Hey everyone! I noticed one thing strange about my FEH and it's also an issue in my wife's HiHy as well. When we drive past those permanent posted radar readerboards the speed given on the sign at which we are supposedly traveling is always 3 MPH less than my speedometer says it should be in the FEH, and it's about 2 MPH low in the HiHy. Are car manufacturers setting speedometers artificially high intentionally to help offset speeding to some extent? Or am I just a conspiracy theorist? Is anyone else's speedometer off enough to be noticeable?

 

Hi there,

The same happens with my Yaris/Echo. The reason is that (in Europe at least) it is illegal for a speedometer to under-read your speed so, to avoid problems should you fit larger wheels and tyres or the like they introduce an errorof - generally - about 3mph. I've checked mine against GPS-based speed measurement devices and the error stays about 5% throughout the speed range.

Hope that's some sort of help,

Alan

 

My wife has a VW Passat that did the same thing. Then I read a post on a VW board about accessing true speed numbers. By punching a code into either the radio or climate control, it would display true speed on the LCD readout and it was always 2-3 mph lower than the speedometer reading in the cluster. I assumed that it was some sneaky trick to keep people driving slower....but the post about it being illegal to under rate the speed might be more realistic.

 

Both of my Escapes read about 2 mph fast when compared to the speed of a handheld GPS as well as to those speed trap signs. My '04 Escape and my '01 Focus all did the same thing. I have read that this is intentional. The odometers are right on though when running through either measured miles or when compared to mileposts over a distance of 10-20 miles. I believe Ford builds this error into the speedometer reading.

I don't know if there is any requirement that speedometers read slightly high (as I have read elsewhere as well) but the odometer in my father's 2000 Buick Century reads exactly the same as the GPS and the radar speed signs.

 

IMHO, to be within 2mph is pretty close.

 

FYI: Speed measurement on a car is normally done by some sensor tapping off an axle or gear directly coupled to the wheel (normally in the transmission on a non-hybrid). That is so the calculation of your speed only includes one variable, the circumference of your tire.

The same sized tires from different manufacturers usually have slightly different circumferences. Since auto (make that most) manufacturers hate to be dependent on one supplier, they almost always buy from multiple vendors. I suppose the auto manufacturer could calibrate the speedometer to each set of tires they offer (and some probably do) but I would bet that most set the speedometer to one generic calibration that works reasonably well with all OEM tires offered with the vehicle.

Plus, as tires wear the circumference decreases. Tires typically lose 2 to 3% of their circumference over the life of the tire (exceptions: off road tires lose as much as 6%, ultra high performance tires may lose less than 2%). The loss comes from the usable tread depth (in the US, usually 1/2 inch on normal tires) that wears away as you drive. Therefore, even if the speedometer reads precisely correctly when your tires are new, it will become more and more inaccurate (i.e. it will read faster and faster) as the tires wear.

I also believe there is a subjective rule of thumb used in the auto industry that the speed will be set to read 2 to 3 miles per hour faster than you are actually doing with the OEM tires that are shipped with a new car (it might be a regulation, who knows). After all, it's better to read slightly faster than slightly slower; at least as far as law enforcement officials are concerned.

Maybe in the far flung future we will onboard speed measurement in our automobiles using some method not related to the car itself (onboard radar perhaps?).

 

i noticed that the other day too. the road i was on is 35 mph and i was right on the line in EV but when i passed the marker, it said i was going 33 mph. with a load of cars behind me who cant pass me i raised my speed to 35 but the ICE turned on (dang) i fig that if i am going to slow everyone else down (most people go 45 on the road) i should at least be doing the speed limit. anyhoo for the 9 miles they have to follow me, i am helping raise there gas milage.

 

That would be true if this were a mechanical system which in newer cars, it is not. The computer can be told to make one revolution of the tire in one second 10 mph, or 60 mph. They can make the speedometer dead on accurate or they can make it so it reads fast or slow if they want to. The speedometer can also be recalibrated with nothing more than a command to the computer.

 

Whether the sensor determines your tire has rotated one complete turn by mechanical means or electronically is immaterial. The distance you travel for each rotation is directly dependent on the circumference (i.e. the distance around) your tire.

Obviously the computer could be told to make one rotation of your tire in one second any arbitrary speed. However, if you want an accurate reading you would calculate the speed using the distance you travel in one rotation. Car tires in the US are normally measured in inches, so using a 25 inch tall tire for an example, the tire would be 25x3.14, or 78.5 inches in circumference. Therefore a 25 inch tall tire would travel 78.5 inches in distance per rotation regardless of your speed. If you made one rotation per second, you would have a speed of 78.5 inches x 60 sec x 60 min / 12 inches per foot / 5280 feet per mile, or 4.46 mph.

I agree, for a given circumference. However, your tires are constantly wearing down, so the calibration would have to be updated constantly if you want the reading to stay accurate.

True, but not on the fly while you are driving down the road. And in any case, to do so the process would have to measure the tire circumference in real time (or height, although height is less accurate) in order to recalibrate.

 

I don't disagree with anything you are saying. I was just saying that tire circumference, radius, diameter, whatever is not the only variable in the equation. The second variable being the conversion factor that equates one rotation of whatever shaft to a particular speed. It used to be by gears - now it is by sensors and computer programs.