All right, a last word or two, addressing some things I have seen in other threads.
Changing up at 2000 rpm is never going to be good for a modern car. That's below the torque peak for just about anything, and below the point at which the turbo spools up for a turbocharged car. Poor mpg and short clutch and gearbox life will be the result. On the open road, go to 3-4K before changing. Of course, if you can find an open road, don't tell anybody about it or it will be full the next time you want to use it
In town traffic and stop-start traffic jams there isn't any real answer as your speed is set more by those around you and the road conditions.
Best mpg is always going to be a compromise. Any increase in airspeed makes consumption worse, so slower is better, but you need to get there before next week. Low rpm is better, less fuel used per foot travelled so cruising in high gear is better. But see what I said above about turbo cars - low rpm, below turbo spoolup, will make things worse, as the turbo increases your effective compression ratio which makes the engine more efficient. Primary factors in high-mpg motoring are aerodynamics (no roof racks! keep the windows closed!), low rolling resistance (skinny tyres) and gradual acceleration. Rabid mpg freaks will do things like slipstreaming a big truck with their engine turned off. An accident waiting to happen
Diesels are different, power drops dramatically above about 4000 rpm, mainly because the fuel burns relatively slowly. Try to rev too high, more and more of the fuel goes out of the exhaust valve as soot (partly-burned) or unburned. So you can't keep upping the rpm limit. The only other way to improve power is to improve the compression ratio, which is why diesels came back into road cars in a big way when turbocharging became affordable in a production engine.
Changing up at 2000 rpm is never going to be good for a modern car. That's below the torque peak for just about anything, and below the point at which the turbo spools up for a turbocharged car. Poor mpg and short clutch and gearbox life will be the result. On the open road, go to 3-4K before changing. Of course, if you can find an open road, don't tell anybody about it or it will be full the next time you want to use it
In town traffic and stop-start traffic jams there isn't any real answer as your speed is set more by those around you and the road conditions.Best mpg is always going to be a compromise. Any increase in airspeed makes consumption worse, so slower is better, but you need to get there before next week. Low rpm is better, less fuel used per foot travelled so cruising in high gear is better. But see what I said above about turbo cars - low rpm, below turbo spoolup, will make things worse, as the turbo increases your effective compression ratio which makes the engine more efficient. Primary factors in high-mpg motoring are aerodynamics (no roof racks! keep the windows closed!), low rolling resistance (skinny tyres) and gradual acceleration. Rabid mpg freaks will do things like slipstreaming a big truck with their engine turned off. An accident waiting to happen
Diesels are different, power drops dramatically above about 4000 rpm, mainly because the fuel burns relatively slowly. Try to rev too high, more and more of the fuel goes out of the exhaust valve as soot (partly-burned) or unburned. So you can't keep upping the rpm limit. The only other way to improve power is to improve the compression ratio, which is why diesels came back into road cars in a big way when turbocharging became affordable in a production engine.
