# How are digital speedometer readings typically calculated?

This seemed like the most suitable SE site, I do apologize if this shouldn't go here.

How are digital speedometer readings (for things like average speed calculations, etc) typically calculated - i.e. how accurate are they?

I understand there is some 'play' in the dial-reading of speed to cater for other factors. With my car (Seat Exeo - just a rebadged 2007 Audi A4, basically), when the speedo is reading 54mph, if I set the cruise control and reset my avg speed computer, it reads a constant 50mph.

How would this value be calculated? I assume it's not GPS, as that would stop it from working should I not have a signal (tunnels, etc?).

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## 1 Answer

The speed is very simply calculated from number of rotations of the axle multiplied by the circumference of the tyre. All that onboard computer then does is then divide by time to get average speed.

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So how accurate might that computer be? Do I assume that the 54mph showing on the speedo is just 50mph (the speed shown by the computer) marked up to compensate to give drivers a bit of 'headroom'? I just want to know what speed I'm actually going! lol. And would the tyres being over/underinflated impair readings? – Anonymous May 19 '12 at 9:12
The tyre pressure does impair the readings, which is one of the reasons all speedos over estimate the speed: legally it is easier for them to say as long as the speedo is at or below the limit your actual should Bree below there limit. The actual variance will depend on the headroom the manufacturer built in. – Rory Alsop May 19 '12 at 12:03
@Anonymous, my experience with VWAG cars (VWs and Audis) is that the analog speedo reads fast, while the digital odometer and trip computer read pretty accurately. Your comments seem right in line with what I see in VWAG vehicles I've driven. – mac Nov 8 '12 at 15:32
@mac - Yeah. Wish my car had a real-time digital speedometer that I could compare to speed on GPS. I guess I could always set cruise control at 50mph on the speedo and compare it to the value read on the trip computer. Or time a distance.. but then I'd expect it to be an increasingly larger differential the higher the speed... Ah well, it's no big deal. :) – Anonymous Nov 9 '12 at 10:53
@Anonymous - sometimes the differential doesn't vary like that. Certainly on analogue speedos, it may vary. Mine is quite accurate at 30, gets further from the truth up to about 90 then starts to creep back towards the correct speed – Rory Alsop Nov 9 '12 at 12:12