Your speedo more than likely reads over to begin with, so a modest increase in diameter would serve only to even things out.
If you feel like playing, there is a pretty good app available for Android devices called DigiHUD Speedo that gives you real-time GPS based speed, mileage and directional readouts. Seems to be fairly accurate for checking a vehicle speedo against.
I should, of course, by way of a disclaimer, add that it should only be used with a hands-free device
Using Boba's link to the calculator it shows that challengerunning's speed would be increased by 5.2%, as most if not all speedo's over estermate by around 10% you will not have any problems (run along a straight flat section of road with a satnav working and see the speed difference actual v indicated).
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