Citroen Specials Club - the club for all lightweight cyclecars

Is it legal on my 2cv based trike special to run a 125x15 tyer on the rear wheel, with 135x15s on the front wheels? as I cannot fit a 135 on the rear without having to deflate it as insufficient room between hub and central mudguard, but a 125 is half inch narrower and just slots in.

It'd be legal as far as I'm aware. The tyres on the same axle must be the same size and as the 125s are on the front axle and the 135 is on the rear axle, you meet that requirement. Take a look at the number of custom cars with a skinny tyre at the front and fat tyres at the rear. My three-wheeler trials car is currently running 135 tyres on the front and a 145 tyre on the rear. The spare for the rear is a 125 tyre so my rear tyre will be changing as and when.

Every Smart Fortwo car on the road has different size tyres front to back.