Driving habits: I have done some experimenting, and I can increase fuel efficiency in any vehicle by 20% or more by going easy on acceleration and braking, and especially by dropping my speed to 60 mph / 95 kph when it's safe to do so. I hardly notice a difference in travelling time.

Tires: I switch from soft and grippy ice tires to harder-compound summer tires every year, and there's a noticeable change in mileage. If I was driving a truck/SUV as a commuter vehicle, I'd ditch the knobby off-road tires for efficient ones and throw a light set of chains in the back.

Regarding clogged filters: I think a badly clogged filter would give an excessively rich mixture, which would blow unburned hydrocarbon out the tailpipe. This would be worse with a carburated engine, since the mixture is fixed. A modern fuel-injected vehicle with an oxygen sensor and computerized mixture control should be able to compensate to some degree.