I'm still tickled by the fact you actually have a GPS device and you're serious
Let's all get our cars up to a dyno - battle of the dubs style
I bought the GPS without actually thinking through any reason to buy it. 
Checking the speedo against the car has been very worthwhile. I checked my last car (Fiat Bravo) and found it to be accurate within 0.5% all the way to 115.
I even got stopped by a policeman who reckoned i was doing 58 in a 40 zone. I told him flat - i was doing 50 buddy. He said "are you calling me a liar?" so I replied "no, just short sighted".
Anyway, out popped the GPS and I challenged the validity of his car, or eyesight.
After alot of haggling, and insults, I let him off with a reprimand.
However, checking the Golf with a GPS is a real let down. It spoils your day, not to mention hundreds of wonderful stories about daring speeds you actually didn't get to.....
Tin