I think the air resistance is the main factor. Even with the roof bars/cycle carriers on I notice maybe 5-10 less average mpg. They’re the Thule aero ones which

I think the air resistance is the main factor. Even with the roof bars/cycle carriers on I notice maybe 5-10 less average mpg. They’re the Thule aero ones which are designed to reduce drag! Also my car has a 6th gear for more efficient cruising on the motorway, but you can’t tow in 6th, it just sounds like it’s struggling.