Would a blowing exhaust cause my petrol consumption to reduce from 25mpg to 20mpg?

There is a split in the backbox

 

it could easily cause that as it'll play havoc with back-pressures, etc.

Doesn't mean it is definitely that, as it would be possible to get lucky and actually make your exhaust more efficient by having a hole in it - pretty unlikely though.

I'd see if you can get it patched up to see if that makes the difference, and then maybe consider replacing.

Cheers

Simon

 

Usually when my consumption drops from 25mpg to 10mpg...i tend to blame it on my heavy foot...

 

Very informative SAM, thanks for the reply

 

....must be the hot weather...