Right I'm probably going to look a complete fool now as I've probably got this all wrong. This was prompted off something I've just read on Pistonheads.

I'm assuming that the speedometer works out the speed based on the revolutions of the wheels, if this is the case and you've put bigger wheels on your car surely your speedo will always be reading slower than you are driving?

Feel free to correct me

 

Carrie IIRC you are right - the speedo is calibrated against diameter and number of wheel revs therefore changes of rolling radius (i.e. not wheel / tyre ratio changes) alters speed.

In theory even tyre wear can make a car slower.


I await a huge flaming

 

You are right Carrie - it takes a reading from the VSS wire. Changing the final drive also has an effect - hence the Yellow Jacket calibrator I have

 

hew: we can't both be wrong

 

Ok - you know you have to explain all that ........ right?

 

The VSS wire runs to a little sensor under the car:



Notice how I've tapped the wire for my Yellow Jacket



The revs on the driveshaft give out a regular blip, the Yellow Jacket knocks out a number of blips per second and then sends it back along the wire to the ECU and the dash speedo. The speedo is calibrated with the stock wheel radius and final gear in the diff and so by combining the frequency of the blips and the assumed wheel radius and final gear it gives the speed reading.

 

thanks Craig. I wonder how many people don't think about this.

 

Err... Surely if you change to bigger wheels you move also to lower aspect ratio tyres to maintain the same rolling circumference.

If you get your sums right and it is within a few % it's only going to effect the speedo by a few %

 

That's right (more so with a changed final drive) but even if I had a stock car, I think I'd probably run a yellow jacket now - knowing exactly my speed (having calibrated using my Origin B2) is very comforting when trying to avoid triggering speed cameras whilst going as quickly as possible (e.g. M42 near me).

 

My speedometer shows a faster speed than my tomtom so I know it gives me a little room for error

Oh, and the VSS wire, is that data coming from transmission, front or rear wheels, ABS sensors?