Speedometers?

PermaBanned · 31 Dec 2009 at 20:20

How do they actually work? I used to think it was to do with the RPM, but then it depends on the surface and incline etc... so what makes them so much more accurate than say the speed calculator on a SatNav?

Fr0dders · 31 Dec 2009 at 20:21

i believe they count revolutions of the wheel, which is why increasing or decreasing tyre size can knock your speedo out.

If you know the rolling radius of the wheel with tyre fitted to the nearest .1 of a mm or whatever, then if you can count how many times the wheel turns, you can get a very accurate reading of how fast you're going.

PMKeates · 31 Dec 2009 at 20:26

Superewza said:
How do they actually work? I used to think it was to do with the RPM, but then it depends on the surface and incline etc... so what makes them so much more accurate than say the speed calculator on a SatNav?

RPM doesn't change depending on surface or incline - it's the number of revolutions per minute of the engine

As MrLOL says, speedometers are mostly based on revolutions of the wheel.

mjt · 31 Dec 2009 at 20:27

PMKeates said:
As MrLOL says, speedometers are based on revolutions of the wheel.

which is why i was doing 60mph on ice whilst actually not really going anywhere

PermaBanned · 31 Dec 2009 at 20:29

Okay, cheers

Lashout_UK · 31 Dec 2009 at 20:31

It's worth remembering that in most cars the speedometer is driven by an output (be it an electrical sensor or a mechanical drive) from the gearbox - hence the accuracy issues that arise when using different size tires and wheels.

PMKeates · 31 Dec 2009 at 20:34

Lashout_UK said:
It's worth remembering that in most cars the speedometer measurement is actually taken from the gearbox - hence the accuracy issues that arise when using different size tires and wheels.

That's a good point, I'd forgotten all about that.

If you're genuinely interested Superewza, post your car and we can probably determine exactly how your car measures it!

PermaBanned · 31 Dec 2009 at 20:36

PMKeates said:
If you're genuinely interested Superewza, post your car and we can probably determine exactly how your car measures it!

I would, just that i don't own a car...

Pete19s · 31 Dec 2009 at 20:49

My current and previous car are driven by a feed from the gearbox. I wouldnt be supprised if a lot of newer vehicles take there readings from the ABS sensors. I know that when they facelifted the Megane they changed from gearbox driven speedo's to readings taken from the ABS system on the front wheels.