Thanks all for the discussion. (this is an '89 2.1 WBX by the way)

I don't understand all of the responses, for example I am not sure what torque has to do with my question. Sure there are some torque differences in 3rd vs 4th gear, I can't understand how that might affect fuel consumption.

But what David wrote makes good sense. In 3rd gear there would be more losses in the engine itself (piston / conn rod weight reversal, valve springs, compression events, oil/water pump). So that suggests that the MPG would be lower while driving in 3rd gear.

But then as a counter argument (if I read it correctly), the engine might run more efficiently at higher RPMs, at 3400 in 3rd vs 2400 in 4th.

I think we need to find some graphs of fuel consumption vs engine speed. Ideally they would be for the WBX, but perhaps all engines have similar characteristics. About 5 different plots, from 1000 RPM to 5000 RPM with each plot being a different load on the engine. I think this is what I am searching for / asking about. It is almost like reverse engineering the ECU.

Roland

On Sun, Mar 25, 2012 at 3:25 AM, David Beierl <dbeierl@attglobal.net> wrote:

> At 09:56 PM 3/24/2012, Roland wrote: > >> consumed in 3rd gear vs 4th gear? Now the immediate answer is none! It >> takes the same energy to drive at 45 mph in 3rd or 4th. The wind >> resistance is the same, the drive-line loss is the same, other friction >> like tires are the same. >> > > But it takes more energy to run the engine faster. The inertial losses > from reversing the pistons' direction of travel, the frictional losses from > scrubbing the piston rings up and down in their bores, the energy lost by > compressing the fuel/air charge and operating the valve train, the > additional output of the water and oil pumps - these parasitic losses all > increase more-or-less linearly with increasing rpm. > > It would be interesting for someone with an OBDII engine to do some > calculations of no-load operation at various rpm, using the rpm and engine > load numbers to determine the relative amount of fuel required to merely > spin the engine at those rpm. I wish I'd done it while I still had my > Honda. > > Yours, > David >