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I just got a 2012 Ford Fiesta S (5-speed manual) a couple days ago. It's running great, but I noticed the speedometer reads about 3 MPH too high at around 55 MPH when comparing to the speed on my phone/GPS. The speed reported by my phone exactly matched the speedometer on my old Cavalier, so I'm assuming the Fiesta is a bit off.

The Fiesta reads 0 MPH while stopped, so the zero point is correct.

Is there some user-servicable way to adjust the calibration? If it involves buying a $100-200 tool to connect to the OBD-II port, it probably wouldn't be too hard to talk me into that, since I'm sure I'd find other useful things to do with it. I have an Actron scanner, but I don't believe it's capable of changing any ECM parameters, apart from clearing trouble codes.

Update:

The tires are 185/60R15, which matches the label inside the door.

I pulled up the digital speedometer (which is hidden in the instrument panel test mode), and also hooked up the Actron, and those two readings agreed with each other, not surprisingly. Both showed within 1 MPH of the speed displayed on my phone, whereas the analog needle was another 2 - 3 MPH higher than that. So it seems like it's just a needle issue, meaning my odometer and MPG calculations won't be way off at least.

The needle's zero point is correct, though, so I don't think it's just a matter of ripping the dash apart and repositioning it on the stepper. Is there any way to adjust the scaling of the needle's movement?

  • How is the tread on your tires? Also, are you sure the tires and wheels are OEM? I'd love to see answers, but I've never heard of a way to calibrate a speedo otherwise people would calibrate them down as much as possible to make the odometer move slower. – JPhi1618 Mar 18 '16 at 15:10
  • @JPhi1618 Good point, I'll have to check the tires against the specs printed in the manual. It was a Ford certified preowned, so it's probably not too far off, and the treads looked pretty good. – db2 Mar 18 '16 at 15:12
  • my speedometer reads different from summer tires to winter tires, just because they are different sizes. There are certain allowances around speed limits exactly because of things like that... ;) – Erik vanDoren Mar 18 '16 at 15:23
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    The speedo should be factory calibrated, I'll bet you're using different sized wheels/tyres than what came from the factory. I believe on some newer cars the dealer can make changes to the ECU to accommodate the speedo calibration for a different wheel size. It's normal for the speedo to have some error either way, though usually it leans to the lower speed side. – I have no idea what I'm doing Mar 18 '16 at 15:28
  • Are you sure the phone is accurate? Also my car vs. my phone gps shows about the same difference, but to me that's a reasonable error margin since tires wear out, and I also have different tire sizes from summer to winter. Also, the viewing angle for the needle may cause a difference. – Alin Purcaru Mar 18 '16 at 16:00
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UK type approval states that a speedometer may read with upto a 10% error provided that the speed shown is either true or over. A speedometer may not under read when a car is new. Most manufacturers therefore calibrate their speedos for a slight overread.

You must also remember that the difference between tyres with new tread and those worn to the UK legal limit may be as much as 6mm difference.

According to the OBD-II spec here, Mode 01 PID 0D should report vehicle speed in km/h. It may be worth checking this against the readout your dashboard is giving you as there may be some difference here.

I do not believe that there is any scope for recalibration within standard OBD-II CAB-BUS protocol but am happy to be proved one. As for Ford specific protocols, I'm afraid I don't know.

  • Marking this as answered, since you appear to be correct that this can't be done by anybody that's not an automotive engineer (or at least ace mechanic/tuner), based on the lack of other answers in 2.5 years. :) – db2 Nov 26 '18 at 19:57

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