Car MPG – Computer vs Actual, Why the Difference?

Car computers display “instantaneous” MPG

Well, mine doesn’t. It displays the average MPG since it was last reset. If I reset it when I fill up it’s usually within about 1-2% for the tank which is good enough for me.

I reckon that the difference is because the average mpg is calculated by the accelerator pedal rather than fuel use.
My car (Skoda) shows 0 mpg when I’m travelling at 70mph on the motorway when I’m not pushing the accelerator pedal. Other readings can be 198mpg when lightly pressing the pedal. So if a car computer takes the mpg readings from my pedal position and then uses an equation to determine the mpg. The difference between the real mpg and computer guessed mpg depends on the data supplied by the pedal. My only example I can give was a 300 mile motorway drive where the on board computer said 60mpg but the full to full fuel tank reading was 50mpg.
I am not an expert in these matters!

My car (Skoda) shows 0 mpg when I’m travelling at 70mph on the motorway when I’m not pushing the accelerator pedal. Other readings can be 198mpg when lightly pressing the pedal

I believe that’s because they don’t actaully add any fuel at all when you’re off the pedal at speed.

So they end up with a speedo that’s accurate +/- 10% gathering data that can vary another 2/3% depending on the tyres – so calibrate the speedos to over-read by 13% so at best they’re bang on and at worst they’re over reading

. All the VW’s I’ve had since owning a satnav have consistently over read by about 10% at 30mph, less at 70%

Mate claims his BMW is bang on.

the kind of person who monitors their fuel consumption

The kind of person who says “monitors”?

When i sit stationary in my car with the engine running my engine adds fuel to keep the car running without my foot being on the accelerator pedal, I travel zero miles but my car uses fuel.

On the motorway when I’m coasting down hill I would of thought my car would of added fuel the same as the nominal tickover rate, maybe more if the engine revs are higher. But during this time I’m travelling some distance so whilst the computer for the accelerator adds nothing the odometer adds miles and therefore thats where the discrepancy occurs between the actual mpg and the computer mpg if the equation used doesn’t account for these ‘free’ miles.

In the same instance the fuel used to power lights, heating and the radio isn’t taken into account either so a long cold journey at night listening to radio 4 with the heater on will have a different reading than the same trip without lights, radio and heating driven during the day.

On the motorway when I’m coasting down hill I would of thought my car would of added fuel the same as the nominal tickover rate, maybe more if the engine revs are higher.

No, (apparently) the car will know that it can keep the engine going without any fuel (the wheels are moving, the wheels are connected, eventually, to the engine) so it will stop putting any fuel into the engine. So you really are doing ?mpg…until you reach the bottom of the hill.

The engine doesn’t try to keep up with the wheels when you’re coasting, instead, it lets the wheels push the engine along instead of the other way round.

No hypermilers on STW then?

My E-class is pretty much bang on down to less than 1mpg based on pump fill ups.

Not a million miles off the official figures either with 61mpg on my latest 220 mile motorway trip despite 50 miles of crawling in stand still traffic, 50mpg when careful on my commute up the hills here, through Bristol, up the M32 and then the ring road or 42-46mpg coming back cross country having the odd bit of fun.

I am trying to stop watching the mpg though as I am obsessing about it. I am learning to drive more efficiently though and I seem to be adopting an driving style that will work well when I get an electric car next.

I seem to be adopting an driving style that will work well when I get an electric car next.

Watt will the MPG equate to when you get a leccy car?

*see watt I did there?*

groan 😉

Mines cock on and yes on a long journey the game is to get as high a fuel efficiency as possible 70mpg being the benchmark.