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Car fuel consumption

Nigel Goodwin

Super Moderator
Most Helpful Member
So how does everyone find the calculated MPG and range in their cars these days?.

I've never been terribly impressed in the cars I've owned, or ones I've driven at work.

However, my new car (which I bought in December) seems pretty good.

I last filled up on 22nd May, and then went away for a few days to Chester (30 Mat - 1st June), so had a reasonable 'run' then. When I filled up I let the pump automatically stop, and then zeroed odometer A (it has two odometers, and I've got it set so this resets the average fuel consumption).

Then I ran it until going to work this morning when it was claiming a range of 26 miles, then the range changed to dashes and the low fuel light came on - I presume this means that the low fuel light is set at 25 miles?.

So coming back from work I called to fill up, at the same petrol station, at the same pump, on the same side - again, I filled until it auto-stopped.

It held 41.85 litres and I'd done exactly 410 miles, which works out to 44.5 MPG - which interestingly (and amazingly) was what the display in the car was showing as well. However, as the tank capacity is 47 litres, either there was more left than it seemed, or a full tank is more than the auto-stop point?.

So I'm quite pleased with the display accuracy :D
 
I've not calculated mine from filling vs mileage, but the long term displayed average I get is 54mpg.

That's a Mondeo, 2L Duratorq (diesel) engine, & I usually drive on cruise control set at the speed limit (from GPS), when traffic and conditions permit.

The displayed range varies somewhat, I think that is calculated from a shorter term average. It peaks at 700+ for a full tank.

It is somewhere near that, as a friend borrowed it for a camping trip to Scotland; he got back without having to fill it since setting off, with roughly 800 miles done on one tank - but he does drive slower, typically 60 at most.
(He had a heavy four wheel drive SUV with terrible fuel consumption, which is why he wanted to use something more economical & also why he's used to a lower speed).
 
I've not calculated mine from filling vs mileage, but the long term displayed average I get is 54mpg.

That's a Mondeo, 2L Duratorq (diesel) engine, & I usually drive on cruise control set at the speed limit (from GPS), when traffic and conditions permit.

The displayed range varies somewhat, I think that is calculated from a shorter term average. It peaks at 700+ for a full tank.

It is somewhere near that, as a friend borrowed it for a camping trip to Scotland; he got back without having to fill it since setting off, with roughly 800 miles done on one tank - but he does drive slower, typically 60 at most.
(He had a heavy four wheel drive SUV with terrible fuel consumption, which is why he wanted to use something more economical & also why he's used to a lower speed).

Nice :D

Mine's a 1.4L Petrol Suzuki Vitara 4WD.
 
1971 Datsun 240Z. 16 Smiles per gallon.

Side.jpg


2019 Dodge Challenger. 20 Smiles per gallon.

2019 Challenger.jpg
 
I had an acquaintance who wrote automotive-related firmware. He told me that most of the average MPG displays are calculated with a rolling average algorithm. Similar to what is used on digital scopes.
However the sample length and the amount of samples in the calculation differs amongst different auto manufacturers. And similarly to a DSO, these parameters will determine the accuracy and detail displayed. Although I think that in a semi- steady state situation, like driving on a highway, the results will be quite accurate.
 
One car I had with an mpg display showed the result to one decimal point, for instance 42.7 mpg

Unfortunately, the result was rounded to the nearest 0.1 litres / 100 km, so it would not go up in increments of 0.1 mpg in normal use.

For instance:-
42.7 mpg (actually 6.6 litres / 100 km)
43.4 mpg (actually 6.5 litres / 100 km)

It was impossible for an figure between 42.7 mpg and 43.4 mpg to be displayed. I guess that there was a CANbus signal somewhere in units of 0.1 litres / 100 km that the instrument pack had to display.

I also drove an American car that showed mpg to the nearest whole number. So far so good. Unfortunately, when "metric" was selected, it showed litres / 100 km to the nearest whole number as well, so 8 litres / 100 km could mean anything from 31.5 mpg to 27.9 mpg (US)
 
Best I've managed in mine ...... 2018 Skoda Octavia 1.5 turbo petrol (not hybrid). 150bhp / 250Nm / 0-60 in 8.2 seconds too.

This was a round trip to pick up a 100kg cabinet in really warm weather so aircon on as well. Generally I'll get around 55-58mpg on the 20 mile commute in to work (and back) and 65-70mpg on a run. I do drive economically though ...
 

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I run a 2003 Mitsubishi Magna, 90% of travel is highway running at the speed limit (110km/h) and it shows as 8.8L/100km (32mpg) which I'm pretty happy with as it is a big car with 3.5L V6 engine (and I'm not known for a light touch on the go pedal :eek:).

Doing the fill and refill at the same petrol station/pump shows it to be very close to the displayed figures.
 
I don't really care what the mpg of my car is. If I were to replace my car to get better fuel economy, I would be way behind financially. I don't drive enough miles each year for gas money to get near the top of my annual expenses. I focus on what matters.

Also, if I'd change cars for better fuel economy, I'd be way behind on my happiness factor.
 
It held 41.85 litres and I'd done exactly 410 miles, which works out to 44.5 MPG - which interestingly (and amazingly) was what the display in the car was showing as well. However, as the tank capacity is 47 litres, either there was more left than it seemed, or a full tank is more than the auto-stop point?.
At least around here, the gas pumps shut off well before the tank is actually full. I spend a few extra minutes nursing the pump to get that extra 1 - 1.5 gallons into the tank.
 
Why don't you just tell me why that is a bad idea. A 15 gallon tank should hold 15 gallons. If the pump shuts off at 14 gallons then that is the fault of the pump not the tank. So many of those nozzles add so much air to the fuel stream that they shut off way too early. Pump more slowly and you get more in the tank.
 
Why don't you just tell me why that is a bad idea. A 15 gallon tank should hold 15 gallons. If the pump shuts off at 14 gallons then that is the fault of the pump not the tank. So many of those nozzles add so much air to the fuel stream that they shut off way too early. Pump more slowly and you get more in the tank.
You can only add 14 gallons to a 15-gallon tank if the tank is completely empty. I suggest you Google why it's a bad idea to overfill your tank (adding after it automatically shuts off).
 
Why don't you just tell me why that is a bad idea. A 15 gallon tank should hold 15 gallons. If the pump shuts off at 14 gallons then that is the fault of the pump not the tank. So many of those nozzles add so much air to the fuel stream that they shut off way too early. Pump more slowly and you get more in the tank.
Because it causes heart failure in environmentalists o_O :banghead::D
 
why it's a bad idea to overfill your tank (adding after it automatically shuts off).

If you fill too near the top of the tank neck, liquid fuel can get in to the vapour capture vent & system and wreck the vapour canister ("Charcoal canister") and other parts in the EVAP system.

If the tank gurgles for a few seconds after the first cut out as the air escapes, like my previous car did, then one more try to make up for that with the nozzle still fully in the tank, should be OK - but never try to defeat the auto cutoff by slowing the fill or partly removing the nozzle!

A few of the thousands of references to this online:




(Edit to fix bad link)
 
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Because it causes heart failure in environmentalists o_O :banghead::D
That's the minor issue. Fuel injection doesn't work well when you have no headspace in the fuel tank. The fuel injectors need to know how much back-pressure is in the tank to fire properly. The sensor is generally near the charcoal canister and does not work when saturated with fuel. This will cause the "service engine soon" or "check engine" light comes on. Additionally, the warning lights will come on when the gas cap starts leaking (which it will when overfilled) - more because of the sensor in consistencies it causes than for environmental reasons.
 

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