Funnily enough, I tried the very same thing tonight. It's tricky to
say for sure because I had to keep glancing at two things (as well as
the road of course.. if I had cruise it might have helped), but I
eventually decided that 70mph on the dash was 66mph on the GPS.
I always use a Garmin GPS and have found the Audi speedometer to read so close
to the GPS at all speeds up to 85 mph that I can not tell if there is any
difference at all. This was in three T44s and one 98.5 A4.
The GPS is supposedly accurate to 0.1 mph and updated each second.
Join a sports car club that has an accurate speed radar (preferably K
or Ka band). Take your car to a safe venue (track or closed airfield)
and let her rip. Have someone paint you with the radar. Have another
brave soul in the car with you to take a reading at the trap. Just be
aware that at that speed you need lots of room to stop (145 mph is 213
Dave (been there, done that with an RS6)
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