Difficulty getting good mileage on motorways and dual carriageways.

[Jocko]

Irrespective of what I do I have great difficulty getting good mileage on motorways and dual carriageways. I have the instantaneous fuel read out from my ScanGauge E but no matter what I do I cannot get the numbers up above 55 - 56 mpg. On country roads I can get 60 mpg but give me a stretch of motorway and no matter what I try I cannot get high figures. About 55 mph/2,500 rpm seems to be best. I also monitor my Manifold Absolute Pressure (vacuum, effectively) to make sure I don't overdo things under acceleration or when climbing steep hills.
I think a lot of the problem is such a small engine and a car shaped like a brick. Add to that the fact that there is little opportunity for anticipation or driving without brakes. All the stuff that gets me good mpg around town is not available on the motorway.
Tucking in behind trucks or buses (not too close, I don't want to compromise my braking distance) is a help. The bigger they are the better.
Today I did 183 miles of mainly motorway and dual carriageway driving, I had a top speed of 59 mph and averaged 39 mph (several long sections with a 40 mph speed limit). I managed 56.8 mpg. Rather disappointing.
Anyone any tips or advice?

[peteo48]

I am not sure I have quite the same issue as you. I can get my best mpg on the motorway but only if I cruise at around 60 mph (in fact what I tend to do is drive as fast as the fastest lorry if that makes sense). A recent trip to Cornwall bore this out. Cruising at 70 where possible but also involving some steepish climbs on the A30 I only averaged 47 mpg (measured - not computer) on one tank. Throttling back a bit on the way home I got 53.2 out of my last tank on that trip.

My best ever mpg was also on the motorway - 56.2 - but the volume of traffic kept my speed down.

Round town I'm high 30s.

[peteo48]

Of course mine is a Mk2 which has better official mpg figures than the Mk1 in any event. I reckon your figures are pretty good and testament to your eco driving skills!

[sparky Paul]

First thing that struck me about driving the Jazz was the low gearing, ours shows 3500rpm at 70. It's geared for tootling around town, rather than motorway runs.

8 valve petrol engines tend to produce peak torque around 2500 revs, which (very) roughly coincides with peak engine efficiency. I guess it's not surprising that motorway speeds make a dent in the mpg, with the additional wind resistance from the brick shape.

[culzean]

It is not only the Jazz that is low geared.  My 1.8 Civic with 140 ponies  and 6 speed box is about 3k at 70, and is at its most economical at around 60.

My first Civic, a 1996 with a 1.5 engine and 90 ponies was less than 2.5k at 70, I used to get around 55mpg from that on a run - something funny going on I reckon when a larger more powerful engine has to rev more.

And my wife used to get better mpg from her 1.4 GD than she does from her present GE 1.4 (which should has to rev more both to pull away and to make decent progress) something funny going on for sure.

[John Ratsey]

I recall that aerodynamic losses increase with the square of the velocity (effective wind velocity - vehicle speed + any headwind / - any tail wind) so 70 mph has about 50% wind losses than 60 mph. A sleek aerodynamic design (low Cd) reduces these losses but results in a less practical vehicle.

I had an example of the headwind / tailwind effect a few weeks ago in my HR-V when I did a return trip between Swindon and Oxford along the A420 (mainly 50 mph / 60 mph speed limits) on a day with a modest SW breeze. On the outgoing leg (tailwind) the vehicle reported 52 mph overall while on the journey home (headwind) it dropped to 46 mph. Assuming a 10 mph wind on an average of 50 mph gives effective wind losses for 40 mph (outgoing) and 60 mph (return).

I wonder what reduction in road transport CO2 would result from restricting lorries (very unaerodynamic shapes) to 50 mph.

[culzean]

I wonder what reduction in road transport CO2 would result from restricting lorries (very unaerodynamic shapes) to 50 mph.

Speed limits for lorries was raised in 2015 from 40 to 50 on some roads,  I just wish lorries would overtake other lorries faster, we must have all been on motorway when a lorry has signalled to overtake another lorry and proceeded to crawl past at no more than 0.01mph speed difference, taking a couple of miles to get past, only to pull straight back in front of the other lorry and stay at same speed WTF!

Must be so boring driving a lorry that they have to find other sources of amusement other than playing with their mobile or watching a video, what better than to play a game to see who can get longest line of cars behind when overtaking?

You can get better mpg by tailgating a lorry, but not for the faint - hearted.

[Jocko]

Wind resistance increases as the square of the speed, so keeping the speed down has a huge effect. The 1.2 Jazz doesn't have enough power to cope with changing gradients at 40 mph, and with experience I have found about 50 - 55 mph gives me my best figures. Wind speed has a huge effect. My Saturday run has effectively two legs, as I travel west towards the Queensferry Crossing, and east back towards the city. Then the same on the return. I can clearly measure the difference wind has on mpg, especially here in the windy Central Belt.
What really annoys me about my motorway mpg figures is the fact I can more or less equal them on my morning commute. I do 4 miles each morning, with a cold engine and in town, and can get almost as good mpg as I can on a long run on dual carriageways!
Tucking in behind HGVs can make a big difference. I always leave at least a 2 second gap but that is no guarantee. Yesterday, I had a car cut straight in between me and my "tow" (to take the next off-slip), just as the lorry hit the brakes. I braked safely, but had to brake hard as the to**er had stolen my braking distance. At least I had my dash-cam if the need had arisen!!

[VicW]

The CVT equipped cars have higher gearing than manual boxes so my 2011 model is geared at about 32mph/1000rpm when in top ratio. 70mph is about 2200rpm.

Vic.

[Jocko]

Today, my wife wanted to go shopping. It was teeming down with rain, the roads were busy, I had two car parks to negotiate, (including waiting for another driver to manoeuvre out of a spot so as I could get in, as there were no vacant spots), and umpteen red traffic lights. My 6.6 mile total, three separate legs, returned 47.6 mpg. And that's without the car ever achieving operating temperature.
When you compare that to yesterdays jaunt in ideal weather conditions, you can see why I am disappointed with yesterday's results. (All mpg figures courtesy of ScanGauge. Maybe not spot on, but consistent.)
Don't get me wrong. I am overjoyed with the figures my Jazz returns. Compared to the Volvo it replaced, with a lifetime average of 29.8 mpg, the Jazz does brilliantly. I just wish I could squeeze a little more out of it on motorway journeys.

[guest5079]

At least on a motorway, when the lorry drivers play silly beggars you have a chance with three lanes, that is until the car overtaking the lorries refuses to exceed 69 miles an hour and then it stretches for miles. On dual carriage ways you don't stand a chance, often travelling for some considerable distance before the overtaking lorry gets past. A couple of weeks backon the A30 a 'Foreign' lorry pulled out to overtake and after a short distance realised he couldn't get past so indicated and pulled back behind the lorry he was attempting to overtake. I have NEVER seen one of ours do that.
I do have a problem with the motorist who pulls out to overtake a car travelling only a smiggin under the speed limit and proceeds at 69/70 mph which is as bad as the lorry driver.
The Highway code states when overtaking complete the manoeuvre as quickly and safely as possible.
It was how they drummed it in to us, if it means speeding up for a few yards so be it BUT if your speed is only marginally faster than the car you are overtaking then don't bother. BUT then you have the BMW and the Jazz parable. The Jazz poodling along at 70mph BMW driver with his fistful of points cant bear a Jazz in front so he does his creep past because he can't stand the chance of plod catching him.
Trying to get decent mileage these days is a case of luck. I am hoping that now the A30 is finished and the road quietens down I will have a go at some decent consumption figures.

[peteo48]

What I sometimes do when I'm a bit disappointed about my fuel consumption figures is go on to Fuelly.com and look at similar cars and even different makes and the mpg they are getting. Always makes me feel better!

[Pine]

I am not sure I have quite the same issue as you. I can get my best mpg on the motorway but only if I cruise at around 60 mph (in fact what I tend to do is drive as fast as the fastest lorry if that makes sense). A recent trip to Cornwall bore this out. Cruising at 70 where possible but also involving some steepish climbs on the A30 I only averaged 47 mpg (measured - not computer) on one tank. Throttling back a bit on the way home I got 53.2 out of my last tank on that trip.

My best ever mpg was also on the motorway - 56.2 - but the volume of traffic kept my speed down.

Living in the Westcountry I find that I always get better MPG when I am heading north or east than I do when I do the same journey back home.  It's because heading back west I am travelling into the prevailing wind (even if it doesn't feel windy) which is generally from the southwest.           

[Jocko]

What I sometimes do when I'm a bit disappointed about my fuel consumption figures is go on to Fuelly.com and look at similar cars and even different makes and the mpg they are getting. Always makes me feel better!

Yes. When you do that it certainly shows you that my figures are nothing to get upset about. I am currently returning the best lifetime mpg figures of the 32, 2006 Jazz, they have recorded, and considerably better than 2006 Fit figures.

[John Ratsey]

Honest John's RealMPG website https://www.honestjohn.co.uk/realmpg/honda makes interesting reading and highlights how theory and reality diverge. Overall, older vehicles tend to get closer to their claimed MPG, probably because they weren't optimised to give the best test results in the way the new models are. A good example of this is the 2017 Civic for which the actual MPGs are much worse with the 1 L turbo than with the 1.5 L non-turbo.