Car motors on 7 cells are what gave E-flight a bad name all those years. What makes a car go does not necessarily make an airplane go.

Take a look at the Gary Wright Model Products Kwik-E (http://www.gwmp.net, though it may be .com. I know one is Gary's site, and the other is a porno site or something...) That runs on a car motor and 6-7 cells. It might give you an idea of what you can do.

For sleek, fast, light airplanes, you don't need a gearbox. I'd say if the plane is under 2lb and very aerodynamically clean, you have a winner.

A bigger, floaty airplane would do well with a 2.5:1 or 3:1 gearbox and a 12x8 propeller. I have a Hobby Shack "Flying Start" converted to electric that weighs 3lbs, and floats around well on a Speed 500, 7 CP1300 cells a 3:1 gearbox and 12x8 Master Airscrew folding propeller.