As I'm driving a diesel Elegance estate atm and regularly getting 50mpg on short journeys, it looks like I'll be visiting the petrol station a lot more often when I get my petrol vRS in December. Luckily the price of oil is dropping.

Still, I didn't buy the car with fuel economy in mind. I bought it to have fun!!

Errmm, I think I need to check my tyre pressures.  Just did 200_ mile motorway run and got 35.3 mpg indicated ( that will be about 0.5 optimistic) when I was expecting ~40.  I've not checked them for ages and the colder weather will have affected them I guess.... Think the 95 Ron probably made no difference after all.  Oops.

I checked the tyre pressures last night and they were down to about 32psi all round, pumped back up to 35/36 now. A bit surprised that a few psi seems to have made a fairly marked difference to my mpg, maybe there was something else going on to affect it.... Time will tell.

Just filled up again and mpg this fill works out at 27.66mpg. Done 1850 miles since new after 6 months ownership, driven on normal mode this fill, great car but skoda way off mark with quoted mpg about town at 36mpg. At least 17% off by my calculation.

Check your consumption over at least 1000 miles or 4 or more refills then the calculated figures will mean something.

 

I would like to have seen you have a conversation with an ex-colleague of mine. He insisted that you could only calculate it on one refill, as you can't guarantee that you top it up to the exact same point, and the errors compound over multiple tanks and invalidate the result. I tried telling him that the percentage error would reduce over multiple tanks, but he wasn't having it, and he was an engineer!

 

One day I told him that you could half fill up, and as long as you'd filled it the tank before and you filled it the tank after it wouldn't stop you doing an mpg calculation. I've never seen a person look that frustrated before or since (well, apart from my 5-yr old lad!).

Great car but skoda way off mark with quoted mpg about town at 36mpg. At least 17% off by my calculation.

 

Is this thread a wind up? Surely everybody is aware that there are virtually no cars out there that achieve the EU combined cycle figures in the real world. I would say 15% error seems to be a typical sort of value, but I simply can't believe that anybody expects to achieve the quoted figures any more.

 

Loveman, you're yanking our chains!

Geek42, whether the tank is filled to the same point or not is irrelevant.  Sometimes I may add 30 litres and sometimes 50 litres.  There is no error to compound.  It is total distance (in my case over 15K miles) divided by TOTAL of the fuel added to cover that distance.  If your friend is an engineer I hope he doesn't design or maintain anything on which peoples' lives depend!

Edited 4 November, 201411 yr by philbes

I would like to have seen you have a conversation with an ex-colleague of mine. He insisted that you could only calculate it on one refill, as you can't guarantee that you top it up to the exact same point, and the errors compound over multiple tanks and invalidate the result. I tried telling him that the percentage error would reduce over multiple tanks, but he wasn't having it, and he was an engineer!

Well presumably a corollary of that is that since all commercial passenger flights are different - different aircraft, different fuel loads, passenger loads, flight crew experience, weather conditions, age of plane, thoroughness and competence of servicing etc may vary, so working out the safety of an average commercial passenger flight can't be done by just averaging out the crash records of lots of flights, it can only be done by monitoring one flight. And if the first flight you monitor happens to end in a crash, then the logical conclusion is that all commercial passenger flights will end in a mass of charred twisted wreckage.

 

What on earth sort of engineer was he?

Geek42, whether the tank is filled to the same point or not is irrelevant.  Sometimes I may add 30 litres and sometimes 50 litres.  There is no error to compound.  It is total distance (in my case over 15K miles) divided by TOTAL of the fuel added to cover that distance.

Well, to be a little picky about it, it it is the total distance covered divided by (the total of the fuel added less whatever amount of fuel is left in your tank). Although I'll grant by the time you've done 15,000 miles, you've probably put in around a couple of thousand litres, so the error introduced by ignoring the 20 or 40 or 50 litres you may have left in your tank will be small but not insignificant. 20 litres in 2000 is only 1%, 50 litres in 2000 is 2.5%, which would be an error of around 1 mpg if you're talking of figures in the mid-to-high thirties mpg.

Geek42, whether the tank is filled to the same point or not is irrelevant...There is no error to compound.

 

As JJJ66 says, if you want to be accurate you need to correlate what you put in with what you actually used. The easiest way to do this is to make sure that you have the same amount of fuel in at the end as you did at the start, and the only practical way to do this it from full to full. The error is the variation in cut-off point of the pump.

 

If you do this over 1 tank then the error in the pump cut-off point can be significant, but over multiple tanks it becomes proportionally less significant.

 

Over 15k miles I'd say it's totally insignificant, and you are correct that it doesn't matter one bit whether you filled or half-filled in between, it is only the start and end points that would matter anyway.

Well, to be a little picky about it, it it is the total distance covered divided by (the total of the fuel added less whatever amount of fuel is left in your tank). Although I'll grant by the time you've done 15,000 miles, you've probably put in around a couple of thousand litres, so the error introduced by ignoring the 20 or 40 or 50 litres you may have left in your tank will be small but not insignificant. 20 litres in 2000 is only 1%, 50 litres in 2000 is 2.5%, which would be an error of around 1 mpg if you're talking of figures in the mid-to-high thirties mpg.

I totally agree.  My explanation of my method was at fault.  I started my mpg calculations when the car was collected with a full tank and the then mileage noted.  Intermediate calculations are made when tank has been filled and total mileage read.  As the total mileage covered is almost 16,500 miles and fuel purchased approaching 375 gallons any slight difference in actual fuel in tank when car collected and when last filled is tiny (5 litres difference would only be .3%, which on my 43mpg calculation is 0.13mpg).

 

My method is accurate enough for me - and getting more accurate the longer I keep the car!

Fuel consumption doesn't matter !

 

How much I enjoy the drive matters. 

 

I drive the car, my wife earns twice the amount I do and fuel is purchased from a joint account with proportional input.