# Is the amount of gas used by a car engine based solely on RPMs?

Suppose that I’ve determined that my car uses 2 gallons of gas running for an hour at 60 mph and 3000 rpms. Does it follow that if I’m idling at 500 rpms I should use 1/6th as much gas or is there more than that to it?

Don’t change two variables at once. If you can drive at 60mph with your engine at 3000rpm for one hour. Then you have put the car in neutral and rev it up to 3000rpm and see it takes more, less or the same amount of time to go through through two gallons of gas.

I suspect load is going to play into this. The faster you go, the more air resistance you have and the harder your engine has to work. It’s either going to need to spin faster or have each combustion cycle produce more power. If the RPMs are the same (because you’re in a higher gear), the engine is going to have to dump more gas into the cylinder to increase power.

Assuming your car has a modern computer controlled engine, the system will feed just enough fuel to keep it ticking over at idle. However, even at that speed, it has to make enough power to keep all the parts moving, including the belt-driven machinery that generates electricity, circulates water, and pumps steering fluid even when you aren’t turning.

As Joey P said, the work load gets factored into the computer’s decisions on fuel usage.

Pretty sure it’s going to be completely based on throttle, not RPM.

If you are sitting still, then flooring your gas pedal is going to put your RPMs into the red and beyond (unless it has some sort of governor).

If you are going 60 MPH, flooring your gas pedal isn’t going to push your RPM up nearly as much.

Look at it this way, the entire point of having a gearing system is to keep your RPMs in about the same range at whatever speed you are going. You get better gas mileage at lower speeds, even if they are the same RPM.

OTOH, I would be surprised if you are using much less than 1/6th the gas at idle than at speed, as cars are designed to be efficient at driving, not ad idling. That’s why many newer cars actually shut the engine off at stop lights, or switch to only running half the cylinders.

Flooring it while climbing a steep incline is going to drink lots of gasoline without making many RPMs — it’s trying but each additional RPM is expensive.

Not flooring it but keeping the accelerator pushed down enough to maintain speed, running in 2nd gear and holding the engine at 7000 RPM, is also going to drink a lot of gas, without operating at a particularly high MPH.

Not flooring it but keeping the accelerator at the maintain-speed position while cruising at 135 MPH is yet again going to consume lots of fuel, even in a very aerodynamic sports car.

There are multiple factors, in other words.

If you have a car with a standard transmission and a roof box on going 60 MPH it’s going to get better mileage when you take the roof box off. The engine will be turning exactly the same RPM at 60 MPH with the roof box on but it will be getting worse gas mileage. The increased drag from the box makes the engine burn more gas.

For gasoline engines, it’s proportional to the mass of air going into the engine. To a first approximation, the engine control unit puts the same amount of fuel into every kilogram of air going in.

The mass of air is controlled by the throttle and RPM. The RPMs dictate how many volumes of air get pulled in per unit time. But the throttle controls the pressure of each volume, and thus the mass. So you need both inputs to determine the total mass air flow.

It is possible to use less gas than even at idle–if your car is cruising downhill, the throttle can be fully closed, so virtually no air goes in. No air, no fuel. Even if the engine is spinning at 3000 rpm. On the other hand, if the throttle is fully open, you may be using quite a lot of gas even at a relatively low RPM.

Diesel engines are different–they are designed to run with fuel mixtures that are leaner than the “right” ratio (called stoichiometric).

In past years some cars were equipped with Manifold Vacuum Pressure gauges. Maintaining max pressure equated to highest mpg. I suspect modern computer controlled fuel injection systems do the work for us.

That’s equivalent to saying that you get better mileage at low RPM with an open throttle than at high RPM with a more closed throttle. Less air resistance and lower frictional losses for the same power.

Modern control systems indeed do the work, but it’s more on the part of the transmission. They run in the lowest gear that they reasonably can given the power requirements. That means the engine runs at a low RPM, which means the throttle is as wide open as it can be, which means you’re at the closest to ambient pressure (ignoring forced induction here, like turbochargers).

Modern automatics tend to get better mileage than manuals, mostly because they are extremely aggressive in picking a low gear.

Perhaps not today, but I recall the days when manual transmissions were capable of better mileage. I believe it was because there was a slight slippage related to torque converters. This was eventually resolved with lock up converters.

Yes, modern automatics get better MPG than manuals. Besides all of the better electronic controls, automatics now have 7-10 gears, allowing for more efficient gear selection.

Gasoline engines also do not always run on stoichiometric ratio. E.g. when accelerating, they run rich to keep things cooled down.

Diesels have a different problem to deal with : soot or particulates.

You meant to say “highest gear”, surely?

Assuming no aftermarket mods, WOT enrichment for a gasoline spark-ignited engine happens under very limited circumstances (typically upper half of RPM range + beyond 90% load) and to a very limited degree, like maybe a few percent past stoichiometric. The typical passenger car engine doesn’t spend many hours of its life operating in this range.

Diesels operate over a wide range of lean, anywhere from just 70% excess air at full load up to 1000% excess air at idle.

late-model cars with dashboard MPG indicators know the instantaneous fuel flow rate for your engine at all possible combinations of RPM and load because that was all documented during the R&D for that engine. Feed it your current speed, and yep, miles-per-gallon is some easy math for them to do.

If you’re talking about the work of figuring out how to achieve the best MPG at any given moment, then Dr. Strangelove is right: the computer basically selects the highest gear consistent with acceptable driveline durability because that gets the engine into its most efficient operating mode. Reducing the RPM helps improve engine efficiency, but the other factor is increasing the torque output (which must be done to deliver the same power output at a lower RPM). Gasoline engines tend to give best efficiency at fairly high load and fairly low RPM. Here’s a gasoline engine efficiency map. For this engine, the operating range of best efficiency is some low/middling RPM, maybe 2000-2500 RPM, combined with maybe 60%-75% of full load.

Yes, thanks for pointing that out. I tend to mix that up, probably because I’m thinking in terms of the ratio between engine speed and wheel speed, and that would be lower for the higher gears.

Like I said, “to a first approximation.” As Machine_Elf noted, the deviations are small and short-duration compared to a diesel.

No, the amount of load is a very large factor as well. Generators run at a fixed RPM (often 1800 so the outgoing frequency is 60hz). Fuel consumption running at 1800rpm with no load is far less than fuel consumption running at 1800rpm while powering multiple different electrical loads.

Right. But it should also be kept in mind that “using less fuel” does not mean the engine is running more efficiently. At no load, the fuel consumption is minimal, but the efficiency is terrible (near 0%).

My truck gets 21 mpg unloaded at 65 mph and 1600 rpm. It gets 11 mpg pulling my 8000 lb. camper although RPM and speed are unchanged. Strangely, it gets around 15 mpg pulling my 12,000 lb. boat, so I assume most of the change is due to drag instead of weight (boat is more aerodynamic).

Yes. The “steady state” work of your engine is, for the most part, pushing the air out of the way as you move down the freeway at a constant speed. So drag plays a very large part. (This is oversimplified of course as there are always other factors.) Note the big difference in fuel economy as you drive faster. I get better economy at 55 MPH than 60, and a lot worse over 60 MPH. This is all due to the increase in drag, which goes up steeply the faster you go (it is non-linear w.r.t. your speed).

Efficiency is fuel consumption divided by mechanical power output. At no load, power output is zero, so efficiency isn’t just near zero - it’s exactly zero.

Weight really only matters to fuel economy if you’re operating in hilly terrain or doing a lot of stops/starts (e.g. city driving). For highway fuel economy, where you’re cruising at steady speed and hardly ever touching the brakes, aerodynamics (and to a much lesser degree, tire rolling resistance) determine how much power is required; it’s then up to you, or the car’s computer, to figure out what the best gear is to get the engine to most efficiently provide that power.

A Honda Fit weights about 2800 pounds (with driver) and gets about 40 MPG on the highway. My motorcycle (with rider) weighs about 800 pounds and gets 42 MPG on the highway, a mere 5% better than the Fit. Part of the problem for my bike is that the power-to-weight ratio is much higher than the Fit, so my engine is operating at a very small percentage of full load, and so is relatively inefficient. But at least a big a problem is the drag: motorcycles are terrible for their size when it comes to aero drag.