80MPG BEEZA
the insurance company will still be liable to pay for third party costs if you have undeclared mod's
however, they can try and reclaim costs from you
don't think the police will care if they pull you over if you have undeclared mods for this reason.
dvla might though, as they should be extracting more tax from you
tax stays the same when putting 172 in a 1.2 trust me end of comment