All other things being equal, gas mileage is dependent on three major factors: vehicle weight, aerodynamics, and engine RPM’s. It’s counterintuitive to think a vehicle would get better mileage by shifting at a higher RPM. Just as it’s counterintuitive to think that a vehicle would get better mileage at a higher speed. So if you want the best mileage, shift early and often. Keep the pressure off the accelerator. You want just enough power available to accelerate at a very modest rate using very little pedal. Its not fun being passed by everyone, but it’s the price for mileage. For the best mileage you will be shifting at well under 2k revs. The first to second shift will happen even before you get through the intersection. You can even skip 1st and start in 2nd. And you will have to use 6th gear regularly. If you are cruising at a steady rate for any length of time 6th gear will save you gas (although the diminishing returns below 40mph make 6th gear unnecessary below that speed).

Now as to the issues from lugging the engine, I wouldn’t worry too much about that. Just because it is running at a low RPM doesn’t mean you are lugging it. If you are trying to accelerate and find the engine at low RPM and your foot well on its way to the floor, downshift. But other than that the engine makes more than enough power to handle the pace of city traffic without going much beyond 2K rpm.

Carbon build-up is not a serious problem in today’s modern fuel injected engines either. Carbon build-up is generally caused by too rich a mixture at some point in the fuel map. It’s more associated with carbureted engines or poor engine tuning.

Still not convinced? Then look at the automatic equipped Xterras. The tranny is programmed to progress through the gears as quickly as possible at light throttle. Nor is it shy about using its top gear in steady state cruising.

-Old Army