## Fuel efficiency

The qwustion is:
2 identical cars with their engine rev 2000rpm. The only difference is that car num #1 is in 4th gear and car num #2 is in 3rd.
Which car will use more fuel per minute?
Considering they are driving on leveled ground.
And does it Make a difference if they are going uphill or downhill instead?

Was arguing with a friend and I would be glad if you could help out

Thanks alot

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 We need to know the speeds of the two cars before we can answer that properly. For example, are they both going the same speed or is one going 30 mph in 3rd and the other at 40 mph on 4th?
 I don't agree that we need to know the speed since the question is how much fuel per minute and not fuel per distance. There should be more vacuum on the vehicle in the lower gear since it is more lightly loaded. Engine vacuum is just about ALWAYS a relative indicator of fuel economy. So maybe I have given you enough clues to figure it out yourself?

## Fuel efficiency

The car in 4th gear will most likely take more fuel per minute than the one in 3rd gear.

First, if the cars are identical, the one in 4th gear will necessarily go faster at the same rpm. More speed means more power required. If we assume that the 3rd gear ratio is 1.26:1 and the 4th gear ratio is 1:1, the speed will be 1.26 times faster in the 4th gear and it will require 2 times the power to fight the aerodynamic drag (= 1.26³).

The fuel consumption rate (r) per power produced (P) by an engine is the BSFC. So:

r = P * BSFC

In a typical engine - at 2000 rpm - the BSFC will probably be lower in 4th gear, around 15% less than in 3rd gear. So the fuel consumption rate should be around 1,70 times greater in 4th gear than in 3rd gear (= 2 times the power * 0.85 times the BSFC).

This is assuming no acceleration and neglecting rolling resistance, which should lower the difference between the two because it is not related to speed and mostly constant. If the speeds are small, it might leads to a lower fuel consumption rate in 4th gear because the power needed will be nearly the same in both gears (0.85 ~ 1 times the power * 0.85 times the BSFC).

Going uphill or downhill will simulate a constant acceleration or deceleration. Going uphill will further lower the difference between the two - just like rolling resistance - because the power needed is also independent of speed. Going downhill should have the reverse effect.

I had this nice discussion in another thread that might interest you.

 So the assumption is that the speeds are different; reasonable assumption but I wasn't going to make that. Better to have a clear description of what the proposed scenario is before answering, but maybe that's just me.
 2 identical cars each in different gears and both engines spinning 2000 RPM, how could you assume anything BUT they are traveling at different speeds?
 Easily, given the way people ask questions. Seen it quite a lot, have stopped assuming that what they type is what they mean. "Oh, I meant identical other than the final drive gear ratio! My buddy and me were wondering whether having a numerically higher differential gear with the transmission in direct drive would be more efficient than a lower diff gear and the transmission in 3rd (same car speed for 2000 rpm). In other words, how much difference is there between the efficiency of bevel gears and hypoid gears."
 The answer has been given here already, but as a little addition it is worth noting that VERY low loads can cause poorer specific fuel consumption due to immense pumping losses experienced due to an almost completely closed butterfly valve on the throttle. Though in this example I'm sure this doesn't need to be taken into account as traveling along at 2000RPM in either 3rd or 4th gear should provide ample butterfly valve opening for this phenomena to be irreverent. I have attached a little JPEG of an experiment I performed recently demonstrating this effect. Attached Thumbnails

 Quote by The Chase The answer has been given here already, but as a little addition it is worth noting that VERY low loads can cause poorer specific fuel consumption due to immense pumping losses experienced due to an almost completely closed butterfly valve on the throttle. Though in this example I'm sure this doesn't need to be taken into account as traveling along at 2000RPM in either 3rd or 4th gear should provide ample butterfly valve opening for this phenomena to be irreverent. I have attached a little JPEG of an experiment I performed recently demonstrating this effect.
Actually, the pumping loss is most extreme at this engine speed in a lightly loaded cruise mode- it's ALL about throttle angle- so if both were at WOT up hill the loss would be insignificant. If you were to take a vacuum gauge and run it at idle then at 2,000, I think you will see what I mean.

At 2k only the load will be different for this test- they will both use very nearly the same amount of fuel, with the 4th gear engine using slightly more.

 Actually, the pumping loss is most extreme at this engine speed in a lightly loaded cruise mode- it's ALL about throttle angle- so if both were at WOT up hill the loss would be insignificant. If you were to take a vacuum gauge and run it at idle then at 2,000, I think you will see what I mean. At 2k only the load will be different for this test- they will both use very nearly the same amount of fuel, with the 4th gear engine using slightly more.
/

Did you check the picture I posted? Clearly, pumping losses are NOT at the most extreme at mid-cruising load, throttle angle is further reduced at zero load under tickover situations, therefore pumping losses are the most extreme here.

 Quote by The Chase / Did you check the picture I posted? Clearly, pumping losses are NOT at the most extreme at mid-cruising load, throttle angle is further reduced at zero load under tickover situations, therefore pumping losses are the most extreme here.
I don't care what your graph shows, really, anyone can make a graph to represent anything. Use a vacuum gauge- or even a scanner, on a properly running engine and get back to me.

There is a reason EGR valves were introduced on vehicle engines besides the blending of a neutral gas. The introduction of the EGR necessitates the increase of the throttle angle to maintain the same power level- reducing the pumping loss.

 Recognitions: Gold Member Science Advisor mylar..not cool...no need to get personal...no one is going to dummie up a graph to thwart you.. .the Exhaust Gas Reg valve is on the engine for one reason..so cars can pass the smog test ..i.e. stupid regulations for car emissions...makes engines run super lean and super hot...
 The tests were conducted within an enclosed engine test cell, which includes vast amounts of digital measurement equipment (yes, including digital measurement of intake manifold pressure). I'm not here to get into conflict, only share knowledge with like minded people, if you have data to prove your stance on this issue then please, I'd love to hear it. But currently all you have is your opinion - and that doesn't really account for much.
 Chase, my deepest apologies if you thought I was assaulting you in any way, shape, or form. I am too new here and didn't know your credentials, my apologies. But after 35+ years of working on cars, I still have to disagree with your tests in a real world platform. Oh my, I just looked again at your picture and noted that it's based on DIESEL performance- NO WONDER I disagree! Diesels don't have a throttle plate to induce pumping loss. Ranger, that is a very popular misconception about Exhaust Gas Recirculation- it was ORIGINALLY used in that way, but was found years later to have the added benefit of reduction in pumping losses. As EGR flow is introduced into the cylinder, a wider angle of throttle opening is required to maintain the desired power level- thus the reduction in pumping losses. Believe me, it took a while for me to wrap my head around it as well. There are many things we, as technicians, were taught incorrectly and have been forced to "unlearn"!
 Recognitions: Gold Member Science Advisor thank you mylar..i will look into this..maybe ya ca n teach old dog new tricks??
 Traditionally no they don't, modern designs are trending to do so however in order to achieve suitable intake manifold vacuum for EGR operation and to avoid excessive lean out and therefore NO and NOx production at higher loads. Now please, let this lie here and allow the thread to die like it should have done several posts ago.