How to calibrate your car's speedometer - dead easy | Auto Expert John Cadogan

Correction: In the video above I said the regulations demand under-reporting by a maximum of 10 per cent. The actual rule is 10 per cent plus 4km/h. This means at an actual speed of 100km/h your speedo can be indicating anything from 100km/h to 114km/h. Sorry about that minor gaffe - essentially it just means you can be going even slower than I proposed. Worst-case scenario: at 100km/h indicated speed, you might be going as slow as 87km/h. (I said 90 in the video.) In practice, most speedos are not that far out.

Susan commented recently on the website, echoing the frustration of many slightly OCD, somewhat innumerate, unpragmatic car owners with no serious concerns in their lives:

“I’m extremely frustrated that my 2016 Honda travels around 10% less than the posted speed limit. I’ve had numerous conversations with Honda Australia saying I want my speedo calibrated to the actual speed I’m travelling at. Their response is that due to the ADR it is set at 10% under the actual speed and they reckon they can’t change it unless it was showing a faster speed than what I’m travelling at. Any ideas for getting my speedo calibrated to the actual speed? I tried Honda because it’s under warranty. They were no help, except to say put a GPS in my car. The car came with a navigation system. I’m not sure why they’re think I’d have a spare GPS lying around…” - Susan

Newsflash: Most speedos are like that. They under-report your actual speed so that you don’t get pinged unwittingly for speeding. That’s just how it is. In fact, it’s regulated.

We have these - essentially - globally homogenised compliance regulations for almost everything on cars, and the speedo one for most modern cars demands under-reporting by a maximum of 10 per cent.

That means you could be on an indicated 100 and doing any speed between 90 and 100. In practice, most speedos under-report in that 5-6-7 per cent range.

Therefore, if your speedo is doing that, it’s not defective. So the manufacturer is under no obligation to replace your speedo. As for recalibration, forget it.

And if you’re secretly enjoying demanding this resolution in (quote) “numerous” conversations with Honda Australia, spare a thought for the poor bastards, or bastard-esses, in the customer support team, and perhaps instead, just bash your head against a brick wall in private, with your phone powered down.

It’ll feel roughly as satisfying, and it’s just as likely to get you that recalibration result you claim you want.

If it really does get under your skin, and you are that OCD over a few kilometres per hour, do this:

Go to the Google Play app store (or iTunes, if you use one of those third-rate phones) and download one of the hundreds of free GPS speedometer apps to your smartphone.

Find a nice, quiet, relatively level piece of backroad and do a few tests. (Level is important here, because being on a significant slope can dick with the Z-component and impact on the accuracy of GPS speedos.)
Drive at an actual 50 kays an hour on the GPS, and note the indicated speed on your car’s speedo. Repeat the test for 60, 80, 100 and 110. (You might need to go on the freeway for 100 and 110.)

You will then have done your own recalibration, of sorts. 105 indicated equals 100 actual - kind of thing. Dead easy.

Anyway, then you’ll know what the correction factor offsets are. Thus, you can drive bang-on the limit everywhere, if that is your wont. And you won’t have to pester the shit out of any more lowly paid customer care dudes or dudesses, for a resolution you have a snowball’s chance in hell of ever achieving.

In short, your speedo is not defective - it is compliant with the regulations. You just don’t like it, and clearly you don’t have enough really bad things going on in your life to detain you, so that’s nice, when you think about it.

And (glace cherry on the icing of this particular cake) hacking the accuracy is free, and easy to do.