Speedo not accurate

[Kremmen]

I know illegal, but I used to overtake them in the inside lane whilst doing true 70

When level, a quick glance and a shake of my head

[nowster]

Tyre wear will also affect the speedo reading slightly.

Brand new tyres will read closer to the actual speed and as they wear the reported speed will creep faster. The difference in diameter is only about 10-15mm from brand new to MOT Failure (assuming about 7mm tread depth when new).

The 185/55R16 tyre on the Mk4 is 610mm diameter when new, dropping to about 595mm when worn down beyond the markers.

Circumference is (pi × d). However when comparing circumferences as a ratio, the factor of pi cancels out, so it's as simple as (15/610) ≈ 2.5%

Most of us will change our tyres long before that point, so the effect on reported speed is going to be less than 1.5%, or about 1 mph at 70mph.

[Kremmen]

That will explain why my under 5k Jazz was only 1mph out

Plus I checked tyre pressures regularly

[Neil Ives]

I know illegal, but I used to overtake them in the inside lane....

I think that it is not illegal as long as it's done safely. In heavy traffic, it's not unusual to see vehicles passing on the left of a slower flow. If you've ever driven near the Heathrow area of the M25 you will have seen this

[Kremmen]

Heathrow / M25 - know it well

I'm talking about the 60 to 65mph centre lane owners club idiots who I passed at true 70

One M25 trick, western section, clockwise, once you have passed the M3, get into the very inside lane, then come off at J11 and J12 and immediately rejoin the inside lane again which always moves quicker in heavy traffic

[Downsizer]

70 mph v. 65 mph saves an average of 2 minutes a day over 10,000 miles in a year. Not worth stressing about!

[zdan]

5mph difference is about 4minutes for each hour of driving at constant speed
10mph obviously double that