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I have been wondering about the correctness of the average MPG displays on modern cars. Mine in particular is a Renault Modus. What I did is fill the tank, drive around and then refill the tank to see how much fuel I used.

Based on my calculations I made 557 miles (roughly) on 50 litres of diesel... which I agree is very good and comes down to roughly 50.6 mpg. (using the UK gallon here).

The display, at the time of refuelling, was showing 53.4 mpg average.

My question here is, how do these displays work and how much off can they be? Is it expected to be a good 3 mpg off over a 550 miles trip?

Thanks everyone!

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2 Answers

I think that the computer's fuel consumption statistics don't actually come from the tank float. The consumption estimate comes from the injector duty cycle and fuel pressure.

The computer probably has a map or graph between duty cycle and fuel pressure which tells how much fuel the injector will spray at any given cycle and pressure. Using this it continually estimates how much fuel it has sprayed since the mpg calculator was last reset. It then takes this amount and divides it into the miles traveled since reset to give you the miles per gallon.

There are definitely going to be inaccuracies in the graph between the expected fuel spray and the actual spray that occurred so the MPG estimate will never be 100% accurate but I wouldn't expect it to be anymore than 5-8% off.

Also as previously mentioned when doing your own calculations you shouldn't use your entire tank capacity as the fuel consumption. It would be better to use the amount that you actually refilled. As long as you refill to the same point in the tank every time this should give you an accurate reading of mpg. Understand that you might not get the exact same fuel level each time though which means there will be slight error in this calculation as well but for all intents and purposes is accurate enough.

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A couple of points to remember: 1. the inaccuracies in distance measurements will apply to all organics calculations (could try GPS for true distance over ground) 2. filling to the same point on the tank is nearly impossible to do accurately (you'd need to drain the tank and then add a measured gallon of fuel). –  Bob Cross Jun 28 '13 at 16:55
Neither of those points add anything to the problem at hand. –  Mike Saull Jun 28 '13 at 17:39
The inaccuracies in the distance will not factor into discrepancy between the calculations the odometer and computer ecu would both run off of the output shaft rev counter of the transmission. I mentioned point #2 in my answer and as I stated before it would be accurate enough for any regular purposes. Emptying the tank would be overkill. –  Mike Saull Jun 28 '13 at 18:22
Your assumptions as to how the consumption is calculated is correct. –  Juann Strauss Jun 29 '13 at 15:56
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Consider that your gas tank has a reserve reservoir, which is not measured by the float in the tank and also that you can overfill your tank by putting fuel in the chute(or whatever it's called). The most accurate test you can do, is to fill up your tank as completely as possible and then drive around until your car stutters to a halt because your tank is empty. Fill it up to the brim again, and see what you get.

I don't expect a car's average calculation to be totally accurate, mind you because it is calculated by monitoring the airflow rate in your engine and assumes combustion is always optimal, which it may not be. Toyota has a sensor over their fuel injectors which is pretty accurate, but other manufacturers expect the fuel to be a certain quality and the combustion to be optimal (complete).

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Of course this method is hypothetical as you never want to fill the tank past the pump automatic shutoff point. Overfilling the tank can result in damage to the evap/emission system. I would fill the tank until the pump shuts off. when it is time to refuel return to the same pump and fill the tank again only until the pump shuts off. –  mikes Jun 28 '13 at 13:00
I think the car average in this case is simply done mathematically, meaning that it shows me "fuel used" and "miles" ad then the average is simply calculated. I will try again without going into the reserve to see if it's a problem with the float not measuring it. –  Tallmaris Jun 28 '13 at 14:00
The salient point is that you need to fill the tank up to exactly the same level as before. –  Juann Strauss Jun 29 '13 at 15:57
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