About the tires: Tire Rack shows the revs per mile for the OEM tire = 844, while the Michelin Pilot Super Sport in 225/40-18 = 829 revs/mi.
I calculate that you will be going 1.777% faster with the just slightly larger diameter tire on your car. Ignoring any speedometer error, this means that if the speedo reads 118 kph, you're actually going 120.1 kph, or 1.777% faster than the speedo indicated speed.
Your tires, therefore, played only a small role in any speedometer error. This did put you 0.1 kph over their tolerance level, according to you.
Sometimes on downhills the cruise control may allow the car to stray a bit above the set speed. Might that have been an issue here?
Perhaps he gave you a ticket for a false 129 kph thinking that you would go to the court and plead it down to a lower speed, which would then work out to be fair and just. Who knows?
Had it been me and an officer told me he clocked me at, say, 10 mph higher than I believed I was actually traveling, I'd ask to walk back to his squad car and have him show me the reading on the radar unit; it should still be up on the display. If the officer was correct, I'd investigate my car's speedometer. Then again, if the officer appeared to be the type who might beat me senseless for challenging him to provide proof, I might abandon that plan. There
are bad police officers, more than we would like, unfortunately.
By the way, if you drive 15 miles at 81.5 mph, it will take you a tick over 11 minutes. If you drive 15 miles at 69 mph, it will take you exactly TWO MINUTES longer. If the distance is less, the time saved by speeding will be even less … but the ticket will still cost as much, the points on your driving record will be the same, and your insurance rates will rise the same.
That can prove to be an expensive TWO MINUTES.
On the Interstates of the U.S., I set my cruise control for four mph over the limit for regular drives. I drive completely relaxed without fear of an Enforcer hiding over the next hill crest, round the next bend, or up on an entrance ramp hidden from my view. During rush hour commutes I "go with the flow" as needed, which could easily be 85 mph out in the left lane. At 69 mph, I'd be trampled out there!
