buy the tools to reprogram the car, but they will cost about $250 to $350. The dealer should be able to reset the system for under $100.
The speedo is electronic, with the pick-up on the output shaft of the trans if it's rear wheel drive, and on the transaxle if it's front wheel drive.
Ray