Your Emails: Why speedometer tech is intentionally wrong
Cars
Brian Cooley here from ONCARS.
Another one of your emails about hi tech cars and modern driving this time from Daniel Van Di in Australia and he says between the speeds of 40 and 100 kilometers per hour my 2011 Volkswagen GTI's speedometer and my GPS speedometer on his application show a 3-10 kilometer per hour disparity, pretty notable.
With the GPS, he says, usually showing the slower speed.
Which one should I trust?
And that matters when you don't wanna get tickets, doesn't it?
Ok, Daniel, here's the thing.
Speedometers like yours are never accurate.
I've never seen one that reads right and they tend to read higher the faster you go, an overage, if you will.
They're optimistic, if you wanna put it that way.
Here's how it works in the US.
You're allowed to have your speedo read between two and 5% above truspeed.
And again usually a higher percentage, the higher speed you go.
The European standard is a little different.
It's one-tenth of the speed + 4 KPH is the allowable maximum additional, that the speedo can show and still be considered Acceptable if not accurate.
It's an auto maker tradition that goes way back.
Part of it is they don't want to be on the hook for you getting a speeding ticket because their speedo said you were doing less than you were.
They instead show you doing more than you actually are.
I don't know why they don't just make the damn things accurate.
But they have been doing it this way forever.
By the way if you're concerned about what this all means to the age of a used car you buy and its true mileage.
Don't.
Odometers need to be accurate.
Speedometers are the ones that will run high.
If you do want your VW to be dead on there is after market software you can get called VCDS and it allows you to change a bunch of parameters In your engine and automotive computers, and one of them is the speedometer calibration.
It's 3 to 500 bucks, I'm not sure if it's worth it to you, but you can do a lot more then just tweak your speed up if you're inclined to do so.
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