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Better MPG at lower speeds - forced test on M6

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raybarrow
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Better MPG at lower speeds - forced test on M6

#156076

Postby raybarrow » July 30th, 2018, 11:24 am

We know that lowering your speed gives better MPG, but never get a chance to try it in real life. Went Birmingham to Manchester twice on Friday and Saturday (shan't bore you with why but involved a family wedding).

M6 is a series of roadworks, smartmotorway upgrades, the journey being 50mph for probably over half that distance. I normally drive at about 65+ but reckon I did more like 55+ on average. MPG at least 10miles extra (50+mpg), but the interesting thing was how relaxed the driving was. Very few people 'desperate' to get somewhere, the traffic kept moving steadily onward. I actually enjoyed the journey apart from coming back Sunday afternoon, my second return trip, and then we ground to a halt as there were some loose sheep on the M6 near the M54 junction!! What can you do? it was that sort of a weekend.

By the way car is 15 year old (I've had it for 10) Mk 4 Astra 1.6 8 valve. They don't come much slower, but very comfortable and roomy, sadly its time has come to be be replaced, but please don't tell it.

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Re: Better MPG at lower speeds - forced test on M6

#156100

Postby Hardgrafter » July 30th, 2018, 1:00 pm

Generally mpg is optimum at around 55 mph. i.e drive in the slow lane with the trucks, which are limited to 56 mph. Perhaps surprisingly, at 40 mph the economy is worse.

See http://www.mpgforspeed.com/ , which is US data.

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Re: Better MPG at lower speeds - forced test on M6

#156105

Postby bungeejumper » July 30th, 2018, 1:25 pm

Hardgrafter wrote:Generally mpg is optimum at around 55 mph. i.e drive in the slow lane with the trucks, which are limited to 56 mph. Perhaps surprisingly, at 40 mph the economy is worse.

Sounds likely. I get a much better mpg spinning along an A road at 60 than I do when I'm stuck at 45 behind a campervan. (1.6 petrol Toyota.) That'll be due partly to the fact that I'm in sixth gear, of course, and partly because I can drive more smoothly and anticipate the road better when I'm not following somebody else's bumper.

I'm about to head off for 2,000 miles of French autoroutes, loaded to the gunwales at mainly 130 kph. Will report back, but not expecting less than a 15% improvement on my daily average mpg.

BJ

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Re: Better MPG at lower speeds - forced test on M6

#156109

Postby PinkDalek » July 30th, 2018, 1:36 pm

bungeejumper wrote:… I'm about to head off for 2,000 miles of French autoroutes, loaded to the gunwales at mainly 130 kph. Will report back, but not expecting less than a 15% improvement on my daily average mpg.

BJ


It might have been you who takes a westerly route but do you expect your daily average mpg to be impacted by the French Truckers' and Farmers' intermittent blockages?

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Re: Better MPG at lower speeds - forced test on M6

#156121

Postby bungeejumper » July 30th, 2018, 2:18 pm

PinkDalek wrote:It might have been you who takes a westerly route but do you expect your daily average mpg to be impacted by the French Truckers' and Farmers' intermittent blockages?

That's going a bit O/T :lol: , but we've had them before. The protesting farmers will generally stop the motorway solid for an hour or so, during which time your engine's off and your mpg becomes irrelevant anyway. After that, the gendarmes will suddenly put down their coffee and stop chatting genially with the blockagers, and will instead start assaulting them with pepper spray and the occasional clutch of passing cyclists, if available. It's a kind of summertime sport, really. All part of the fun.

BJ

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Re: Better MPG at lower speeds - forced test on M6

#156213

Postby BT63 » July 30th, 2018, 9:41 pm

Hardgrafter wrote:Generally mpg is optimum at around 55 mph. i.e drive in the slow lane with the trucks, which are limited to 56 mph. Perhaps surprisingly, at 40 mph the economy is worse.

See http://www.mpgforspeed.com/ , which is US data.


Remember that US cars tended to have large-capacity, high-cylinder-count, lazy, naturally aspirated engines coupled with torque-converter-type automatic transmissions and rear-wheel-drive which cause considerable losses of energy between engine and wheels. For such cars, 50mph or so may be the most economical speed.

However, for a typical econobox with small-capacity, three-cylinder engine with manual transmission and front-wheel-drive, often the most economical speed is approximately 30mph.

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Re: Better MPG at lower speeds - forced test on M6

#156257

Postby marronier » July 31st, 2018, 9:15 am

Driving 100 miles at 75 mph effectively increases the cost of fuel to £2.08p per litre from a pump price of £1.30p such is the increase in fuel consumption over the optimum at 55 mph.

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Re: Better MPG at lower speeds - forced test on M6

#156259

Postby swill453 » July 31st, 2018, 9:27 am

marronier wrote:Driving 100 miles at 75 mph effectively increases the cost of fuel to £2.08p per litre from a pump price of £1.30p such is the increase in fuel consumption over the optimum at 55 mph.

I agree with BT63, the optimum isn't 55mph, it's a lot lower. Usually the lowest comfortable speed in top gear, which will vary according to gearing of the car, but will probably be not much more than 30mph.

Scott.

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Re: Better MPG at lower speeds - forced test on M6

#156261

Postby chas49 » July 31st, 2018, 9:29 am

I have also noticed (Volvo D3 auto) that cruise control makes a (positive - i.e. beneficial) difference to consumption. Even more so when you set it to approx 56mph.

I have always assumed this is because the mpg test figures report on a constant 56, so it makes sense for manufacturers to optimise at that speed. Cruise control on a motorway set at 56 is probably as close as real like gets to the rolling road test.

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Re: Better MPG at lower speeds - forced test on M6

#156264

Postby swill453 » July 31st, 2018, 9:38 am

chas49 wrote:I have also noticed (Volvo D3 auto) that cruise control makes a (positive - i.e. beneficial) difference to consumption. Even more so when you set it to approx 56mph.

I have always assumed this is because the mpg test figures report on a constant 56, so it makes sense for manufacturers to optimise at that speed. Cruise control on a motorway set at 56 is probably as close as real like gets to the rolling road test.

But the rolling road test doesn't have wind resistance to overcome.

Scott.

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Re: Better MPG at lower speeds - forced test on M6

#156268

Postby chas49 » July 31st, 2018, 9:46 am

swill453 wrote:
chas49 wrote:I have also noticed (Volvo D3 auto) that cruise control makes a (positive - i.e. beneficial) difference to consumption. Even more so when you set it to approx 56mph.

I have always assumed this is because the mpg test figures report on a constant 56, so it makes sense for manufacturers to optimise at that speed. Cruise control on a motorway set at 56 is probably as close as real like gets to the rolling road test.

But the rolling road test doesn't have wind resistance to overcome.

Scott.


Accepted - but the constant speed probably makes somewhere around the 56mph figure as sensible as any other for the optimal mpg to be tuned.

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Re: Better MPG at lower speeds - forced test on M6

#156286

Postby Lootman » July 31st, 2018, 10:42 am

BT63 wrote:
Hardgrafter wrote:Generally mpg is optimum at around 55 mph. i.e drive in the slow lane with the trucks, which are limited to 56 mph. Perhaps surprisingly, at 40 mph the economy is worse.

See http://www.mpgforspeed.com/ , which is US data.

Remember that US cars tended to have large-capacity, high-cylinder-count, lazy, naturally aspirated engines coupled with torque-converter-type automatic transmissions and rear-wheel-drive which cause considerable losses of energy between engine and wheels. For such cars, 50mph or so may be the most economical speed.

However, for a typical econobox with small-capacity, three-cylinder engine with manual transmission and front-wheel-drive, often the most economical speed is approximately 30mph.

The "large, lazy lump" characterisation of US auto engines is a bit dated now. Sure there are plenty of those old gas-guzzlers around but modern American cars tend to be more efficient with smaller engine capacities and higher tuning. Not to mention the trend towards hybrids and electric vehicles.

During the 1970's oil crisis a nationwide speed limit of 55 mph was introduced to reflect the idea that 55 mph is (or rather, was) the optimal speed for consumption.

In practice I got good fuel consumption numbers driving in the US in those big old cars for a couple of reasons. Firstly the long-stroke, low-compression engines had tons of torque, so you could use top gear at relatively low engine revs and road speeds. Secondly there is often far less traffic around, outside of the cities anyway, so less of the stop-go driving necessitated in the UK. It is constant acceleration and braking that burns up fuel.

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Re: Better MPG at lower speeds - forced test on M6

#156287

Postby swill453 » July 31st, 2018, 10:46 am

Lootman wrote:During the 1970's oil crisis a nationwide speed limit of 55 mph was introduced to reflect the idea that 55 mph is (or rather, was) the optimal speed for consumption.

I'd say it reflects the idea that going slow uses less fuel than going fast. But it was/would be unrealistic to expect everybody to drive at 30mph on the motorway (freeway, whatever).

Scott.

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Re: Better MPG at lower speeds - forced test on M6

#156307

Postby tea42 » July 31st, 2018, 12:23 pm

It would be interesting if people posted the minimum and maximum mpg they have experienced on their vehicles..

My current petrol 1.4 litre turbocharged Seat Ateca 37mpg and 41mpg ratio 1.11

My current petrol aircooled VW T25 1980 Devon Moonraker Camper 14.5mpg 18.9mpg ratio 1.30

My old 61 plate turbo diesel Mitsubishi ASX 37mpg and 43mpg ratio 1.16

My old petrol 6 cylinder Merc E350 TE estate car purred along doing 22mpg regardless?

Obviously worth doddling along in the camper, and thats part of its charm…

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Re: Better MPG at lower speeds - forced test on M6

#156324

Postby redsturgeon » July 31st, 2018, 1:00 pm

tea42 wrote:It would be interesting if people posted the minimum and maximum mpg they have experienced on their vehicles..

My current petrol 1.4 litre turbocharged Seat Ateca 37mpg and 41mpg ratio 1.11

My current petrol aircooled VW T25 1980 Devon Moonraker Camper 14.5mpg 18.9mpg ratio 1.30

My old 61 plate turbo diesel Mitsubishi ASX 37mpg and 43mpg ratio 1.16

My old petrol 6 cylinder Merc E350 TE estate car purred along doing 22mpg regardless?

Obviously worth doddling along in the camper, and thats part of its charm…


Over what timeframe are you measuring, a tankful?

John

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Re: Better MPG at lower speeds - forced test on M6

#156332

Postby DrFfybes » July 31st, 2018, 1:23 pm

You can't generalise - it depends on the car.

My sister's BMax is more frugal at 40mph than on the motorway, where it drinks the same as the wife's 3.2L BMW. Modern low revving small capacity turbos will sip fuel on a trickle of throttle at 30 or 40 mph.

We took the Z4M to Assen the other week - 30mpg average from Dover back to Devon at motorway speeds, but has managed 35 mpg on a run up the M5/M6 to Manchester through the interminable roadworks and an average nearer to 50 than 70, Then again I'd happily pay 10% more on fuel to shorten that trip by 30 mins.

The Maser is a different beast - UK m/ways gives 20mpg at about 75mph, continental cruising at 85 has given 22mpg although I use 98 Superunleaded over there as it has less ethanol. I've no idea what it does at 55mph, although it drops into 5th gear in auto mode as it is about 2200 rpm. Mixed cross county A-roads gives the claimed 18mpg, winter urban commuting will empty the 95L tank in under 200 miles. Still costs less in fuel than servicing though :(

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Re: Better MPG at lower speeds - forced test on M6

#156337

Postby tea42 » July 31st, 2018, 1:34 pm

redsturgeon wrote:
Over what timeframe are you measuring, a tankful?

John


Yes, mostly, I note the milage trip reading, then zero the milage trip and fill right up. 4.54 litres per gallon, a simple calculation. The displays that purport to indicate fuel consumption on the Seat and Mitsubishi are wildly inaccurate. The fuel gauge on the T25 is terrible, owners all rely on the milage trip to tell them when to need that (frequent) refill.

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Re: Better MPG at lower speeds - forced test on M6

#156400

Postby staffordian » July 31st, 2018, 6:55 pm

tea42 wrote:It would be interesting if people posted the minimum and maximum mpg they have experienced on their vehicles..

My current petrol 1.4 litre turbocharged Seat Ateca 37mpg and 41mpg ratio 1.11

My current petrol aircooled VW T25 1980 Devon Moonraker Camper 14.5mpg 18.9mpg ratio 1.30

My old 61 plate turbo diesel Mitsubishi ASX 37mpg and 43mpg ratio 1.16

My old petrol 6 cylinder Merc E350 TE estate car purred along doing 22mpg regardless?

Obviously worth doddling along in the camper, and thats part of its charm…


1.2 normally aspirated 2015 Nissan Note 40mpg worst, 53mpg best. ratio 1.33

It's mainly used for pottering, hence the none too impressive figures...


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