Tyre size - effect on computer


New Member
I have just purchased a GIANT FCR4 as my winter bike, it came with Kenda Kontender 700x 26c tyres, My Polar CS200CAD will only allow me to set 25c tyre into its 2nd bike set up memory. What effect will this have on the accuracy of my speed/mileage readings? Will the 'puter show me going faster than I actually am, or slower?:biggrin::biggrin:


Ride It Like You Stole It!
South Manchester
Should be very minor difference - not noticeable !


Legendary Member
Cant remember which way it goes Motir, but its VERY negligable.
I worked it out a couple of years ago, i'd had the 'standard' setting entered on the computer. When i did an actual measurement and worked out the difference, it was a paltry 1/2 mile lost or gained in 45 miles.

Elmer Fudd

Miserable Old Bar Steward
Smokin Joe said:
He'll stil be lying to himself but he won't be so happy. He should go the whole hog and fit two magnets to the wheel.
I've got a spare ..........

Elmer Fudd

Miserable Old Bar Steward
If your computer says you are doing 20mph or kph you will actually be doing 20.09mph or kph.
A discrepancy on speed and distance of 0.47%
If you want to be really pedantic, get a calculator and multiply your mileage by 1.00469 to give your real mileage (same for speed).


Senior Member
Quite right, you put a bigger tyre on then you cover more distance per revolution and the computer will under-read.

Saying that tyre sizes are largely nominal, it's perfectly possible your 26c is in reality the same size or smaller than a 25c, you'll also lose a mm or two over the life of the tyre. It varies from manufacturer to manufacturer and even different products made by the same people.

Anyway AFIAK, the 25/26c refers to the width of the tyre and not the profile which would be the significant factor here.


New Member
Thanks for all the replies, so basically I will look like I'm going slower, but I'll go further :angry: OK as stated it's so marginal as to not be a major concern, well certainly not for me at my level anyway!:blush:
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