Modern Vespa

I recently fitted a 130 front and 140 rear Michelin Pilot City tires and wondered what the new discrepancy in the speedo was. Jess did this way back in 2005 with stock tires [topic40] I didn't make a graph, but you can see that the tire difference closed the difference down a bit, but not nearly as accurate as I would have liked.

I got ahold of a Garmin GPS unit and went out and did some comparisons with the speedo. There was a bit of discussion about this on whos running 140's on their rear 250? and so I found a GPS unit and here is what I found.

1st number: speedo reading
2nd number: GPS reading

40mph-37.5mph
45mph-42.5mph
50mph-48mph
55mph-53mph
60mph-58mph
65mph-63mph
70mph-67.5mph
75mph-72.5mph
80mph-76.5mph
84mph-81mph (only could achieve this speed in a full tuck)

small vespa windscreen, topcase, PM pipe, 5525 miles, 155lb 5'10" rider with gear and 81 mph in full tuck. I can hold 77-80 indicated on flat ground without tucking. YRMV.

good day, jason

Thats a lot more accurate than with the standard tyre.

Basically (in the UK anyway) for a speedo to be type approved, and therefore your vehicle to be road legal the speedo must NEVER read below actual speed. It can however read above actual speed up to 10% + 2.5 mph. That is why all speedo's read a little high. Piaggio are just aiming for the middle of the tolerence band with their innacurate speedo.