S.A.F.E.D

[warren t claim]

Safe And Fuel Efficient Driving.

 

It's not often I have a car with a credible fuel computer so I thought I'd try a little experiment and share the results with you.

 

I had to do a journey of mixed motoring taking in towns, motorways, A roads, B roads and a 20 minute heavy traffic jam. To make it interesting I chose to drive the entire journey not exceeding 3000rpm which translates to 68mph in 5th. Rather than use hard acceleration I simply built up speed swiftly and constantly without dawdling and being a hazard to other traffic. As I'd paid good money to get up to my chosen speed I did my best to keep all available momentum by correctly anticipating and need for change of speed or direction and did my best to read the road ahead. I allowed my speed to drop slightly on uphill gradients but I reduced throttle input on the downhill stretches. Yes I was occasionally overtaken but I did also have to do the odd overtake myself as well so I can't of been driving that slowly. My journey time was about 4 minutes less than the sat nav said it would be.

 

Here's the result after 100 miles.

 

Now, the 1.8 406 isn't exactly a torquey unit and can feel a bit gutless under 3000 revs so I'm somewhat surprised by that figure, I know that if I had an HDi the figure would probably be 8mpg or so higher but 47mpg is not just a better average figure than my 106 it's actually a smidgen higher than I used to get with my NA 1.9D 306!

[CreepingJesus]

Tidy result. Disciplined driving goes a long way to frugal motoring. Especially given the cost of go-juice lately!

I worked out recently, that mine's more economical at 80 than 70 or 60. It must be 'cos it's pulling 3000rpm at 80, and is near peak torque.

SAFED course for the CPC tomorrow, so I've paid for the privilege of being lectured on the bloody obvious for 7hrs. Huzzah.

[warren t claim]

I suppose the lesson for today is that you don't need a 55mpg diesel if you're doing less than 50,000 a year or do loads of urban driving, the extra complexity of high pressure fuel pumps and DPF issues aren't worth it. If your finances are so bad that 55mpg (of more expensive fuel) is critical over 47mpg of cheaper unleaded than maybe a bus ticket is a better bet.

 

Shit! I'm starting to sound like Pete-M!

[Spiny Norman]

That's a pretty tidy figure for a decent sized 1.8 saloon, but it shows what's possible with just a small change in driving style. I once got close to that figure (about 40-45mpg) from a BMW 520i when the throttle linkage fell apart in the middle of nowhere (well, near Oban.. ) and my cobbled-together repair permitted 2500rpm and no more. Still managed 70mph though!

[skattrd]

I did similar recently in my VW bora diesel.

Driving normally I average 56-58mpg, I do mainly long runs. Slightly more careful driving sees 62mpg. Very careful (incredibly dull) driving sees 70mpg. These are calculated figures, not ones provided by the computer, which is usually a little optimistic.

 

I have found that slow acceleration uses slightly more fuel than say medium acceleration as the engine is under load for longer. Accelerating slowly say you're doing 25mpg for 20 seconds to get up to speed, harder acceleration (but not flat out) mpg drops to say 20mpg but for 10 seconds .... I spend 20+hrs a week sitting behind of the wheel of the car and quite often get bored.

 

I did similar tests in my mk2 Scirocco a few years back and Shell v-power worked out cost effective as I got a 10% increase in mpg for a 7% increase in fuel cost

[HillmanImp]

Have you checked the reading with how much fuel you are actually putting in? How accurate is it?

[pompei]

I love it when I've driven 50 miles with the feather pedal and the computer tells me that my range is longer than when I set out

[warren t claim]

Have you checked the reading with how much fuel you are actually putting in? How accurate is it?

 

It seems pretty accurate cos I never really put much fuel in, certainly a hell of a lot less than in the W124.

[jakebullet]

I tried playing truck driver @ 56 mph in my disco. Same route (Hull & back) gave 3 mpg difference. Conclusion: Driving something shaped like a brick cannot be economic, and 3mpg isn't worth the fear of someone twatting you up the arse.

[willswitchengage]

mine's more economical at 80 than 70 or 60. It must be 'cos it's pulling 3000rpm at 80, and is near peak torque.

 

Are you sure? !!CAUTION !! NERD ALERT!! Thermal efficiency in a diesel stays broadly the same across your rev range - petrols it peaks at about 5000rpm (why you start to lose power afterwards) as they're very limited by flame speed. Thermal energy is lost at lower RPMs by conduction through the cylinder block walls which represents a limiting factor on running an engine too slowly, but the biggest factor are your friction/pumping losses - ie the enormous drag induced on your engine by it simply spinning. An idling 2ltr engine will require about 5hp just to tickover - and this will increase linearly throughout its rev range sucking out power. Here's an example to show the "sweet spots" for gasoline and diesel engines respectively:

 

 

As you can see it's about 3500 for the pez and 2000 for the diz - these are a compromise between pumping losses (at the high end) and thermal losses (mainly at the low end) in an engine. So, that's the efficiency at the flywheel. By far the biggest impediment is your speed, however, and the drag force acting on your car is proportional to the square of its velocity - meaning that the force at 80 isn't 33% greater than at 60, it's be more something like 50%. Peak torque sounds like it should be important - but this is pretty much limited to HGVs who operate at around 100% load factors on only minor gradients. In practically any car you'll only ever be operating at about 75% load factor (or in English - how far you press the loud pedal down) even at 70 - you'll only get to utilise peak torque when accelerating, not when cruising. Sorry but I recommend you re-look at your sums ! (if you care that is ) Here's a generic mpg-speed graph if anyone's getting a bit lost here:

 

[Conrad D. Conelrad]

Speaking of pretty charts, I've made one to reflect my own experiences with fuel economy:

 

[willswitchengage]

Add stranded with the fuel light on and all-empty filling stations for fifty miles due to fucking idiots panic buying to the top there.

[dollywobbler]

A car is never going to be more economical at 80mph than 70mph because you need much more power to do 80 than 70. You might be nearer peak torque at 3500rpm, but you're still using more fuel to get there.

[warren t claim]

A car is never going to be more economical at 80mph than 70mph because you need much more power to do 80 than 70. You might be nearer peak torque at 3500rpm, but you're still using more fuel to get there.

 

Is your 2cv more economical at 70 than 80?

[dollywobbler]

Actually, the 2CV probably is more economical at 80mph - as it's either falling off a cliff or being recovered by something very fast!

[AeroNautiCal]

Actually, the 2CV probably is more economical at 80mph - as it's either falling off a cliff or being recovered by something very fast!

 

Or both!

[Pete-M]

I noticed in the XJR that it appears no worse on fuel at 120 mph than it is at 80 mph. That seems to be its 22 mpg cruise zone. The XJR isn't very aerodynamic with 0.37cd, but the supercharger is possibly in a happier place at 120 mph than it is at 80. That's the only way I can explain the MPG. Long gearing probably helps, 28 mph/1000 rpm in top.

 

The Subaru does 28-30 mpg on a 70 mph motorway run. 28 on a quiet A or B road and 26 in town. Drops at high speeds because the gearing is pretty short, 22 mph/1000 rpm in 5th.

 

604 seems to average 17 in town, 24ish on a run. Gearing is short on this too, no longer than the Subaru by the looks of it.

[Ross_K]

Here's the result after 100 miles.

 

The wife's 307 has a similar trip computer. The mpg reading you've got there is the cumulative total and I doubt it's accurate.

 

Fill the car to the brim and zero the cumulative total by holding in the button on the end of the wiper stalk.

 

Do that three or four times and see what the mpg is like. Keep track of your fill-ups on fuelly and compare the numbers.

 

If you stick in a tenner here and a tenner there you won't get accurate readings.

 

Full tank (65 litres) on a 1.8 16v Xantia (same engine) used to get me 440 miles of Paisley -> Grangemouth motorway driving at 60-70mph. That's what, 31 mpg?

 

I wouldn't expect the mpg on a 406 1.8 to be that different.

[CreepingJesus]

Thermodynamics of heat machines paper

 

Cheers, Will! Now while I'm every bit as much of an engineering geek (well, as much as the next guy with a string of A+ and merit marks in uni coursework on thermodynamics etc., ), there's one vital flaw in your working. Mine takes spark plugs, not glow plugs.

Taking Ian's assertion that 'it can't possibly be true', and Pete's Jag as a case in point: in the real world, it's all about torque and gearing. Generally the torque curve on an engine is broadly mirrored in its' Specific Fuel Consumption curve (see Will's graph). When the engine's on song around peak torque, it's making best use of the energy in the fuel, and usually produces its' best work. Hence SFC is a measure of 'productivity' if you will. (Given that it takes a predictable amount of energy to move a mass over a distance, it's how you do it that counts)

Pete's supercharged Jag, probly has a wide, flat torque curve (very typical of supercharged petrol engines). So in the tall top gear, it's as happy at 80 as 120, because I'll bet that brackets where its' SFC curve bottoms out. Try driving it at 80 (or 120) a gear lower, and it'll still require that same amount of energy to move it along, but the engine won't be in its' happy zone. So relative inefficiency will be the outcome. Same in top gear at 30.

In my case, a workmate got talking about 5-cyl Volvos; he's had a couple, and swore to me that the best economy on his S80 was at about 90, on a run back to his native S.Yorks. He put it down to the long geared auto 'box, much like Pete's. I wasn't sure about this at all, as in fairness, it seems counter intuitive. But then I've done a number of bloody SAFED courses, and often turns out to be the case, that counter-intuitive is correct.

I put it to the test on a 543mi round trip, from here to the North East, to Wigan and home. Lotta motorway in that. Almost all of it tackled at 80 or more. I reckon it did 40mpg on that run, but thanks to scientific analysis (Fuelly), I can prove it did 35 overall for that tankful (which included 2 economy sapping runs from here to Larbert along shit A-roads) which is 5-6 better than usual. It would probly do even better if it was just me in the car, 'cos I wouldn't stop for as many breaks.

Why did it do this? 'Cos it was sitting nicely in the lower end of its' torquey goodness sweet band. Altho', trying to explain torque to a 2CV nut, is analogous to me explaining to my mum why Slayer are better than Richard Clayderman...

[PhilA]

Agreed. Coming from driving a 899cc 41hp/41lbf fiat which was happiest about 3000-3500 rpm (that was about 55-60 in fifth) to a 5328cc 270hp/315lbft which is happiest around 1500 rpm, it still makes a noticeable difference how I drive.

 

Admittedly the Cinq would routinely return 65mpg+ and the truck a heady 18mpg, they both suffer the same with driving style. The cinq would be flat out everywhere and the truck generally begins to sound strained over about 4k (80mph is about 1800 rpm so that's a bit academic)- makes me wonder what gains having an engine tuned for good torque at low RPM (truck will pull away happily at 600rpm, the Fiat wouldn't even idle below 900) versus a smaller engine which has to work hard.

 

somewhere along the line there seems to be a happy balance- the 3.5 in the wife's daily cruises at about 2200 rpm at 80 and on a 2600 mile round trip to Tennessee in May it averaged 31 US mpg (37 UK mpg) whilst taking on some mountains (3000'+) and freeway and city driving and a little bit of traffic jam. It's about Granada sized.. I'm going to try see what economy I get from the Renault- European figures for the 1.4 are about 45 UK mpg and people who own these things with the 2.0 in say, driven safely to mingle with modern day traffic it'll return easily 35-40 US mpg (42-48 UK mpg).

 

I'm going to try using Fuelly for it because it doesn't have a trip computer, and the diagnostics port doesn't have a speed output.

 

Be interesting to see how it does. I have a feeling the air con is going to impact the economy significantly. In the wife's daily it makes marginal impact, surprisingly.

 

--Phil

[wuvvum]

My old Cadillac STS was more economical at 70 than at 60 - I think it's quite a common thing in cars with with huge engines - the gearing tends to be so high that at bimbling speeds the engine hasn't really got into its stride and is therefore not operating at its most efficient level.

[Pete-M]

I think with big heavy stuff they're happier at 80 mph than 70 because at 80 they're in the happy zone where they're not fighting their way up hills, the engine is happily sitting in its torque curve so to go up a hill at 80 is less hassle for the engine than pulling up the hill at 70.

 

Some 2.0 Cavaliers used to be funny for it, GLi models mainly. In 5th at 70 they'd not quite pull happily up a long motorway incline, but at 75-80 the same hill wouldn't trouble the car at all. A car that's out of the happy zone trying to pull a car up a hill will need more loud pedal than one that is in the happy torque zone and just cruising.

 

P38a Range Rovers are another one. If you can keep 'em between 70-75 mph on the motorway you'll get 22-24 mpg from 'em. Do 65 and you'll be pushing 18 mpg as the gearbox will drop to 3rd more often than not....

[warren t claim]

I think with big heavy stuff they're happier at 80 mph than 70 because at 80 they're in the happy zone where they're not fighting their way up hills, the engine is happily sitting in its torque curve so to go up a hill at 80 is less hassle for the engine than pulling up the hill at 70.

 

Some 2.0 Cavaliers used to be funny for it, GLi models mainly. In 5th at 70 they'd not quite pull happily up a long motorway incline, but at 75-80 the same hill wouldn't trouble the car at all. A car that's out of the happy zone trying to pull a car up a hill will need more loud pedal than one that is in the happy torque zone and just cruising.

 

P38a Range Rovers are another one. If you can keep 'em between 70-75 mph on the motorway you'll get 22-24 mpg from 'em. Do 65 and you'll be pushing 18 mpg as the gearbox will drop to 3rd more often than not....

 

For some reason the Cav SRi was always more economical the the Cav GLi. Dunno why though.

[gtd2000]

 

Be interesting to see how it does. I have a feeling the air con is going to impact the economy significantly. In the wife's daily it makes marginal impact, surprisingly.

 

--Phil

 

I've not found that to be true - in some cases it might have lost me 1~2 MPG but on averages it makes no real difference.

 

Try driving with the windows open to save using the aircon and you'll use even more fuel above 30MPH due to drag.

[CreepingJesus]

Cav SRi had a CR 'box, GLi didn't. From memory, they evened out past the ton...

[Cavcraft]

Aye, certainly with the Mk2 1.8 they had a different, and far better, box than common or garden models. Unfortunately a lot of the tech talk on this thread has completely gone over my head, so any mpg figures for mine are just guess work or what the computer tells me:

 

The Merc (3.2 V6) tells me it'll do anywhere between 13-ish (tickover stuck in traffic) and a best of 31.9 which was oddly enough going over the Snake Pass in the Peak District. Whanging the cruise on on the motorway sees (alleged) late 20s no bother, and dropping it down a couple on the tiptronic box then wellying it can see low teens. The trouble is it seems it can take a while for the computer to catch up so mixed driving makes it go a bit mad.

 

The Honda (1.4i) seems bloody brilliant on juice, sitting at 75-80 in it's element and it's also quite 'punchy' in this range too. I'd think probably low-mid 30s in town and mid-late 30s on a run in top gear.

 

Son's 1.0 Corsa: Mid-late 30s round the doors, must be doing over 40 on a run. It's fantastic on fuel (in fact cheaper to run than the Audi 80 TDi was!) despite the fact you have to rev the bollocks off it to get anywhere.

 

Poohrot 1.9 n/a diesel 306: Bit early to say (only had it a day or two) but considering we lobbed a tenner in it to get it back from Buxton and I've been round the doors in it since the gauge has hardly moved. I think I'll brim the tank (missus has money off fuel coupon from a supermarket] and then top it up after 40 or 50 miles and see what the 'real world' figures are like.

[HMC]

my old n/a saab 900 would do about 30 mpg at 70, whereas the turbo s used to give 34-35, partly i suspected due to the higher top (2.7k at 70 rather than 3k) my old land rover 90 td5 (quite useless on the motorway) would do 27mpg at 70 and 22mpg at a flat out 82mph. My now departed w126 500sel at 2.2k revs with the cruise on at at 70 would deliver 28mpg, which i didnt think half bad. My best ever mpg was 55mpg from a 405 1.9td, the worst 9mpg from a rusty, mouldy and incontinent xj12

[Spiny Norman]

my old n/a saab 900 would do about 30 mpg at 70, whereas the turbo s used to give 34-35, partly i suspected due to the higher top (2.7k at 70 rather than 3k)

 

My 900 turbo sits at 80 doing about 3000rpm and feels far happier at that speed than it does at 65-70 for some reason, though I don't to enough long runs in it to get an accurate idea of mpg, all I know is it does about 30mpg in day-to-day running and a bit better if it's getting more motorway/dual carriageway runs. The "yippee - turbo" effect means that blasts on the twisties brings the fuel consumption down to less than it does in town.

[willswitchengage]

that counter-intuitive is correct.

 

Sadly however it just isn't true, sorry! Yes, engine's obviously do have a point where specific fuel consumption (L/KWh), but how much this varies is tiny compared to the enormous effect of wind resistance. Between 60mph and 90mph, aerodynamic drag increases by 125% but velocity only increases by 50% - the increase in effort is 2 1/2 times the increase in productivity. Sure, the engine may be moving closer towards its "sweet spot" for BSFC, but moving between say 2500 and 3750rpm will only change BSFC by about 20% max, which is obviously nothing compared to that vast increase in effort to overcome wind resistance. Just google images things like "bsfc rpm" to find out about where you'll use the less fuel over a wind range of load factors and engine speeds, but most importantly just look for "mpg speed" to see that all cars regardless of how many turbos they have, have mpg peaks betwen about 30 and 60.

 

Fuelly is interesting but I don't trust it to affect my future driving style, it's given me the most bizarre readings from my car and all I've really assimilated is that driving in town = bad. Probably the most efefctive way to see for yourself is just find a modern car with a trip computer and log down the instantaneous readings every 10mph or so on a quiet and flat stretch of motorway. Don't think to yourself, as I used to, OMG SUPER 98 GIVES ME 50MPG!!!11 when it's probably that that was 100% steady motorwaying where my previous rubbish 95s will have have involved giving fat people lifts around town journeys generally less than 1000yds (which I actually did do for about a year living with a lazy turd who loved to freeload from my reluctant philanthropy ).

[warren t claim]

that counter-intuitive is correct.

 

Sadly however it just isn't true, sorry! Yes, engine's obviously do have a point where specific fuel consumption (L/KWh), but how much this varies is tiny compared to the enormous effect of wind resistance. Between 60mph and 90mph, aerodynamic drag increases by 125% but velocity only increases by 50% - the increase in effort is 2 1/2 times the increase in productivity. Sure, the engine may be moving closer towards its "sweet spot" for BSFC, but moving between say 2500 and 3750rpm will only change BSFC by about 20% max, which is obviously nothing compared to that vast increase in effort to overcome wind resistance. Just google images things like "bsfc rpm" to find out about where you'll use the less fuel over a wind range of load factors and engine speeds, but most importantly just look for "mpg speed" to see that all cars regardless of how many turbos they have, have mpg peaks betwen about 30 and 60.

.