condorman said:My 1986 Ford Econoline 150 van currently has 235/75R15 tires that need replacing. I am thinking about going up a few sizes to get better gas mileage. Is there any downside to this besides less torque and the speedometer being off?
They're most efficient where the horsepower and torque curves intersect - regardless of rpm. The point I was trying to make was that a particular engine/gear/wheel combination may not have enough oomph to climb a hill in, let's say third gear, whereas a smaller diameter tire may. With a significantly larger tire than normal, you may find yourself not being able to use top gear in many normal hill climb situations; in extreme situations you may not have enough even in the lowest gear to climb a mountain pass.Generally speaking, internal combustion engines are more efficient at lower RPMs.