So as the title says ... can any of you clever people out there tell me :- What is the percentage increase in the rolling radius when you fit the 55 tyre ? Its just that before fitting mine the speedo was really accurate . At a dyno 100 mph my speedo did read 102 . Now it could be under-reading as the tyre will travel more distance per revolution of the tyre .
I'd say the % increase is 10% for the tyre but quiite clearly it's only showing 2% on ur speed runs. Could b down to a lower tyre pressure on the 55 profile tyre? I'm only taking a guess though.
If your running a 190/50/17. The tyre height is 50% of 190mm=95mm, diameter of the wheel is 17"=431.5mm, diameter of wheel & tyre=431.5+190=621.5mm (each rotation is 1952.5mm) 190/55/17. The tyre height is 55% of 190mm=104.5mm, diameter of the wheel is 17"=431.5mm, diameter of wheel & tyre=431.5+209=640.5mm (each wheel rotation is 2012.2mm) Therefore the inaccuracy will be 3%. If your speedo was previously reading 2% high, it will now be 1% low (with the 55 ratio tyres), i.e your speedo will be reading 99MPH when you are actually doing 100MPH. Try taking a GPS unit such as ROAD ANGEL etc out on the bike to compare. Hope that helps.
Unfortunately I don't know anyone with a GPS to lend it to me .. but your maths looks sound and I do remember a previous post about rear sprocket in 44 and tyre size at 55 basically cancelling each other out . Or did I dream it ..
Put in your bike's parameters and work it out here:- Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
