To give you an idea I went from Wellingborough to Putney SW London using M1, M25, M4 then A roads.
Total mileage 95.5 miles.
Started with 100% and arrived with 55% and that is with air con on KERS on step 2.
About 70% was doing around 65, the odd 70 but also hitting pockets of traffic on the M1...