I always wondered if my gauge speedometer worked correctly since i would do 0-60 runs with a stop watch and get times in the 6-7 second range. I know my car is faster than that so i questioned it. I have the correct tire size and gear ratio so what couldn't be it. I did a datalog and the point i get off the gas and thought i hit 60 was showing up at 67 on the datalog. Using this as my gauge i just counted the 15 bits per second to calulate my 0-60 times and it was in the 5's like i thought. Is my dash gauge off or do they all do this? I also just realized i can do this for quarter mile runs at the track (if the clocks are off) or a closed course somewhere. This would be very accurate and beat using my stop watch so i can keep both hands on the wheel and all my attention on the road. I had 2 different cars pace me at 55 mph to verify the cruise speed so maybe the gauge is slow to react to the rapid acceleration.

 

how much taller or shorter are the 315/35/17's then a factory tire? enough of a difference could affect your speedo i do beileve.

 

The 315/35-17's are the same diameter as the stock 245/50-16's (Z-rated option), and slightly smaller in diameter than the 235/55-16's that came on the base V8 cars, but not enough to cause a 10% speedometer error.

I think he's saying that at a steady speed, he has proven the speedo to be accurate. Its just when accelerating, it seems so lag by a significant amount.

 

^^ ^^
What he said exactly.