I've done a search and come up with heaps on this but all seems to be related with diff changes.
mine has had no change in driveline at all but the speedo is off by about 10-15 Ks on the freeway.
Its not the original dash cluster although i'm led to belive this wont matter as it is a speedo gear on the box that tells the cluster how fast etc.
so if its not the diff
not the cluster
What is it? faulty speedo gear? sounds unlikley but i could be wrong any help would be great cheers

myabe ut had a different diff sometime in the past then changed back to stock but forgot to change the sender gear back to stock.

could be any number of things causing it

i think ill just pull out the gear and check ay.

Had a change in wheel sizes? If you want to fix it up, jaycar sell a kit called the "Speedo Corrector" That might help you out.

Yeah I was gonna ask how big your rims are...

17" xr6T rims with 255/45 rubber.

That would do it !!!!!!

From the factory the wheels were 205 60 R15.... (I think)
Because of the change of wheel size, it has caused the speedo to be incorrect....

There is nothing wrong with the car, speedo or anything like that....
Also, Ford purposely put a speedo error on the Guage to make the speedo read faster than what the actual car is travelling at... (To take into account things like tyre pressure differences, guage inaccuracy, etc...)
This is so that no matter what, the manufacturer cannot be held accountable for inacurate speed readings if someone was caught speeding.....

Only way to fix is to...
1) Play around with Speedo Gears (Use this page for calculations)
http://www.fordmods.com/forums/documents.php?doc=30
2) Get a Chaser Cluster, where you can program the Speedo Variation.

In any case... Have a look at that link, as it explains things a lot better than what I can explain here

i got 235 / 45 / 17's an mine is erfectly fine, within 1km

i did that calculator before and it only came up with about a 2km/h difference

Sorry but you are wrong!

Firstly the standard tyre was 205/65/15

Using this tyre calculator you get a error of only 2.1% not 15%!

Check this calculator out for more info http://www.miata.net/garage/tirecalc.html

and the spare is a 16" so i'm assuming they had the tickford wheel upgrade from factory so would make even less difference.

Hi All,

I've got the exact same problem with mine.

No diff ratio change.
EB GT Wheels with 235/45R17.

Getting an over-read of 10 km/hr.

Is it possible for the speed-sender unit to wearout?

The gear that is mounted on it is only made of plastic - could the gear be wearing down / out?

Either way, doesn't it mean that you've got to pull off the rear end of your gearbox to get at it?

Will

Dont forget this !!!!!!!
Also, Ford purposely put a speedo error on the Guage to make the speedo read faster than what the actual car is travelling at... (To take into account things like tyre pressure differences, guage inaccuracy, etc...)

Place the cluster in diagnostic mode, and you will find the LCD speed reading to be 10% out from the needle reading....
Again, the reason being is because of an inserted speedo error from the factory....
The rest is due to tyre size difference, and tyre pressure.....

Its like an elevator.... If it is rated to carry 9.99999999999 average weighted people, they dont mark it as being able to carry 10 ppl.....
It quite possibly could handle 10 people... It may even handle 11 people...
But if the thing breaks when loaded with 10 people... Who is in the wrong...
The 10 people for getting in to the elevator, or the elevator manufacturer...

Its quite common in engineering to have a TOLERANCE factor inserted for safety concerns.... A tolerance can be used for anything such as a resistor, to metal fatigue loadings, temperature limits, viscosity, in fact anything that cannot be guaranteed to be 100% accurate......

Ford has inserted a tolerance factor of roughly 10% JUST IN CASE some tool gets pinged for doing 102Km/Hr in a 100 zone, and tries to blame the manufacturer that the speedo calibration was out, and that he/she/it was not speeding according to the information that was directly or indirectly provided by the manufacturer.......

Remember, earth is an imperfect place... Nothing can be taken for granted, least of all anything that relates to engineering. Ambiant temperature, tyre pressure, tyre temperature, all add up to make a fraction of a difference here and there.... Its much easier for Ford to play it safe and say, make it read 110Km when the vehicle is only travelling 100Km

The point I am trying to make is, for wheels to change the speedo by 15% you need to change the circumference of the wheel by 15%. Now think about it. The standard wheel has a circumference of ~2034mm to increase that by 15% would increase it by over 300mm! Thats a hell of a lot. A massive change in wheel/tyre diametre would be require. Something that would not fit in the wheel arches!


Also there is no way Ford puts as much as a 10% error into the speedo. If yours is then you also has a problem. The ADR's may state that a speedo has to be accurate up to 10% either way but generally they are much better than this. Every car I've owned has a slight error around 1.5%-2% usually in my favour, ie I don't get booked.

I suggest you don't try and use the "my speedo must be out" as an excuse next time you get booked. Cops wont wear it, and niether will Ford!

As for elevators they would have a huge safety margin built into them just like cranes. They also quote a SWL in kgs not just people. Also their physical size prevents them from being overload.

and also ford is'nt the only one who build in a certain margin for error so when a mate is driving along side at 110 goin past me pretty quick and i'm reading 110 then i think i have an issue.