Short answer is yes but will vary to the ratio you choose.... My ratio of 15 / 46 (lowered gearing) makes the speedo over-read the speed, and I'm guessing it would actually make the odometer click over the miles in a shorter distance, but that's a guess as I haven't bothered to check that.

When my GPS is on the bike the difference between a satellite indicated 60 mph and the speedo is 6 mph (speedo showing 54). The faster I go the the bigger the difference between actual speed (GPS) and indicated (speedo) . This is assuming a GPS is NASA standard accurate as well. The original ratio 15 / 45 will still be over reading, set by factory standards, but not as much I suppose as part of 'Type Approval' safety margins. From memory the maximum allowed margin is 10%. With my ratio I am at 10-11%

Changing up the gearing, making it higher (say going to a 44 T rear from a 45 T rear would make the speedo under-read, and guessing again would actually bring the factory margin of difference, (between actual and speedo reading) more in line with actual speed so what your speedo shows will be closer to reality.

It's a personal thing but I wouldn't understand why someone would want to up-gear the 660 because it would be less manageable in slow maneuvers requiring the use of 1st instead of 2nd more often and would make the bike jerky in response to minor throttle grip adjustments whilst in 1st. The only advantage for this, I guess, would be if someone does a lot of constant high speed road commuting to bring the revs down (arguably improve MPG) but then they're probably on the wrong type of bike. Like I say it's personal and some may find this OK and report accordingly.

Having said that the Aprilia Pegaso Factory 660 uses the exact same engine with the exact same internal gearbox ratios but Aprilia put a 15 on the front and a 44 on the rear, so it must work for some.

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