Just a question:
is it more economical to drive at D4 at 40-45 mph with around 1,600 rpm, or change it to D3 with around 2,500 rpm?

Generally speaking the first option is the clear answer. The more revolutions the engine turns, the more time is gets a sip of fuel and thus the more fuel it will burn.

Having said that, there most likely is a point of diminishing returns where the RPM's get too low and you get way out of the engine's torque band and there is much wasted energy just trying to maintain speed.

If you have an automatic transmission, it should keep the revs from getting too low. If you have a manual transmission, running too low RPM will cause you to lug the engine, which is not good for it at all.

YMMV! [img]smileys/smiley1.gif[/img]

I have 4 speed automatic. for my understanding engine takes more fuel during too low rpm (below 1,500) than mid-range (2,000-2,500rpm).

I've driven the manual and i know it's really risky to drive on too low rpm's, however I don't have so much experience with automatic transmissions.

I generally suspect, that it might be more smart to change to D3 mode in the city to drive more ecomically, than just rely on the automatic changing of gears in D4 mode.