Just FYI... on my STX with 255/65-17 tires.

 

I got 18" Goodyears with 3.73 rear end and those machines that the police setup to show you your speed match my odometer exactly everytime I pass them.

 

Same setup as yours except my speedometer reads 2 mph slow.

 

I think Ford engineers did not consider the fact that there is a speed reduction in the differential gearing. Speed signal pickup is normally integrated (magnetic) on the output shaft on the transmission.

In other words, the tire size (overall diameter) should have been adjusted to compensate for the 3.55 LSD.

On the other hand, the radar machines (using doppler technology) are really inaccurate to a certain degree - so I would not really trust them. Two major factors include angle of approach, and the type of material it is focused on.

There was a test in our area where 2 of those machines were setup to trap the speed of a single test vehicle... one machine registered 3 mph faster than the other - enough to get you a speeding ticket!

That is why if the basis of your speeding ticket is from a radar, it is highly possible that you can contest that in court and get your case dismissed (i.e., last calibration, comparator equipment, conditions, etc)

 

Every vehicle I have ever used my handheld GPS has had an error over 60mph. Heck, there is even an article in Car and Driver a while back about the inaccuracy of vehicle speedometers.

Really, nothing to be concerned about. It is not a lack of programming comensation for the tire size or differential ratio. If you were to check the software in the PCM, it would read more accurately.

 

Just FYI -

Not really concerned, but just stating facts.

European cars like Mercedes Benz and BMW cars (though equipped with purely electronic instrumentations) always register accurate readings based on 3 speed-determining equipment (vehicle speedometer, CD/DVD-based vehicle GPS, and portable MMC-based GPS) even when going 140+ mph.

 

Having lived in Germany for four years and driven many German autos on the Autobhan (including the higher end Porches and Mercedes) at speeds approaching 200 I'd have to tell you that I wasn't focused on the odometer. You concentrate on the road before you. Now here in the states you are on the lookout for the highway tax collectors better know as the police and as stated earlier my machine matches their tax collecting devices. So I'm happy with the accuracy of my odometer... red

 

Just an FYI:

DOT spec for speedometer accuracy is +/- 7% of actual speed.

 

Just stating facts, Jack.

Because of moving obstacles driven by people who eat, brush teeth, talk on the phone and worse, put makeup on while driving, I would not even dare half my normal Autobahn speeds on US public roads.

And I will be really surprised if those highway tax collectors try to collect tax inside of a controlled private testing facility.

 

My other cars are all gray market with non-DOT European equipment.

I guess that explains why they're accurate. (joke only).

 

They may register accurate readings in the PCM, but their speedometers do not necessarily correlate.

C&D article

 

I'll have to remember that one:

Officer - Do you know how fast you were going?

Me - Yep, 60.45mph...

Officer - Nope, you were doing 69.55mph...

Me - Hmmmmmm maybe on am on the low side of the +/- 7% and you are on the high side, damn speedometers LMAO

 

Okay Red looks like that C&D article busted your european accuracy dominance theory.....

 

Actually, that is wrong. All MB cars are NOT equipped with GPS speed calibration.

 

Not really. That article was published in April 2002, and could well perhaps been written as early as October 2001. It's almost 2004 now.

With the advent of on-board GPS-assisted eletronics when navigation units became standard on high-line vehicles (including Lexus, Infiniti, etc), I think these cars now take an average of all readings it knows through the CAN-bus system - from wheel sensors, transmission output, and satellite data - all to compensate for the potential errors caused by outside factors that the industry has realized (i.e., worn tires, etc)

As for the GPS speed calibration on MBs, the COMAND (Navi) and the vehicle speedometer are independent systems of each other - and moreso, the separate handheld GPS receiver.