I'm looking for suggestions on a low rpm, "realistic" gas engine.

[modelman]

The short answer is-NO. Small gas engines are designed to run at hi-rpm. You could look at a 4-stroke engine like this- http://www3.towerhobbies.com/cgi-bin...&I=LXXRH2&P=ML. These engines are made to run at lower rpm and have good torque. They are pricey but very good quality. The biggest problem you will have is cooling. All these motors are air-cooled. In airplanes they work okay but even in R/C cars running fast they still get very hot which causes short engine life. Also you will never get realistic"truck" sound from a gas engine, even the 4-stroke. Not saying that it can't be done but there will be lots of problems. Just my opinion- Good luck