Speedometer accuracy

[Hack]

So - have any of you checked the speedometer accuracy with a phone app? I checked mine and I think it reads about 2 mph higher than my actual speed. I did cross-check on the GF's 2013 Escape and that was right on at highway speeds.

I have the rims and snow tires from my 2011 Brembo GT on the car, so I was worried the OD might be a little different than the stockers. Looking at the tires I thought they would have the opposite effect (make the speedo read slow rather than fast).

Sponsored

 

[DickR]

More specs from the factory service manual CD:

Speedometer

The PCM receives the wheel speed data from the GWM over the HS-CAN1 . The GWM receives the wheel speed data from the ABS module over the HS-CAN2 . The PCM uses tire size stored in the vehicle configuration file along with wheel speed inputs to generate a vehicle speed signal.

The IPC receives the vehicle speed message from the GWM over the HS-CAN3 . The GWM receives the vehicle speed message from the PCM over the HS-CAN1 .

The IPC provides a tolerance that allows the speed indication to display between 3% below and 7% above the actual vehicle speed. This means that with an actual vehicle speed of 97 km/h (60 mph), the speedometer can indicate between 94-103 km/h (58-64 mph). Incorrect tire size or tire size configuration could potentially affect the speedometer accuracy.

One easy way to estimate likely speedometer and odometer error from a tire change such as snow tires is to compare the revs per mile spec frequently shown in TireRack's specification data for the tires. Then do the math.

 

[Hack]

It's within the tolerance spec you found in the CD.

 

[15EBStang]

All late model Ford, GM, and Chrysler cars that I've driven are 2-3 mph fast compared to a GPS.

 

[MagneticMan]

Set your cruise control to exactly 60 mph and drive a mile between mile markers, start a timer at the first marker and stop it at the second one. If it takes 1 minute your speedometer is on the money, if it's shorter then you're faster than the speedo, if it takes more than a minute your speedo is reading fast.

 

[Hack]

Thanks for posting methods to calculate actual speed or differences between tires.

Really though what I was asking about was whether anyone else had checked their actual speed in comparison to the speedo readout.

 

[WestRace]

Most commercial portable NAV should have some real-time speed sensor. That's how I was able to measure my vehicle speed vs. its own speedometer.

 

[JimmyTwoTimes]

Wheel size can impact accuracy.

Sponsored