Why is GPS speed different?

[Bigrig]

sometimes the speedo drive gear at the gearbox end has an incorrect ratio. A cynic might say that if the Ks click over on the odometer more quickly than they should, that is a good thing for those who offer a warranty. I have seen this with new factory fitted tyres and rims in a GQ with 16s.

In the case of my Mav, however, I suspect it is because it was set up for 750 R16s and changed in aust to 10rs

The legislation for Aussie sold motor vehicles changed a few years back (and for most countries I'd assume). Speedos in the vehicle used to have to be calibrated to within plus or minus a certain percentage - I think it was actually 10% which is alarming - but now all new vehicles MUST overstate your speed - that is, if you're doing 100 on the speedo with factory setup, then you're actually doing less - probably more like 95 or 96. As said, changes to tyre circumference will affect this and the larger the tyre, the more understating of speed that occurs as a result.

[the evil twin]

sometimes the speedo drive gear at the gearbox end has an incorrect ratio. A cynic might say that if the Ks click over on the odometer more quickly than they should, that is a good thing for those who offer a warranty. I have seen this with new factory fitted tyres and rims in a GQ with 16s.

Maybe years ago, but the Odo's tend to be fairly accurate as it is the Speedo that is required to overread by ADR's. If I had factory tyres on my truck the Odo would be almost spot on (underreads by 6% with existing tyres) and the Speedo would be out by 9 KPH at 100 (overeads by 3% at 100 KPH).

Underreading Odo is a bitch in my case 'cause I have to do 6% more K's to hit my minimum K's for my Lease.

Nissan have a range of drive gears available if anyone does want to calibrate their Speedo's cheaply and easily.