[Neko-]

Drove home yesterday in my 2003GT (which to my knowledge is complete stock) and had the car set to cruise-control on 120 km/h (around 75 MPH) which is the legal speedlimit here. I use the inner-ring that is present on my speed-indicator which depicts the speed as 120 km/h.

I had two cars tailing me (friends) those being an Escort and a Focus. As we stopped for a bite to eat they informed me about the fact that according to their indicators I was consistantly doing 130 km/h, which is over the speedlimit.

Since the equivalent of the 'highwaypatrol' here is pretty dead-set on collecting cash, I'm now trying to ascertain if the indicator indeed is showing an offset (which I plan to do with the onboard GPS (and yes, I know that one only offers an indication of my speed)), aswell as possible angles on how to correct this to a tolerable level.

I'm well aware that the car will always have an offset, so by law the police have instigated a correction of about 3 to 4 km/h before issuing a ticket to allow for the speed discrepancies in the actual and indicated speed. They need to check my real speed with devices that have been checked to indicate the actual and correct speed of the car. Since the car will have an offset the correction is factored into the decision to actually send me a speeding-ticket.

Now my local Ford dealer (who does do the European models (which include the '99 Ford Cougar for comparison), but not focuses on the US ones, could tell me that the speed idication is handeled by the onboard computer (same as with the aforementioned Cougar, which I already was aware of), but they could tell me that this is something that is not alterable in the computer or something so that the indicated speed will be closer to the actual speed.

While searching the forums I did come across a few posts about 'Speed Cals', which I gather are to adjust speed indications after changes to the rear-axle. My car did have work done on it's rear-axle, but it was 'revised' and thus I don't expect this to actually have any impact on the way the car behaves.

So what I'd like to ask is if this is a known problem (do more people have issues with it), and what are possible things I could do to correct this to tolerable levels?

Thanks for any and all replies.

[FunkyBoss]

6 mph off is kinda high, but not extreme. Do you still have your original tires on? Tire size can effect your speedo just as gears will.

[Neko-]

The car has the standard 17" tires under it: 245/45ZR17.

Either my car is showing my speed being too slow, while actually speeding, or the other two cars that were behind me are showing a faster speed on the display then they actually travel.

It's been brought to my attention that cars appearantly are put on the road with a 5% to 10% offset on the counter. This means the Focus and Escort may well have too high a speed on the counter while still driving only 120 km/h. (so they indicate 130 km/h, while they're actually doing 120 km/h)

This offset is mainly to (appearantly) accomodate dutch legislation which (and I was told this, it's not like I actually started to ask questions higher up) states that a car is allowed to indicate too high a speed when driving, but having a lower speed on the thing is forbidden by law.

So if I read that correctly that means that if the thing indicates 120 km/h, and I'm actually doing 130 km/h... I'm breaking the law with every mile I drive. Then again... The car was trialed prior to receiving the go-ahead from the dutch instances to actually receive dutch license plates, so I'm gathering that should've been something they should've checked aswell.

I'm planning on taking some measurement with GPS, and/or taking a look at the hectometer-signs that are placed at 100 meter intervals next to the highways here to try and work out what my actual speed really is.

[Neko-]

Can't edit the post, so I'm adding a new one. Used GPS yesterday and according to that thing the speed-indication is pretty much on the mark.

My guess is that the indicators in the Focus and Escort are off for some reason or another. I'm planning on going off with my father this weekend, so I'll just hang behind his car to see what the speed does when he uses the 120 km/h mark in his non-Ford car.

Then vice versa (him trailing me), so that should provide a pretty good insight to the offset if any...

[FunkyBoss]

Good deal man, I'd trust the GPS reading.