Volvo RWD Forum

Been there and done that and got the tee shirt. Yes it increased my mileage.
I needed to either increase the volume of the tank or increase the gas mileage.
I run 3 to 4 times a year between Pittsburgh and Naples Fl to visit family.
It is a 1177 mile trip one way. One problem I had was having to stop 3 times for fuel. I only wanted to stop twice since I drive straight through. Being that I owned a 1987 745T with an auto and a Cd of 39 it wasn't going to be easy. One problem was it only has a 60 liter, 15.8 gallon tank. At the time I didn't realize that a 75 liter tank might fit. So anyway I pulled the AW71 and went with a M47 since I couldn't find a M46.
I also pulled the 3.91 rear and installed a 3.31 rear. The reason the rear was pulled is because the OD on the AW71 is .69 and the manuals are .78 for the M46 and .82 with the M47. Had I left the 3.91 in the rpms would have jumped to 2800 to 2900 rpms at 65 mph. Also the Turbo's came with a 3.54 and I wanted something a little higher like the 3.31 rear end.
My mileage on the highway jumped from at a best of 26 mpg to 29 mpg at a steady 65 mph with the M47. At 70 mph the mileage went from 23.5 - 24 mpg to 27.5 to 28 mpg.
I swapped out the M47 for a M46 due to the lower OD. It dropped my rpms at 65 another 125 with the M46 over the M47. That raised my mileage another 1 mpg to 1.5 mpg. If I baby it on a fairly level area such as from SC to Naples at 65 mph I can see 31 mpg.
City mileage jumped about 1 to 2 mpg to 20 mpg with the manual.
With the automatic transmission I only got 325 to 340 miles out of a tank of gas on the trip. Today the car gets between 410 and 425 miles per tank.
It also is filled with synthetic Mobil 1 in the engine and synthetic Redline oils in both the transmission and rear end.

Now let me make a point on why my mileage jumped so much. One the auto pulls about 20 HP more than a manual. Second and the biggie, my AW71 did not have a locking torque converter. That sucked most of the gas mileage out of the system on the highway. A locking torque converter is worth at least 1.5 to 2 mpg.
Third installing the manual also lowered the weight of the car by 50 or so pounds. My 97 960 gets what my 87 745T got with an automatic. It weighs 400 lbs more and has a 2.9 liter 6 cylinder engine vs the 87 with a 2.3 liter 4 cylinder engine.
My 745 runs about 2200 rpms at 65 mph and 2400 to 2450 rpms at 70 mph.