Folding Bikes - setting cateye for a bike friday

so I just switched the cateye wireless 7 to my GF's NWT.
the manual has the mm diameter for 20 x 1.75 tyres
she us using schwable marathon 20 x 1.5!
this setting is not on the list in the cateye manual!!
the mm for the 20 x 1.75 is 1515, so I set the cateye at 1512, figured at least 3mm less, but i just dont know.
has anyone got the acurate measurement for 20 x 1.5 tyres? this would be much apreciated!

I have Marathon 1.5s front and rear on the XP, rollout gave me 1520mm (figure also given by one of the gear range calculators).

You can measure the wheel circumference very accurately in the following way:

Choose a flat concrete or paved pathway.

With the bike held up on a side stand, against a wall, or with the help of an assistant mark the centre of the contact point of the tyre on the ground with a piece of chalk and on the sidewall of the tyre exactly over the mark on the ground.

Wheel the bike forward one whole wheel revolution until the mark on the tyre is once again dead over the centre of the tyre contact point.

Make another chalk mark on the ground exactly in the centre of the contact point and exactly below the mark on the tyre wall.

Measure the distance between the two marks on the ground.

Use that distance (usually in millimetres) and enter it into your cycle computer.


This method can be repeated just to be certain that there are no errors in the first measurement.


This way is much better than using some rule of thumb value based on other people's wheels and tyres. You can be sure that you enter the exact measurement of YOUR actual wheel circumference.

Not all computers allow you to enter an unlimited range of values. Quite a lot won't handle wheels smaller than 18".

3 measurements averaged out to 1530!

Well, if you are confident in your measurements, that's the circumference of your wheel.

I find the method very accurate. We have radar speed displays here and while my car speedo always reads high on these things as I approach them, my bike speedos are dead on. I've checked my car speedo against gps and it does read high.

You can roll it forward multiple revolutions and then divide by the number of revolutions. This helps decrease the error associated with measuring the starting and ending points.