Good posts. I see Tom has done gear ratio changes on a variety of automobiles. I hope I'm not misinterpreting if I said Tom was searching, experimentally, to provide a mathematical and experiential foundation in figuring out just what is the optimal gearing ratio is that ultimately can be extended to solve any configuration by extrapolation and/or interpolation; obviously the differences in diesel and gasoline engines being crucial. It seems the primary difficulty in doing this is that for a given vehicle the optimal ratio changes as a function of how the automobile is driven, weight carried, ambient temperaturesetc. The engineers problem is that they have to install a gear ratio(s) that give's satisfactory performance over a very wide range of conditions. A diesel that is used to turn a generator under a constant load presents an easy problem. So, as Tom said, the relationship between diesel engine and gear ratio so as to give the best fuel economy, but not present problems in radically bad acceleration, radically bad drivetrain stresses, radically bad top end speed, etc. Given that, Tom's experience is of great value as it outline's the problem well enough to give some general parameters and how these parameters relate to one another in a general and a limited specific set of cases. Obviously Richard's experience and solutions add much to Tom's original post. My important point, at least to me, is that one has to install gearing that is trading off between a large # of variables, so, in the same car, one driver's optimum gearing is going to differ for another driver in the same vehicle setup.