How accurate is your car's Speedometer?

H2G2

Maybe this is old news to people here, but I (possibly naively) assumed that car speedometers were accurate, or at least 99% accurate. But prompted by a friend of mine we used a GPS to measure the actual speed of movement and compared to the car’s speedometer reading. There is an reasonable error rate. I understand there is lots of possible sources of error; age of car, wheel size, tyre pressure, tyre ware, etc., but the 3 cars we tested are all 2007 low milage.
Eg. For my car (early 2007 Reno Megane) Car 50kph - GPS 48kph; Car 100kph - GPS 96kph; Car 120kph - GPS 114kph
My mates car (mid 2007 VG Golf) is reading between 9 & 11% error.

This raises all sorts of issues re Garda speed camera, etc.
Should I complain to the garage and get it re-calibrated?

Stephen

I think we had this exact thread about 2 weeks ago

At 100kph my Honda Civic's speedo was 6kph faster than the GPS reading.
Overreading is normal in most cars, probably for legal ass protection - so people can't claim their speedo said 60 when they were done for doing 65.

dos30

My 93 Integra is the same, Car 126kph - GPS 120kph

It won't raise any issus with speed cameras as the actual speed you're doing is less than what you think you're doing.

Salmon

Are you sure that the GPS is accurate! They both need to be calibrated to ensure that they are accurate! The only constant in this instance is that in a court of law a gardas speed camera will (should) be in calibration.

prospect

When my GPS reads 100Km/h

My alfa's reads 98/99Km/h
My dads alfa reads 98/99Km/h
My toyota reads 95/96Km/h

Tipsy Mac

I suppose to really do an accurate test the car would need to be on a 100% level surface otherwise the GPS is calculating movement over the earth and not calculating actual up and down movement the car has covered over the road. A test on a runway should give an accurate reading, perhaps ask Mr Clarkson if you can have a shot of there's for a lap or two.

testicle

I drove a Lexus for a few days in the USofA lately, and the the speedo was 100& accurate over its range...

commited

100kph real is 106kph on my speedo. That's what I go by

dumbyearbook

I just noticed this last night coming in the N4 (straight stretch) 105KM came up on the speedo but 101 on the sat nav. to be safe ill be sticking with the speedo though.

KOJAK_1

Speedo's are not accurate and can by law read higher than the actual speed. So the speedo can show a speed that is higher than the car is actually travelling. It however should never show a rerading that is less than you are doing. Can remember what the allowed difference is but i think it is 10%.

unkel

KOJAK_1 wrote:

the allowed difference is but i think it is 10%.

As the others said we've done it recently here. Legally, the speedo has to show a speed somewhere between the actual speed and the actual speed + 10% + 4km/h

E.g. an actual speed of 100km/h must be anything between 100km/h and 114km/h on the speedo

Curran

Speedo's are always calibrated so they show a faster speed than you are actually doing!
This, however, all changes if you decide to change your alloys from 16" to 18" (assuming that you havent changed the profile of the tyre), because you have changed the overall circumference.

But Unkel's comment had me thinking (if true)....if doing 100 km/h, it can be anywhere between 100km/h and 114km/h......how accurage is the odometer (i.e. the milage)

johnny_adidas

Curran wrote:

This, however, all changes if you decide to change your alloys from 16" to 18"

as is the case when i changed to 18s. my speedo now reads 72mph when im doing 60mph. compared using roadside speed indicators. least i shouldnt get caught speeding any time soon:rolleyes:

roughan

loads of things can affect your speedo
tyrewear ....different size alloys , high./low profile tyres
etc they are ususlly calibrated to be +_ 10%