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

OP checking back in. So is there any simple and relatively accurate way to calculate my fuel consumption at idle without idling for hours on end?

You can buy an OBDC-II dongle and some software to plug your car into your phone, tablet, or laptop computer. The instantanous fuel flow is available inside the car’s computer.

Whether that particular parameter is surfaced to ODBC in a way that your software can read depends on the make, model, and year of car. Some Googling for your model of car and the word “ODBC” should get you started.


I’d recommend a Bluetooth model. You can get 'em for about $10 (just search for “obd2 bluetooth”). The last time I was into this stuff, the Torque app for Android (maybe iPhone, I dunno) did a great job of interfacing with it, though there are likely good alternatives.

I actually wrote my own app back in the day. And it is in fact easy to derive the fuel flow from the raw data. There are a handful of values that are standard across everyone. Error codes, on the other hand, required technical manuals to implement…

I, too, endorse the Torque app and an OBD-via-Bluetooth device. Very useful.

In response to the OP, I note that engine power (to which fuel consumption is closely related) is approximately proportional to RPM times manifold pressure.

D’oh! Thank you. At least I didn’t say ODBC. :wink:

So it’s just a look up table based on a perfect engine? There’s no actual fuel flow sensor tracking the actual milage usage?

Couldn’t you get an inaccurate MPG due to wear/damage? Like if something wrong with your car that robbed it of 20% of its efficiency - that wouldn’t show up in the MPG indicator?

In the olden times of carburetors, there was a somewhat crude way of feeding fuel under idle conditions. When the throttle plate closed, the increased vacuum under the plate pulled fuel and air through a path called the idle circuit, which doled out a very rich mixture, compared to what the engine usually ran on. It didn’t happen only at idle, either. When you lifted your foot to approach a stop light or descend a long hill, the idle circuit was dumping fuel as if you were idling.

As long as you’re talking absolute pressure, of course (not gauge pressure). And really this just comes down to the old PV=nRT. The mass of some gas (keeping volume and temperature fixed) is proportional to the pressure. RPM is proportional to the volumetric flow, so pressure*RPM is proportional to the mass flow. And since gas engines run virtually stoichiometrically, that’s proportional to the fuel flow and thus power.

A long time ago, I recall reading a review of a pickup truck that had higher tow rating for boats than for other types of trailers. Aerodynamics was exactly the reason.

I’ve used the Torque app for a few years and I recommend it highly. I don’t know exactly how it works but the instantaneous fuel economy readout does not exactly match the same readout from my dash. I don’t know which is more accurate but they are both pretty close to each other. I also discovered that the Torque app didn’t completely accurately report throttle position. That is, the app couldn’t tell when I was flooring it for several seconds even though I know the throttle was completely open. All that said, the Torque app will provide a real time indicator when fuel economy is higher or lower .

Yet another uncited “crap I remember reading a long time ago” post. I recall that the ECM (and by extension apps connected to the OBD-II port) are relying on the number and size of fuel injector pulses to track fuel consumption. The ECM continuously counts the amount of gas being burned in real time. Divided against speed, it can calculate the fuel economy. Presumably this means that if there is a problem with the fuel injector leaking or not injecting enough fuel, the economy gauge would be off.

Now that i think of it, in my last car, the dash readout always reported slightly low fuel economy compared to the amount I calculated based on the gas I purchased and the distance I traveled. The odometer was very accurate, as confirmed by GPS. The reported fuel economy was about 4% lower than actual. Maybe my fuel injectors were lazy.

Here is a previous thread which covers some of the same ground as this discussion: Accelerating and fuel economy. - #23 by Dead_Cat

If they’re not using a lookup table, then they’re using the duration of each fuel injection event, with that model of injector having been well-characterized during development. If you’re doing engine R&D in a test cell you definitely would use a NIST-calibrated external fuel meter, but for the finished mass-produced vehicle, using a look-up table or injector pulsewidth provides sufficient accuracy for a dashboard measurement while avoiding the cost that a dedicated fuel meter would incur.