I have the inline 6 with 31" tires and 3.55 stock gears, but that is aways a decision for me to 3 or 4th around the 35-45 mph range. I like to be doing atleast 40mph to be in 4th. At the bare minimum, if I'm on flat road I will drive in 4th down to 35mph. For a long time I thought that turning less rpms was saving me gas but really, I needed to open the throttle more in the higher gears and was actually using more gas than what I do now. Now I stay in the lower gears, and turn higher rpms. It's actually harder on the motor to lug it than to turn higher rpms. Lugging can happen at any rpm. Lugging can happen at wide open throttle; if the engine can't gain rpm, it's lugging. Maybe you want to stay in 3rd gear so you can press on the gas pedal less, that will probably save you more gas.