Measuring MPG

[NormanC]

I'm seeing more and more threads giving concerns about the mpg members seem to be experiencing - not surprising these days!

Some seem to be relying on some calculation over relatively short distances, where the accuracy of measurment of the actual fuel consumed may be low.

Can I presume to offer a little advice, having kept accurate fuel consumption records for a number of cars over 200k plus miles.

In-dash displays are virtually useless for giving accurate results. They are a reasonable comparator for assessing changes in driving style in the same vehicle, but not for cross-vehicle comparisons - there are just too many variables!!

If you go for measuring consumption on a full-to-full basis, you will get a better comparison but a single tank full can give very misleading results. Ideally, you should go for say 5 tanks full of fuel before you can begin to get a reasonably representative result, i.e. a figure worthy of comparison.

As a matter of interest, below are 2 examples of full-to-full measurements for a couple of cars (only one an M-B I'm afraid) which show the massive variation that can happen when comparing single tanks of fuel. These are all the same driver in the same car!!

Thanks.

 

[Roger Jones]

Well said. Any figure I quote is based on either rolling six fill-ups or life of vehicle in my ownership. I once got 37 mpg out of my Golf VR6 on a single trip; its rolling average was just below 30.

 

[wemorgan]

Interesting that for your A3 it took you almost 60000 miles to reach your average that you then maintained until I guess you sold the car. Over what time period was the A3 recorded & did you notice much seasonal variation?

 

[NormanC]

Interesting that for your A3 it took you almost 60000 miles to reach your average that you then maintained until I guess you sold the car. Over what time period was the A3 recorded & did you notice much seasonal variation?

Hi Will,

The A3 was a company car and was delivered new on 15/03/2005 and handed back on 19/05/2008 with a mileage of 78248.

Yes, it is surprising that it took so long to stabilise. No noticeable seasonal variation (but I haven't tried regression analysis!) but much more variation than I'd expected in day to day usage - basically a variation of between 25 to 42mpg from any given tankfull!!

Interesting, the Alfa 156 Selespeed I had before that didn't stabilise until around 82000 miles, taking stabilisation as all variations within +/- 0.025 mpg, i.e. within a 0.05mpg range.

 

[NormanC]

Looking at the above, I've only one real comment to make about myself......

I'm sad, I'm sad, you know it, I'm sad....... (with apologies to Michael Jackson).

 

[BTB 500]

Why the slow but steady decline in MPG with the CLK??

 

[NormanC]

Why the slow but steady decline in MPG with the CLK??

Basically, a change in usage pattern, Bill, in that there haven't been so many long motorway/dual carriageway trips this year. If you look at the graph you may make out a step change in typical tank-fulls at around 47000 miles.

However, Tuesday will see a trip from home to Aberdeen with only a stop in Dundee for petrol. 450 miles of which 448 are dual-carriageway or motorway and a total of 11 roundabouts and no traffic lights or pedestrian crossings.

 

[Mactech]

I agree that MPG is just miles per gallon. How can you get that wrong?
Many ways it seems. The fuel that comes from our pumps is reasonably well controlled by weights and measures. Recording it over a long period (the life of the car??) will make things a little more accurate but we normally want to know how the car is performing over a much shorter period.
The fuel meters in my last few 320CDI cars have all been within 1.5% when checked against 'full to full'
But how to tell the distance you have travelled. Do you believe the mileage on the dash for that? I checked many of my regular journeys against Autoroute, but is that any more accurate?
This was brought home to me when we did a landspeed record. The timing over the measured mile was to the 10,000 of a second. But the mile was measured by a couple of guys from the FIA with hand held GPS

 

[Nasco12]

I'm sad, I'm sad, you know it, I'm sad....... (with apologies to Michael Jackson).

Well, if you're sad then I am too - I've been carrying out a rather similar exercise over the last 6 months. In that time my MPG has risen from 25.5 to 28.5. Why? I have no idea. Could be an oil change, filter change, the car getting used to my driving style. Who knows?

I could improve the MPG figures by driving less in town and more on the motorway. But what's the point in that?

The only real way is to change the car. Buy a Golf do the same miles (8000) and I might save £400. But, again, what would be the point? There is no way I am going to have as much fun in a Golf as I do in my Merc.
　

 

[scumbag]

sad eh?

I keep every petrol reciept and write on the back of it, the total miles, miles between fill and what the MPG the car is saying, (from reset, not point with from start), MPH average and hours run.

I then put this on a spread sheet which show a history of fills, and correctly calculate the MPG between fills so I can see how the car's calc is performing.

Boringly I even make a note of the price pr ltr.

Whoever gets this car when I am done will have a complete history of it.

Knowing how it has done is another reason I an dispointed with the tyre life.

It has had 2 ltrs of oil in 3220 miles too.

 

[renault12ts]

sad eh?

I keep every petrol reciept and write on the back of it, the total miles, miles between fill and what the MPG the car is saying, (from reset, not point with from start), MPH average and hours run.

I then put this on a spread sheet which show a history of fills, and correctly calculate the MPG between fills so I can see how the car's calc is performing.

Boringly I even make a note of the price pr ltr.

Whoever gets this car when I am done will have a complete history of it.

Knowing how it has done is another reason I an dispointed with the tyre life.

It has had 2 ltrs of oil in 3220 miles too.

Thank God...I thought it was only me. At the end of the year I tot up the litres bought, convert to gallons and divide into the total annual mileage...can't get much more accurate than that. You don't even have to fill up at anytime.

 

[macp]

I must admit Im impressed with the fuel consumption of your 3.2. Looks like it averaging 28mpg and the cabrio will be a heavy car wont it ?

 

[NormanC]

Not too bad, thanks. I don't think that it's slow driving either as I always try to be going as fast as either conditions or the law allow. If posssible tend to floor-board it at an upwards change of speed limit until the new speed is reached.

I do use the cruise control quite a lot, but I can be more economical without it (anticipate ahead, only accelerate downhill, let speed bleed off when going uphill, etc.).

Perhaps I'm just lucky!!