I think your issue is that the two sites compare things backwards from each other. Your second site is actually saying that when your speed is 100, your speedo will say 97.56. The tire you're running is a little bigger, so you'll go farther for each revolution compared to the stock size. That means your speedo will read lower than actual (or at least lower than it would read compared to the stock tires). GPS is the way to go, especially to check the odometer (more important for MPG calculation).
Quote:
Originally Posted by pletby
As a strange sidebar my speedometer is optimistic by around 4km/hr, but when I read the speed off my Ultragauge accessing my car's computer it matches the GPS exactly. Weird eh?
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A lot of cars will read a little high on the speedometer, but the odometer and speed from the OBD-II port are right on. Supposedly it's so you can't blame the manufacturer if you get caught speeding.