depends on the calibration of the speedo.  most speedos will read alittle fast.  No dealer wants a customer to come back and sue for a speeding ticket cause the speedo was slow.  So most here can put on a slightly larger 140/90 and have the speedo read dead on.  some will go as far as to doubt what speed someones states they've gone.  And then there's the chain mod group who are 10% above the indicated speed.  

So far, I'm right on with Wolfman, but my only gage of this is hubby riding behind me, comparing to his speedo.  With my stock tire, I always seemed to be a little fast (speedo reading lower than actual), now it seems further off.  I was cruising about 55, he said I was going 61.  Down around 30ish, its only a couple mph off.  I don't have a GPS and haven't been on the highway to do a mile-marker test yet, so I've just been extra careful....  

Oh, we tried going through a "work zone" that had one of those electronic "your speed is...." things, but it didn't pick up the bike.  I guess not enough surface area to see us.

If your speedometer has been calibrated for 140/80 R 15 (24 x 5.51 x 15) tires, replacing them with 140/90 R 15's (24 x 5.51 x 15's) would result in the speedometer reading low by 4.42%.

i.e. you are going 4.42 % faster than indicated at all speeds.

This link is the only tire calculator I found that allows motorcycle sizes.
http://www.dakota-truck.net/TIRECALC/tirecalc.html

Enter 14/80 - 15 in Tire A and 140/90 - 15 in Tire B.
Select "Compare Tire A to Tire B" and click on Submit.

Speedometer error listed from 5 mph to 200 mph in 5 mph increments.

Quote:


The faster you go, the bigger MPH the difference is.

16 inch rear wheel with a 130/90 tire and 16/47 chain conversion. I have no idea how accurate my speedo is anymore really. Dont matter much though. With all the mods Ive done Im speeding 90% of the time anyway   . Luckily...so is almost everyone else around here, so I blend in well.