3400 RPM at 70 mph seems very far off.
Doing the math (3400x24.88)/(340x4.11x0.780) yields about 77/78 mph.
My 98 LGT wagon 5MT would be at just about 3050 RPM at 70 mph.
You might have a car with an Outback 5MT installed. They have a shorter 5th gear ratio. It's 0.871 instead of 0.780 ratio. Doing the math with that figure in place, shows that you are right on the money for having an OB transmission installed.
3400 = RPM
24.88 = OD of tires
340 = constant figure
4.11 = differential ratio
0.871/0.870 = 5th gear ratios (non-OB/OB)
YES, the engine RPM's are slower to recede until the motor warms up. No throttle adjustment issues are at fault here whatsoever.
Around town, you shouldn't really get less than 280 miles to the tank worst case scenario... You do have a problem of some sort there...