fuel consumption differences

Consumption will be different. At the two different engine speeds, the engine fuel efficiency is basically different, 3500 rpm usually being considerably less.

But, if you are driving in 4th gear in that case and 5th in the first case, you are also wasting gasoline. The reason is obvious. Expand the scenario to the ridiculous...drive at 90 km/h in first gear and see what happens.

Assuming the vehicle in question is powered by a conventional spark ignition gasoline engine 5th gear will have the lower fuel consumption.

The major factors are:

1) Throttling losses (or pumping work) Pumping work is a negative work incurred during the intake stroke as the throttle is partially closed to restrict the amount of fresh air inducted. Because your operating condition is at "constant" power, the 3500 rpm mode will have higher throttling losses.

2) Mechanical friction This is the "rubbing friction" of the engine parts due to rotation and reciprocation. As engine speed increases mechanical friction increases in a non-linear fashion.

3) Secondary effects a) EGR: This really depends on the ECU emissions calibration. For many vehicles the Federal Test Procedure keeps the engine at relatively low speeds and low to moderate loads. The quantity of EGR (exhaust gas recirculation) tends to be highest in the FTP region of operation. At 3500 rpm the calibration may not call for EGR at all. One of the benefits of EGR beyond Oxides of Nitrogen reduction is a reduction in pumping work. This would tend to favor the 2500 rpm speed. b) Transmission gearing efficiency: If in 4th gear you are at a 1:1 ratio, the transmission friction losses MAY be slightly lower. This would favor 4th gear. But, this effect is probably very small and will be dominated by items 1 and 2. Whether or not this effect is even there would depend on the specific design of the transmission.

The answer to your question is in the rpm. For every revolution, every cylinder is fired & burns some amount of fuel. So for the case you describe, with all other variables held constant, you will consume less fuel in 5th gear.

What about for a diesel engine? Is it the same situation? And what about the difference between driving/accelerating in a higher gear with a wider throttle or the next gear down with the engine going faster but less throttle because the engine is not working as hard?

Since the described operating mode is a relatively light load, for the Diesel engine higher engine load at lower speeds will be better due to the lower mechanical friction in the engine.

Higher loads (short of when enrichment kicks in) will be more efficient than lighter loads. When the engine is "working harder" is usually a more fuel efficient mode for the engine.

Not to hijack the thread, but what if in the above scenario, it took

80% throttle in 5th gear to maintain 90km/h and say 50% throttle in 4th. Then does that switch the outcome?

Since the original constraints were level road, 90 km/h that would be pretty unlikely.

I suppose if the engine were transplanted from a lawn mower or there was a

150 knot head winds, etc. But even then, the one in 4th gear is burning more gas by virtue of firing more often.

Since we're expanding the thread, my '79 VW diesel used to be at about 3/4 throttle to maintain 75 mph and wide open throttle (pedal to the floor) to do it in 5th. While it sometimes could not maintain the speed in 5th, it always consumed less fuel in 5th.

What does the owner's manual say? What is the optimal speed for each gear?

1986 GMC Jimmy. 2.8L 235/75-15 tires. 50 mph headwind/crosswind. (winter storm.) Vehicle cannot maintain 100km/h in 5th gear. Downshift to 4th, accelerate to 110. Upshift to 5th, watch speed drop back down to 90. I never measured fuel economy, but I can say that if you have enough of a gutless wonder and enough of a headwind, then yes, it's possible that 4th would be a better choice.

Frankly, the only way to know FOR SURE for THAT VEHICLE is to measure the actual fuel used. There are too many variables - you don't tell us what kind of car - let's say it's not a car - 2500rpm is barely fast idle for a motorcycle, but 3500 rpm is redline for a 5 ton truck, so it really depends. If you're lugging the motor, you probably need to downshift, but _usually_ lower rpm = less gas used, but not 100% of the time.

Ray

Perfect analysis. Thanks.

V.M.