: Speedometer Showing Wrong MPH



dretwenty5
04-06-07, 09:58 PM
My speedo is about 12 mph off, how can I get this fixed. Would the speedo effect the odometer readings as well.

eldorado99
04-07-07, 03:30 AM
Yes it will affect odometer readings, first you need to figure out why your speedo is off. Are you using non stock sized tires?

Red_October_7000
04-07-07, 03:31 AM
Put the factory wheels/tires back on it.
Seriously if you have changed them this will do it quite readily.
If you have the stock size, at least, then you have another issue. Is it a consistant 12MPH? Which direction? Digital dash or analog?

ewill3rd
04-07-07, 07:01 AM
On old cars with cable driven speedos, the odo ran off the speedo.
Nowadays the PCM sends pulses to the IPC and the IPC ticks off the miles and runs the speedo based on the same signal.

As stated above, what tire/wheel combo have you got?

Speedos are off by a ratio, that means it is not 12 mph off, but 12 mph off at a given speed. At lower speeds it will be less, at higher speeds it will be more.
Are we talking at 60 mph or something?

dretwenty5
04-07-07, 09:42 AM
I have the factory tires and rims on there. At 60 speedo reads 48, at 65 its 52. Digital dash.

misfit6794
04-07-07, 11:04 AM
Sounds like you are having a wheel speed sensor failure. Is your traction control and abs working properly?, or do you have any warning lights for either? They both need the speed sensor to be working to function correctly. Did you drive through a deep puddle of water? Thats almost always the reason wheel speed sensors go bad. The fact that your speedo is only a little off,relativelty, makes me think its getting a bad reading from one of the wheels. Hope this helps.

JTraik
04-07-07, 12:01 PM
Are you off by a percentage or off by an integer? Sounds more like a percentage. Did you swap gears in the rearend or anything like that?

dretwenty5
04-07-07, 05:34 PM
The car is completely stock, the ABS has started to come on once in a while though.

Rob Benham
04-08-07, 03:09 PM
I assume that the pulse is taken from one or more of the detectors on the ABS system. Is this correct? If so, have you tried wire brushing the castellations on the hubs? Some of mine were clogged with disc metal particles.

How are you checking the speed...GPS?

dretwenty5
04-08-07, 03:25 PM
The speed is being checked by a radar gun. The ABS code is for the front right wheel speed sensor.

Red_October_7000
04-09-07, 03:54 AM
A radar gun? Since speedometers don't error by an integer but by a percentage, I'd borrow a GPS and stick that in the car and see how that looks. Or just get somebody to follow you or drive along side you and communicate with CB radios or Cell phones.

ewill3rd
04-09-07, 08:24 AM
The ABS system is totally unrelated to the speedometer on that vehicle.
I think we are missing some vital information here.
How are you checking the speed with radar? Do you have a buddy that is a cop? I don't know anyone that owns a radar gun.

I'd agree with getting your hands on a GPS or having a friend drive next to you.
If all the equipment on your car is stock, there is almost no way for the speedometer to be off.
This could have only really happened after you changed something.

Motorboat
04-09-07, 10:21 AM
Time your progress between mile markers.

Red_October_7000
04-09-07, 01:50 PM
The ABS system is totally unrelated to the speedometer on that vehicle.
I think we are missing some vital information here.
How are you checking the speed with radar? Do you have a buddy that is a cop? I don't know anyone that owns a radar gun.

I'd agree with getting your hands on a GPS or having a friend drive next to you.
If all the equipment on your car is stock, there is almost no way for the speedometer to be off.
This could have only really happened after you changed something.

FYI you can buy radar guns at Radio Shack. They're not prohibited to own or anything special like that. But I do agree with you that it is a funny way to check speed. I'd get the "Second oppinion" of a GPS or something like that.

ewill3rd
04-09-07, 07:07 PM
I didn't know you could buy one. Why would you really want one?
I guess to see if the speedo is working right in your fleetwood?
:D

dretwenty5
04-12-07, 10:03 AM
I've had three different cars follow me and my vehicle speed is off 13 mph at 60. My speedo says 47 the other cars are all going 60.

jayoldschool
04-12-07, 12:58 PM
Go drive 20. The speedo won't be off by 13.

JTraik
04-12-07, 01:41 PM
I've had three different cars follow me and my vehicle speed is off 13 mph at 60. My speedo says 47 the other cars are all going 60.

Ok, it is off by a percentage, the car doesnt read 13 in park and it wont read 33 at 20. Go find what YOUR speed is at ACTUAL speeds of 20 40 60 and 80.

N0DIH
04-12-07, 09:38 PM
Is it the stock PCM? Or been reflashed and done with incorrect speedo info?

Has the rear axle been swapped?

ewill3rd
04-13-07, 08:40 AM
I have a thought on this but I have to dig out the manual and do some research, not sure when I can find time to do that but....

N0DIH
04-13-07, 10:04 AM
Is the speedo sensor maybe have a missing tooth? Hard to test unless you have an oscilloscope and connect to the speedsensor output and drive or rotate driveshaft.

The output would be:

_||_||_||_||_||____||_||_||_||_||

Where _||_ is one pulse logic low, logic high logic low. So if it missing one tooth for one rotation of the shaft, then that is the problem.

IIRC it is 40 pulses per mile isn't it? At least that is what TunerCat says on my OBD1. I can check the L05 if there is any speedo calibration in the definition file.

dretwenty5
04-13-07, 01:33 PM
There have been no modificatins to the pcm. Stock rear axle.

Rob Benham
04-15-07, 12:00 AM
Got a refurbished Garmin C330 for $269 to the door. Tried it out on my 95 FWB and the error was less than 1MPH at 30. If the thread is still running when I get on a highway, I'll cross check again.

dretwenty5
04-18-07, 08:23 PM
After doing some math I've come to the conclusion that the speedometer and odometer are 25% off, if the speedo sensor is damaged how hard would it be to replace this?