[Berber]

Well I have had my i30 diesel now for 8 months and have driven 10k miles in it.

I have always taken as red that the avg MPG displayed on the dash was accurate until today.

My reading as I pulled up at the petrol station was 62 MPG. The car had done 505 miles and it took 46 ltrs to fill up. When I got to work I sat down with a calculator as I was suspicious of the figures. Given the assumption is that the mileage is accurate, I took the milage, divided it by the litres of diesel and then multiplied it by 4.5 to give me gallons. This came to 49.9mpg, a difference of 10mpg between what I really used as opossed to what the car told me it used.

Now I knew it would be a little out but not 10MPG out!

I do wonder though, if I had travelled the remaining miles the car told me was left in the tank, whether the displayed figure would have been more accurate.

Does anyone else have a similar issue in the difference between what the car tells you and what your real consumption is?

[AlanHo]

The only way you can check accurately is to brim the tank - and I do mean completely brim it until you can get no more fuel in - and make a note of the odometer reading. Also reset your Avg mpg display at the same time for comparison later.

Now use the car until the fuel warning light comes on and refill it again - making sure that you really do brim it. You will probably get more than 50 litres into the tank. Take a note of the odometer reading and the number of litres of fuel just added.

You now know exactly how much fuel was used over an exact distance.

Now calculate your mpg by dividing the litres by 4.545 to get gallons. Divide the miles done on that tankful by the gallons just calculated.

However - even this calculation may not be accurate - my last three cars have all had inaccurate odometers. I check this by comparing the odometer readings on a long 100 plus mile motorway journey by comparison with the kilometre distance boards and my Garmin sat-nav. A bit of maths to convert the motorway kilometre board readings to miles is needed. The odometer of my present car reads 2.5% under. This means that when the odometer shows I have done 100 miles the actual distance is 102.5 miles.

THis converts a calculated 60 mpg to 61.5 mpg when the odo inaccuracy is take into account

[Berber]

Thanks Alan. It is still a little disappointing .

[Asterix]

I allways brim the tank and when car shows 4,9 I often get 5,0 when calculating odo/liters.

I'm happy with that. 

[Dazzler]

Thanks Alan. It is still a little disappointing .

But what Alan is saying (in a fashion) is "do a proper test before getting too disappointed" I found the Trip PC in our previous FD diesel i30 to be just a little optimistic (certainly not 15% though)

[Phil №❶]

What's with the mpg average and litres into the tank, don't you people use Imp Galls any more 

[eye30]

What's with the mpg average and litres into the tank, don't you people use Imp Galls any more 

Because the price for fuel was going up so quickly the price per gallon was changed to per litre.

This gave the impression that fuel was cheaper in a period when fuel was increasing nearly every other day by 10p or more per gallon.
 plus 1p or 2p increase per litre gave the impression price rises were under control!

[Phil №❶]

Really

After seeing the behavior in Blackpool on our TV's, I can imagine the public falling for a trick like that.

[Berber]

Really

After seeing the behavior in Blackpool on our TV's, I can imagine the public falling for a trick like that.

Must have missed that one.

Anyway to get back to the point. I thought the avg mileage display would have been more accurate than that.

[PiL]

I think I heard on the radio today that hyundai and kia usa just got slapped with a massive fine for way over stating their mpg efficiency.
$400 mill or something.
Ouch.

[Dazzler]

Didn't this happen a year or two ago? 

[Phil №❶]

Whether it happened or when, someone in the Hy organization cost the group $400 Meg. I wonder if they still work there 

[Lakes]

I found the economy i got out of my i30 CRDi was more to do with how i drove , where i drove & the speed i was sitting on, also hill's or flat, a lot of down hill driving was the best 
converting L to Imperial Gal 4.54L = 1 imp Gal.

Sounds like a con job on the fuel measurement over there.

the world know they can rip Aussies off, so they do, in USA they can't & if they try & the public find out everyone complain .

[Berber]

My last car was a 2.2 Civic. The AVG display was only 2 mpg out. I assumed therefore, that the Hyundai would be similar. This far out makes the avg display worthless.

[Dazzler]

My last car was a 2.2 Civic. The AVG display was only 2 mpg out. I assumed therefore, that the Hyundai would be similar. This far out makes the avg display worthless.

I don't think it is that far out if you do your measurement of fuel and distance right..

When I did my 2008 FD SLX it was showing 5.4 LPH mostly and calculated actual at about 5.6..

[Lakes]

My last car was a 2.2 Civic. The AVG display was only 2 mpg out. I assumed therefore, that the Hyundai would be similar. This far out makes the avg display worthless.

Berber , i think it takes time to get the average closer to accurate . every car with a trip pc i've had was the same.
& you need to take into account if your driving on a motorway for say 500k at a constant speed of say 90KPH no stop & go no overtaking, then you get to a city with stop start driving, the trip PC is still showing your average from the motorway. you have to do same type driving for say a month without resetting.

[Phil №❶]

IIRC, the actual fuel used per injector cycle is a very precise amount calculated by all the engine variables by the ECU. If the AVG is out in any way, it will be most likely as a result of NOT precisely being able to measure the distance travelled. Remember also, that if you were traveling along a flat plane (the X axis and the other one that is at 90 Deg to X), the distance would be the shortest distance between A & B. However, due to undulations / bumps in the road, the car travels an irregular wave centred about the x axis but hardly ever on it, so the distance traveled is greater. Very small amounts, each time, but over 700 k's, (434.959) miles, it adds up.\

Additionally, ANY hill or valley you travel along will add significantly to the coordinate distance between two points on a map.

[Dazzler]

IIRC, the actual fuel used per injector cycle is a very precise amount calculated by all the engine variables by the ECU. If the AVG is out in any way, it will be most likely as a result of NOT precisely being able to measure the distance travelled. Remember also, that if you were traveling along a flat plane (the X axis), the distance would be the shortest distance between A & B. However, due to undulations / bumps in the road, the car travels an irregular wave centred about the x axis but hardly ever on it, so the distance traveled is greater. Very small amounts, each time, but over 700 k's, (434.959) miles, it adds up.

My head hurts (but you are correct Phil) 

My head always hurts when an X gets a mention though.. 

[The Gonz]

... if you were traveling along a flat plane (the X axis) ...

That would be a line, not a plane. You need to add a y or z axis to make a plane.

[Asterix]

Filled up the old i30 today. (yes, I allways brim it) 

1059,6 km

53,28 liter

Calculated: 5,03 l/100

Display in car: 4,9 l/100

Difference  2,6% 

[The Gonz]

I've never really brimmed for the purpose of checking the economy guage but I always get well over 1100 km to the tank, so my average displayed of 4.1 is good enough for me.

[Asterix]

The purpose of brimming is for me to run longer between fillups. I can fill another 10 liters in the tank after the nozzle click off the first time. 

[Phil №❶]

Me 2

[The Gonz]

Me 3

I just don't wait unti I can see the fuel at the top, especially not in the morning when it will expand during the day.

[Berber]

thnaks for all the replies. It makes sense to me now. I did wonder whether it changes as the tank gets lower.