My best mileage on my 89 505 SW TURBO Automatic was 65.449 mpg coasting
downhill with a 85 mile tailwind with tire pressures at 305 lbs front and 445
rear, all windows closed, AC and radio off (so antenna would be down). Once
I leveled off, reduced the pressures to about 5 lbs above spec and drove like
a normal person the mileage dropped precipitously to about 18.5 around town
and 22 on the highway.
Mort Y.