Hi to all. I have a doubt about the official TT course lenght used by the timekeepers to calculate the average speed of the laps. I hope that this may be an interesting and not boring subject for the ttwebsite articulists (and in particular for the TT experts).
Well, the official TT course lenght is 37.733 miles and all the books seems to agree that this is the only official TT course lenght used by the TT timekeepers since 1922. But arithmetic seems to have a different opinion. Take for example the 2007 TT edition. In this edition the absolute lap record was improved two times in the Senior race by nonother than John McGuinness:
lap 1 - 17m25.77s / 129.833 mph
lap 2 - 17m21.99s / 130.354 mph

A simple calculation allow us to obtain the lap lenght used by the timekeepers:
lap 1 - 37.715 miles
lap 2 - 37.730 miles

Even if we try and take in account in the calculation the rounding to the third decimal place we're not able to remove in the calculation the discrepancy between the two laps lenghts.
Now it comes my question: which is the explanation of that discrepancy? It does not seem to be due to a length correction for the first lap as it's absent in other TT editions (for example the 2006 one when McGuinness had the same racing numer, no.3).
I add that:
- the discrepancy between the calculable lenght values of the race laps seems to afflict many TT editions (also '20s and '30s TT editions).
- the calculation on the TT official absolute lap records data (i.e. lap time and average lap speed) gives (lightly) different values of the TT course lenght through the years.