How Accurate Is My Speedo

[marty32]

was out this morning doing a few bits and pieces, plodding along dual carriageway with flow of traffic at about 77mph, but my sat nav says im doing 72?

is it fair to say that the speedo on my car is not 100% accurate, can anybody shed any light on this

[martin1810]

was out this morning doing a few bits and pieces, plodding along dual carriageway with flow of traffic at about 77mph, but my sat nav says im doing 72?

is it fair to say that the speedo on my car is not 100% accurate, can anybody shed any light on this

That's ok. What it must not do is under read.

[marty32]

as in say your doing 70 when your doing 75

[martin1810]

That's right.... or your nicked mate.

[Fawbs]

I'm sure i heard somewhere speedo's are usually 8% inaccurate or can be, something along those lines

[GreyRacer]

My Celica TS is the same. At 32 on the speedo I'm doing 30 GPS. At 70 on the speedo I'm doing 66 GPS.

This is quite normal and calibrated that way by the manufacturer to read high. This is not to protect you but the manufacturer, so that you could not sue them in some countries by having an under reading speedo.

There can be another reason offcourse to have an excessive difference and that would be not running on original sized wheels and tyres if the running circumference is not the same as original.

[Twizted Nibbz]

My Celica TS is the same. At 32 on the speedo I'm doing 30 GPS. At 70 on the speedo I'm doing 66 GPS.

This is quite normal and calibrated that way by the manufacturer to read high. This is not to protect you but the manufacturer, so that you could not sue them in some countries by having an under reading speedo.

There can be another reason offcourse to have an excessive difference and that would be not running on original sized wheels and tyres if the running circumference is not the same as original.

As far as i was aware all speedo's are calibrated differently! This is why the police allow a 10% buffer zone before they will pull your pants down and shaft you good and hard... i.e. if your travelling at 30 then GENERALLY they wont do you unless your doing 34 at 70 its 77 and a nything higher and your being ver very norty!

Good point about the wheels and tyres though... There is a convertor on the net somewhere that you can use to calculate exactly how much out they are by using the size of wheel and tyre you have against the manufactures stanbard. I will try and find it later but im at work at the mo and will probably get a kicking as im due of my break.

Catch ya later

Brett

[Scarlett Arrow]

My Celica TS is the same. At 32 on the speedo I'm doing 30 GPS. At 70 on the speedo I'm doing 66 GPS.

This is quite normal and calibrated that way by the manufacturer to read high. This is not to protect you but the manufacturer, so that you could not sue them in some countries by having an under reading speedo.

There can be another reason offcourse to have an excessive difference and that would be not running on original sized wheels and tyres if the running circumference is not the same as original.

Good answer :)

Fitting slightly larger size Tyres than stock will actually make the Speedo more accurate.

For example:

215/45/17 or 225/40/18.

The downside though will be a slight decrease in acceleration.