Can modern-day speedos "drift"

Buckster · 16 Dec 2013 at 20:58

up until around 6 months to 1 year back - my speed seemed to read the standard 10% ish over - so 65-66 was about 60 etc - for years of ownership (I've had the car over 9 years) - Honda Civic

yet since then - its been reading much much nearer to "actual" - ie GPS indicated TomTom

my initial thoughts as this seemed to coincide with a large TomTom update for my device that perhaps they'd put some "tolerance" in the updated software - but my wifes car still reads 10% over TomTom

its not a major issue - the car is to be replaced in next month anyway (174k miles so getting on a bit ...) - but I was just interested to know what could "drift" on a modern day car

I thought that the speed was read directly from the ECU - so nothing to "drift" as such

tyres are definitely the correct ones for car

any ideas ? - I'm only interested as I'm an engineer and just wondered what could cause it

62 is around 60 now, 31 about 30 etc - so quite tight

rodenal · 16 Dec 2013 at 21:06

Seems more likely the TomTom software variable is causing it tbh.

I suppose there are a number of factors that could very slightly alter the speedo reading but unless you done something drastic like significantly change the rolling radius of the wheels I couldn't see it making much difference. If there was a speed sensor error it probabky just wouldn't work as opposed to any interference or degradation in signal quality

Dureth · 16 Dec 2013 at 21:13

It could be down to tyre wear/replacement so the rolling circumference has changed throwing the speedo measurement off compared to previously.

bledd · 16 Dec 2013 at 21:17

Tyre wear, tyre pressure. Then faulty vss, a bad vss with give you crazy readings though.

Buckster · 16 Dec 2013 at 21:18

thanks for suggestions

tyres are currently all near new (rears are 4 weeks old - yes I know I'm dumping the car soon but they were borderline and the tyres for my car are quite cheap - so was better to replace and not take the risk)

when the tyres were really worn the speedo read the same as it does now

are the dials themselves effectively analogue driven via a voltage ? could the dials themselves go out of calibration so to speak ?

acemastr · 16 Dec 2013 at 21:57

My speedo is spot on with GPS

bledd · 17 Dec 2013 at 09:59

Assuming your Civic is similar to my Honda, I think the VSS plugs into the top of the gearbox.

It either works or it doesn't

Colbaker · 17 Dec 2013 at 10:46

How accurate is the gps speedo anyway? Surely it cannot be as accurate as the one attached to the car.

MrKnifey · 17 Dec 2013 at 11:16

Gps isnt spot on as we get truck drivers in moaning all the time that their trucks are doing 53 on the gps instead of 56 on the dash but when on the rolling road the trucks are bang on

Scania · 17 Dec 2013 at 11:41

MrKnifey said:
Gps isnt spot on as we get truck drivers in moaning all the time that their trucks are doing 53 on the gps instead of 56 on the dash but when on the rolling road the trucks are bang on

I bet you do!

I find my current one reads 55mph on both dash, gps and tacho yet the 13plate I drove last week was different on each display!