My 1991 Cadillac Brougham has the 5.7 V-8 and weighs in at 4100 pounds. The best MPG on the highway was a good stretch at 27.2 with the A/C compressor off. When it is on and I'm not particular about being ultra-easy on the pedal, it gets 24 MPG on the highway. The worst possible MPG is when the engine barely gets warm, the A/C compressor is on and I wind up with 11 MPG in town. Typical mixed driving with the engine warmed up well sometimes puts me at 15 MPG.
The 2007 Mercury Grand Marquis my friend bought 3 weeks ago was driven by me from the Willamette Valley to the south coast as he had the truck loaded with supplies. My "easy" driving had the 4.6 V-8 in a car weighing 3900 pounds with single exhaust deliver 28.1 MPG with the compressor off more often than not for that trip. His driving which had mixed use slanted toward highway driving with the A/C compressor on sometimes saw him hit for 25.1 MPG. He's not quite as good at "easy" driving as I am. On one stretch of all highway driving over 2-lane with a couple of grades, I had the MPG's at 29.2, so that appears to be the maximum for a 4.6 with single exhaust. I understand getting a standard 4.6 with the dual exhaust option will bring an extra 1 MPG as well as increasing the HP from 224 to 239.
My former Cadillac, a 1984 Coupe DeVille with a 4.1 V-8 and weighing in at 4100, maxed out at 25 MPG with "easy" driving on the highway. Normal highway driving wound up delivering 23 MPG. In town driving with short distance/cold engine would wind up at 10.6 MPG. The A/C compressor was never engaged since the A/C did not work in this car.
Some years ago I had a 1985 Cadillac Sedan DeVille with a 4.1 V-8 which used premium. It was good for 28 MPG on the highway. My friend's previous car, a 1995 Buick LeSabre, basically the same car as the Cadillac but with the 3.8 V-6, got 30 MPG on the highway and 18 MPG mixed use.
My lady friend's 1990 Mercedes 190E 2.6 6-cylinder that burns premium gets 24 MPG on the highway with me driving it normal to fast.
The worst cars I had for MPG were a 1972 Pontiac Grand Prix and a 1971 Olds Toronado, both with 455 V-8's that were low-compression smog motors. On the highway they got 12 MPG and in town they got 9 MPG.
In comparison, my 1969 Olds 98 LS from way back then got 20 MPG on the highway in "easy" driving mode and 18.5 MPG when driven normally. The 1969 455 V-8 was a high compression engine and had no extra smog equipment on it since it was licensed in Oregon. A 1970 Olds 98 LS with a high compression 455 V-8 with a state-mandated smog device since I was in SoCal at the time got 18 MPG on the highway. The 1967 Imperial with a 440 V-8, which was the standard high compression engine, got 18 MPG no matter how hard you flogged it. The 1968 Dodge Polara with the 440 Magnum V-8 got 15 MPG on the highway. The 1966 Cadillac Coupe DeVille with the 429 V-8 got 16.5 MPG. The 1964 Rambler Classic with the 287 V-8 that burned regular got 18 MPG.
Driving a new Chevy Beretta in the mid-80's with A/C saw the MPG's go from 30 to 18 when the A/C compressor was engaged. I believe the engine was the 3.1 V-6. The car from that era that impressed me the most was a 1984 Toyota Corolla GT-S with a 4-cylinder and automatic that had all the options. It delivered 38 MPG on the highway driven hard.
My conclusion is that an underpowered modern vehicle will overwork the engine and bring the MPG's down while an engine that is adequate to a bit more than that, when driven properly, will get the most MPG's from the vehicle in stock form. The 1970's low compression smog motors were the worst offenders by far in terms of ******* gas and not delivering on power. What most impresses me about modern sports cars is that light weight combines with powerful engines, that if carefully driven, will come close to delivering the same MPG's as popular economy cars like the Honda Accord.
Your mileage may vary...LOL!
Rick
