Speedometer shows 2 MPH faster than actual speed

15 inch rims. My speedo is 2mph under what police radar setups show.

 

I wonder if this is also the reason why the puter MPG is higher than actual calculated. I suspect so. I haven't checked via a GPS yet.

--Mickey

 

The MPG number is based on two parameters, trip distance and fuel usage.
The "trip distance" is fairly accurate.
It looks the "fuel usage" is intentionally biased 5%.
The speedometer display is also intentionally biased to meet the regulation.

Ken@Japan

 

you guys know when the tire tread wears down, the speedometer reading is more accurate, right? When the tread is almost gone, the speedometer will be about spot on.... as the circumference changes, the distance traveled is reduced by 4% (or roughly 2 MPH at 50MPH) as it wears out.

 

No. It's opposite.

When the tire tread wears down, the distance traveled is increased, then the speedometer offset is also increased.

Ken@Japan

 

Oops... I stand corrected... I got it backwards.... stupid math....

 

This thread has been around for some time but I just learned how to get in to the Service Menu on my Gen III. One of the items on this service menu is GPS and Sensors page. One of these sensors is the speed sensor.

While driving to work this morning with the CC set at 60 mph, the Service Menu page displayed the reading of the speed sensor to be 57 mph. So my car is actually biased by 3 mph.

 

After installing new tires with a slightly higher rpm than stock (863 vs 855), my wife can assure you the speedometer is calibrated perfectly, right up to 136 kmh.

 

Can you state what tires and size you are using? This 2mph off reading has always bothered me. I've tracked the actual distance traveled and that was off too. Came out to be about 1 mile off for every 50 miles. Which means the mpg reading would be off too for many people. Thanks

 

I replaced the stock Goodyear Integrity (185/65/15) with Goodyear TripleTreds (195/60/15). I would highly recommend a switch for improved traction, handling, and safety, but not just to fix the built-in speedometer error.

 

I have a package V , no options, 17" tires and I am also 2 mph off at 60. Two GPS's show me traveling 58 MPH. Not a huge deal, but something to be aware of.

 

I believe the 195/60R15 (863rev/mile) tires turn more than 185/65R15 (855rev/mile) at the same speed, therefore the speedometer reading with 195/60R15 is higher than with 185/65R15.

We need larger diameter (low rev/mile) tires for less speedometer error.

Ken@Japan

 

True. I'm using the Goodyear Comfort Tread 195/60R15 and it's still 2mph off. I'm closing to needing my second set of new tires, 175k currently, and will probably go with the Goodyear Fuel Max. I'm just not sure what size.