Okay guys... this might be for the mathematicians amongst us LOL.

I've just fitted a Sigma BC 506 bike computer to my treadly. It's working OK (nominally) except that the odometer is measuring significantly less mileage than it should be. Over a measured distance (and confirmed via Google maps to within around 100m)it's reading 18% less than the true distance.

I've got 26 x 1.75 smooth tread tyre at 35 PSI.

As per the Sigma's instruction manual, I've used their 2070 setting on the computer.

...so, have I got a 'lemon' with the Sigma, or can I simply choose another size setting — say for a 26 x 1.9 tyre?

Thanks in advance.