can someone tell me how a speedo works?

just can't seem to think of a way how a car would know how fast it is travelling. surely it must have something to do with wheel revolutions but seeing as there's lots of different wheel sizes and tyre circumferences - it does my head in.

also - how much does changing the size of wheels you have alter the cars performance?

 

The older "mechanical" speedos usually used a rotating magnet which was driven by a flexible shaft that ran from the gearbox through to the dashboard. As the magnet rotates, it generates an electrical current in a coil, which in turn drives the needle. The faster the magnet rotates, the greater the current, and so the further the needle goes round the clock.

The electronic speedo used in most modern cars works on a broadly similar basis, with a pulse generator in the 'box connected to the speedo unit with an electrical wire. The greater the frequency of the pulses, the further round the needle goes.

So basically the speedometer isn't a "speedometer" at all, it's a transmission rotation speed counter, and yes, the graduations on it will be calibrated with a specific tyre size in mind.

If you change the size of your tyres, the speedo reading can indeed be put out of kilter, and the overall gearing of the car will be altered. Put larger diameter tyres on, the gearing will be raised, and the speedometer could under-read. Put smaller diameter tyres on, the gearing will be reduced and the speedo will over-read to a greater extent (it should over-read slightly to start with).

This is why, if you're going to fit larger wheels, you must use a lower aspect ratio tyre to maintain the same overall rolling radius, maintaining the gearing/speedo readings as close to their intended levels as possible.

 

plus most manufacturer retard the speed calibrations on all speedo's so you don't keep getting speeding tickets, this also gives a buffer for tyre wear. Most are up to 10% out.