...Driving 75 instead of 55 saves just over 36% of the travel time but increases air drag by 85%!
Actually it increases the drag by 2.535% compared to the 55 mph drag amount. Drag increases with the cube of the speed, so even a 10-20% speed change adds a lot of drag. Speed KILLS your fuel economy. (20% speed increase = 1.95 x drag increase)
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
As for the ?, will taller tires help, I'm pretty much with bzipitidoo. If you have power to spare, you can drop the engine rpms by changing the gearing or different tire diameter, that should definitely help with FE.
Of course getting a larger diameter tire turns your speedometer into a liar, and could help you get a speeding ticket too.
Getting a skinnier tire of same diameter will also help FE for two reasons. One, a smaller front profile pushing against the air. Two, the narrower tire will have less "squirm" which just wastes the energy you're trying to conserve.
The specs on TireRack.com show the range of flange width(s) you can use with any given tire. Obviously that should match the width of your rims. Sometimes it's stamped on the rim so you can see it even with tire mounted on rim.
As for traction, wider tires don't have more rubber on the road so you lose nothing with a skinnier tire. The weight of car vs. air pressure in tire gets you x square inches of rubber flattened onto the road. The wider tire on wider flanges with shorter sidewalls does give less sidewall flexing and more overall control, but you pay for it with FE lost to the rubber squirming and air drag issues.
that and skinnier tires help traction on the snow (sliding, fishtailing especially) because theres more concentrated weight to sink in the snow down to pavement. but if your in sand, you dont want to sink thus wider tires (also depends on the car)
kinda like a road bicycle (the ones with the 1" width tires) vs mountain bike tires (2-3")
oh and if u think having the4 RPM's at 3K going 75 try 3K when your only going 60...liek my chevette. man do i need some taller tires on that. geared obviously designed as a city car cuz i hit 4th gear at 35mph.
I tried the taller tire thing. My neon had factory 185 65 R14 tires. When it came time to replace I put on 185 70 R14's. I noticed an instant decrease in power. Which will end up being worse FE. But once I maxed out the tire pressure my FE was right back where it was before. So I guess I'm saying it doesn't help any. Not for my ride anyway.
According to all my truck driver / racer friends...if you run taller tires, you have worse performance at slower speeds but better performance at higher speeds. That is why you see shorter tires on "city" trucks and really tall tires on flatland highway trucks.
McIntyre's First Law: "Under the right circumstances, anything I tell you may be wrong."
O'Brien's First Corollary to McIntyre's First Law: "I don't know what the right circumstances are, either."