Fuel Economy With "X" Tyres

Sir,

Your report on the recent fuel consumption tests of vehicles using Michelin “X” tyres was very interesting.

Many of us consider these tyres to be excellent value for money, and I certainly get a better m.p.g. figure when using them. Could this possibly be because a 5.20 x 13 “X” is no less than 3in. greater in outside diameter than an equivalent Dunlop ?

J. E. A. RICH.

Peterborough.

(The effect on overall gear ratio of changing tyres and sizes is worked out from their respective r.p.m. (revs per mile), which in turn is based on the rolling circumference, not the free diameter. Taking 905 as the r.p.m. for the 5.20-13 Dunlop tyre and 892 for the 5.20-13 Michelin “X,” this means the gear ratio is increased by just under 1.5% on “X” tyres, or if 924 r.p.m. is the figure (Goodyear) this means a 3.4% increase in overall gear ratio. However, tyres never run cold, they heat up, and when hot ordinary textile tyres can grow between 3% and 5% according to load, pressure and speed. “X” tyres remain unchanged.

Taking the 3% figure, the 905 r.p.m. would become 878 r.p.m., which would mean the Dunlops would be just over 1.5% larger. By the same token the 924 r.p.m. of the Goodyear would become 896, which is much the same size as the “X.” – ED.)