I have been driving in suburbia for a year and getting consistently between 17 and 19 miles a gallon.

Yesterday for the first time I drove highway miles (140 miles). I filled her up before I left. I was curious about about my highway gas mileage, so after the 140 miles I filled her up again. Only 4,639 gallons went in, so my MPG was 30. I am quite happy about it, but don't understand why there is such a big difference between suburban and highway driving. Shouldn't the two figures be closer and not so far apart?

What kind of condition could cause the poor gas mileage in suburban driving? When in suburbia I do mostly 15 to 30 minutes trip between 30 and 40 miles per hour.

For the highway trip I just took I was at 65 miles per hour for the whole distance.

Any explanations that would explain the relatively poor suburban gas mileage versus the really good highway gas mileage?

My car does have a rough iddle, with some shakes in the gear shift and some vibrations. I have always attributed this to bushings (perhaps both transmission and motor mounts). Of course all vibrations, shakes go away at high speed.

Would a bad fuel line regulator give too much gas at iddle?

Work done to the car since I got it in June of last year at 155K miles :

new plugs and wires
new distributor cap and rotor
new O2 sensor
air, oil, fuel filters changed
new tires
new front brake pads
new mufler and pipes from Cat back
new airbox thermostat
new coolant thermostat
new flame trap, new breather box
cleaned throttle body and iddle control motor
new air intake hose (black acordeon type between AMM and TB)
all new fluids (oil, coolant, brake, transmission, rear differential)

To do :
timing belt, tensioner, all other belts, maybe water pump

Thanks in advance for your input.
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'89 244DL M47 159K miles