Rear tyre aspect ratio
 

Will putting a 120/90 -18 tyre to replace a 120/80 -18 have any noticeable effect on the ride? I read somewhere that changing the tyre height will effect the gearing (ie. akin to going up or down a tooth on the sprocket).
They don't do the tyre I want in the same size as stock and the 120/90 is the closest. Am I overthinking this?
Cheers.

Probably

Try one of the online calculators like tiresize. com/tyre-size-calculator

You are going to raise the seat half an inch and go from 789 revolution ls per mile to 761 (speedo reads 50 at 51.8)

Seat height and mudguard clearance would worry me more than gearing or the speedo.

110/90 R18 is closer if they do it.

Andy

Thanks for the link. I'm not worried about mudguard clearance: plenty of room at the back. Seat is already a bit high, I am tall though. No 110/90- 18.

I'm not sure tyre sizing is an exact science - not in the width dimension anyway. Pick nominally identically sized tyres from different manufacturers and they'll have different rolling radii. So your 120/90 might well be the same as someone else's 120/80. Or it might be loads bigger ...

If it's that critical you could buff off a few mm of tread or reduce the number of revolutions / mile by running it half flat. Both of those are essentially short term solutions though :rofl:

Personally if it was the tyre I wanted and it didn't rub or catch when fitted I'd go with it. Give it a few thousand miles and the problem will solve itself.

Difference
 

The diff between 80% and 90% of 120mm is 10% of 120 mm
=> 12 mm on the radius.

So according to math, You rise the bike 12 mm
Nothing to worry about.

:thumbup1:cheers all.bier