What causes an engine to lug?

[campisi]

Originally Posted by Cobb

If you can tell us the make and model we can do wonders for you. In general either reduce load or increase rpms. Maybe inflate the tires another ten psi? Under drive pulley for the accessories? Turn off the AC? Check for vacuum leaks, do a general tune up.

My example with my sidekick seems to like to be around 3 thousand rpms. Since going up 3 tire sizes I find situations where I need to go from 5th to 4th where as I use to leave it in 5th. The benefits of the bigger tires well out weigh that issue.

I remember being told that lugging the motor pounds the crank bearings (especially the mains?). This is 70's era knowledge though.

As soon as my current tires wear out I'm going up 4.3% in tire diameter and my biggest concern is that, in situations where I used to be able to be in 5th gear, I may now have to be in 4th to avoid lugging the motor. My hope is that the benefit from highway driving offsets this situation. I'm more often in a speed range where I can comfortably be in 5th so I think it's worth a shot.