I AM at a loss to understand Mr Clarke's letter (June 7). It is 55 years since I did any physics so perhaps somebody could explain his figures to me or perhaps Mr Clarke himself could let me know his source.

Surely the most non-polluting vehicles are yachts and gliders, also I do not understand how a bike (foot propelled) is 100 per cent efficient while a foot-propelled pedestrian is only 40 per cent.

Explanation please! Also, he states that trains are 28 per cent efficient.

Does this cover steam, diesel, electric, cross country and local?

Are all these equally efficient? Why does he omit aeroplanes and ships altogether?

Also, Mr Clarke, the fact that cyclists do not cause pollution (a highly debateable point) does not absolve them from obeying the law.

Finally, if cyclists are so efficient why do so many of them not push the pedals round correctly?

I was taught that one pushed the pedals round with one's toes but I see quite a few cyclists I see have their instep firmly planted on the pedals.

Mike Usherwood,

Mendip Close,

Huntington, York.