Hello,

My 1991 420SEL is running strong and smooth but fuel econ is poor compared to what others report for their 420. On a recent 240 mile all highway trip, only 15.1 overall average is 12.2

air filter is clean,
no smoke at anytime,
plug wires are good,
cap and rotor are original

Whay could it be?

Poor! I have a 560 and I get about 13.5 city, 18 hwy. Your 420 should do 19 or so on hwy, 15 overall.

My 560SEL get about the same as Bigshot's, although I do not do much highway driving. Sounds like you checked the car over pretty well, looking for common causes. Tire pressure ok, and transmission shifting right? Are you in an area with lots of hills, stop and go driving too? A/C on hurts too...This could be some of it......But the cars are not econo boxes, all models are luxury cars, not much thought given to conserving fuel. Sorry, not much help here.

Check your oxygen sensor. I had the exact same problems and it was the problem.