I did a search, but got some many hits, it was impossible to find what I was looking for.
I switched from the stock 245's to 285's. Approx. what percentage should I expect my speedo to be off? I will eventually get a programer, but for now would like to know what to expect. I don't have my gps to get a real reading. I don't want a ticket when my speedo says I'm going 65, only to find out I was really going 75 or so if that's the case.

Thanks

Multiply your speedo by 1.0775293965628634190463884222768 to get your correct speed......approximately.

Calculator:

http://www.miata.net/garage/tirecalc.html

I use 7% and find it pretty darn close. That is: mileage reading x 1.07 divided by # of gallons = mpg. or speed x 1.07 = actual mph. Had a cop friend radar me at 60 mph on cruise. I calculated 64.2, he said 64.3. Close enough fo me... Air pressure makes a slight difference too.

1.07 is what I use as well. Seems to pretty accurate.

My 265's calculate out to 1.06.