speedometer calibration?

have a 95 taurus, speedometer is fairly accurate at low speeds (less then 25mph) but reads slower then actual as the speed increase's indicating 48mph, GPS shows 55mph.
tires are stock 205/65/15.
any ideas?
thanks!

I think the speedometer on the 1995 was electronic rather than cable driven, I don't have my service manuals available to reference from here at work. If the speedo is electronic and off by that much at higher speeds, that's either going to be a VSS issue or possibly the speedo head in the instrument cluster.

On the newer cars you can "calibrate" the speedo through the OBD-II port, but even then it's a linear calibration which probably wouldn't solve your issue.

-Rod

thanks for the quick reply, let me know what the books say when you get the opportunity.

For the standard Taurus with an analog instrument cluster, there is no signal connection between the VSS and the instrument cluster. It would appear unless you have the SHO or the digital cluster, the speedometer is cable driven. If your speedometer is off significantly, probably your only option to fix it is to replace the speedometer head once you confirm your tires are all properly inflated.

-Rod

thanks again Rod, it's a lx model, wonder how the cable driven speedometer could be that far off? maybe it's been repaired in the past and the drive gear to the head unit was changed with the wrong one?
I know my '85 F350 had the engine and trans swapped out and is 12mph on the fast side (speedo ) just a matter of getting the right gear for the end of the cable inside the tranny.

maybe the gps is off...

borrow someone else's portable gps & confirm

that's weird that all of a sudden your speed is off when you're using OEM tire size & nothing else changed:2cents:

edit: oh ok you're not the original owner, so that's a possibility that it was replaced with a wrong gear; but i doubt it if the original trans is still there
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more likely the cable may be binding during higher speeds:

Your Taurus is cable driven, go between the firewall and engine and there will be a cable connector with spring clips holding the cable housings into it remove the clip holding the lower cable, slip them apart and try to pull the cable up, lubricate it and reinstall, twist the cable as it gets seated into the drive.

The problem may go away after a few miles, if it don't- lube upper cable or do both at once if your comfortable with it. However the upper one may need to pull the dash, when you put the dash back in be very careful of the connectors! This worked for us on 2 different cars! Both 95's. Good luck.