Magic marker at the new 80mph mark so you don't get a reckless ticket. (the law in Virginia anyway)

BTW there is a neat tire size calculator here... Tire size calculator

Hi Wild Man,

Actually, I'm still not 100% sure on that. My test method (driving past displays reading out my speed) is a somewhat imprecise and hazardous one!

Another simple workaround solution would be a DIY product like this one:

Speedo Corrector MkII Kit - Jaycar Electronics

That's from an Australian store, but it's bound to be available over there too.

One question: Is the odometer off by a similar amount?

It's an easy check. Just go out onto a road that has easily visible mile markers, look at where your trip meter is (including tenths) just as you pass one. Then go 10 miles, and see where you're at on the trip meter. This will tell you how far it is off, accurate to within 1%.

Hi Wild Man,

Actually, the odo's off by a very predictable amount. It's stone dead - and I need to pull the gauge cluster and fix it. That's one reason I was interested in sorting out the speedo question. If it's possible to kill two birds with one stone...