I just found my ciclo bike puter that I though Í'd lost 3 months ago (in a side pocket of my pannier no less!) and I was also trying to find the right circumference mm seting for my bike as I have an odd wheel an old 271/4. I have half decent vredestein tyres (its a commuter bike its an old ten speed racer that I did up cromed and put a shiman nexus 4 is great for getting to work. Anyways on the tyre it has 32 630 then (27 1/4) and I was looking around the web and found this chart...............................TECH.INFO: TYRE CIRCUMFERENCE
What is the exact circumference of my tire?

In order to accurately program a bicycle computer it is generally necessary to enter the exact tire circumference.

The tire circumference varies depending on the rim, the tire pressure and the load on the tire. For these reasons, it is not possible to list exact tire circumferences.

In order to accurately program a bicycle computer, we recommend a simple rolling test with the rider on the bicycle.

The approximate tire circumferences for the most common sizes are listed in the table below.

Inch. ETRTO. Aprox.tire circumference

20" 40-406 1530 mm

47-406 1550 mm

54-406 1590 mm

24" 47-507 1860 mm

54-507 1920 mm

26" 25-559 1940 mm

35-559 1980 mm

40-559 2000 mm

47-559 2020 mm

50-559 2040 mm

54-559 2080 mm

57-559 2100 mm

60-559 2150 mm

37-590 2090 mm

28" 20-622 2110 mm (700c)

23-622 2120 mm

25-622 2130 mm

28-622 2150 mm

32-622 2160 mm

37-622 2190 mm

40-622 2220 mm

47-622 2280 mm

50-622 2280 mm

60-622 2350 mm

32-630 2180 mm

40-635 2240 mm