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Paddy Wagon, I've been testing with my computer readout like this too. I got the idea from a Car and Driver article that did a whole spill on speedometers. In a nutshell, your speedo is off (reads faster than you are actually going) and your odometer is actually much more accurate - as is your avg. mph meter.
Well, I guarantee I'd rather have the speedometer read faster than slower.
A few years ago, I bought a used Ford truck with oversized tires. The 2nd day of ownership and I get 2 speeding tickets in one night. I thought I was 5MPH over the speed limit, when I was actually 12MPH over. I managed to get the judgements lowered with a receipt from the Dyno Guy, but...Sheesh!
I guess I don't have to explain to you why manufacturers do it then It is deliberate and the amount of error varies from car make to make and from country to country.
There is a law in Europe, I believe, that your car cannot read slower than you are actually going so more error is built in so that a wider variety of wheel/tire combos can be used.