Mpg conundrum

wideboybloke

wideboybloke

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I do a 150 mile round trip every month or so from Hampshire to Hertfordshire. It involves the M3, M25 and A1(M) with a couple of miles at either end to get to the actual destinations. From Hants to Herts I average 44 mpg according to the DIS, but in the reverse direction it’s always 38 mpg. I keep the speed at 80 or just under, occasionally straying towards 90 if I need to overtake the odd car I’d rather not follow. I can’t fathom why I don’t get the same mpg figure in both directions, so does anybody have any idea why there should be such a disparity? I’m sure it’s too simplistic to think that it’s all downhill on the way to Herts and all uphill on the way back!
 
Just got my s4 avant one week ago. 250 on the clock. I am not a fuel saver, not a gas padal delineator. Mainly efficicency mode.

30 mins traffic incl limit of 70km/h 60km/h 80km/h, some uphill and downhill. The lowest fuel consumption is around 12L/100km, equivalent to 28mpg to 29mpg. There was a time I hit below 12L/100km(lowest 11.4l equivalent to 26.8mpg) , when going downhill using engine ****** brakes. However when I went uphill, fuel consumption went up to 12L/100km
 
I try to challenge 10L/100km (23.5mpg) since my previous car s3 can easily attain that number when the traffic is ok under the traffic I mentioned above. However it is a bit hard to go under 11L/100km (26mpg) considering a 3.0 v6 engine.
 
wideboybloke said:
I do a 150 mile round trip every month or so from Hampshire to Hertfordshire. It involves the M3, M25 and A1(M) with a couple of miles at either end to get to the actual destinations. From Hants to Herts I average 44 mpg according to the DIS, but in the reverse direction it’s always 38 mpg. I keep the speed at 80 or just under, occasionally straying towards 90 if I need to overtake the odd car I’d rather not follow. I can’t fathom why I don’t get the same mpg figure in both directions, so does anybody have any idea why there should be such a disparity? I’m sure it’s too simplistic to think that it’s all downhill on the way to Herts and all uphill on the way back!
Click to expand...

If wind is in consistent (and behind you coming south), could that explain it ? Tiny average headwind / tailwind makes a significant impact on mpg,
 
Wind, number of corners, should be same but may not be, hills, number of vehicles around the car will also affect it in different ways.
It just shows that mpg is a subjective element and varies, sometime, hugely, from vehicle to vehicle depending upon the road and driving conditions.
 
Do you have a big dinner then drive home?
 
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I get a similar thing on my trip to work. It's a nearly 10 mile trip. I am travelling at 9pm and returning at 6am so there is no traffic but I can always get 4-5mpg more going to work than returning from work.
For me it's down to the variation in elevation on the trip.
You can put the start and finish postcodes into https://www.doogal.co.uk/RouteElevation.php and it will show you this kind of info. This is for my trip to work. Travelling to work I am most of the time travelling downhill even if that is only a small overall drop. The times I am going uphill they tend to be shorter and steeper when going to work. Coming back from work it's more of a steady prolonged up hill sections.

Upload 2017 11 24 12 33 54


Put your postcodes in and see what it comes out with. Prolonged sections of climbing even at shallow angles has a bigger impact on mpg than a short sharp climb with the same gain in height.
As I have been doing this trip for many years I can spot all the hills on the way but the slow rises and falls are not so obvious. So it may be similar for you.
Post up your results should be interesting.

Karl.
 
As an example winchester to Biggleswade.
Upload 2017 11 24 13 3 6


Upload 2017 11 24 13 5 14


If you move the cursor across the height profile map you will see a blue dot on the map so you can spot where the different height features are on your trip. This site is useful for cyclists looking for a flatter maybe easier route :) .
 
Great site! Obvious why my economy is worse since I used to go 20 miles on a trunk road to work. Although I didn't need a graph to tell me that, it's nice to look at.

Elevation
 
I don’t know how you upload the results from the doogal app, but what a brilliant idea! My journey is from Fleet to Baldock, so it starts at about the 50 km mark on Desertstorm’s chart and ends somewhere between the 140 and 150 km marks. What I observe is that from Fleet to Baldock the first 30 km are fairly sharply downhill and then it’s a steady climb to Baldock, finishing at a lower elevation than my starting point. On the return journey it’s gradually downhill all the way, but when I get to the final 30 km the DIS is reading 38 mpg and that figure doesn’t change for the remainder of the journey.

I appreciate that wind direction is theoretically a factor, but my mpg figures have been measured over several repetitions of this journey during the past 14 months so there would be variations in traffic conditions, weather conditions, time of day etc but the mpg results are nevertheless consistent.
 
I presume that by a fuel tank calculation you mean brim full to brim full on the outbound journey and then same again on the return leg. That might be an interesting exercise. I’m due to do the journey again at Christmas so I’ll bear it in mind.
 
Upload 2017 11 24 23 22 22

To upload images I just use the snipping tool to grab a part of the screen and then paste it.
Travelling from Fleet it seems the first 10K is pretty flat, fractionally downhill, makes it easier for the engine when it's cold and gives you a good start to the journey. From 10-14k it's uphill but the next 15k pretty much all downhill. I find that if you get a good start to a journey and manage to run up a good average mpg in the first part of a journey it's easier to hold onto it. Rather than trying to improve a poor start later on.
Your return trip seems to start immediately climbing and is very up and down for the first 10K when the engine is cold. This means you don't get such a good start and your average mpg will suffer . And as I said earlier it seems to me that it's more difficult to make up average mpg when you have had a bad start.
To get the best fuel economy I find not using cruise control is best as it will try and hold the speed up hills. The best thing to do is build a little extra speed on down hill runs and then let that speed bleed off when going up hill. A lot of the up's and downs in the graphs above are small gradients which may not really be perceptible as a climb but if you are ever in a gym try running on a treadmill up a 1-2% grade. It makes life harder.
 
wideboybloke said:
I presume that by a fuel tank calculation you mean brim full to brim full on the outbound journey and then same again on the return leg. That might be an interesting exercise. I’m due to do the journey again at Christmas so I’ll bear it in mind.
Click to expand...
Yup exactly that...
 
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