Yas89

Hey guys, got an interesting question. What is better for mpg, and by how much would you say?: driving for 90 minutes at 75 mph, or driving for 80 minutes at say 85 mph?

RichieST

iTrader: (2)

75mph. Have noticed on my trip computer, stick at bang on 70 mph and i'm doing 30+mpg, at 90, its mid 20s!

nugnah

alot of things can alter MPG, firstly what car, diesel or petrol , stereo , heaters , air con, on or off , behind vehicle or in front etc.
My mate has A4 TDI 130 sport chipped to 185 and at 80 he still gets 50+ mpg

jason

Yas89

so even though by driving faster you are travelling LESS time, the lower speed should still make it cheaper?

Im wondering coz Ive got a focus 1.8tdci and its claimed mpg is 50. Im trying my best to get as close to this as possible. I really wish my car had 6 gears!

steves24V's

iTrader: (7)

what year is you focus and what have you managed to get ot of it ours gets 43 mph (08 plate) how every its driven.

Benni

60mph in 5th will be the most effecient I would say from when I owned a diesel. Keep a steady speed and look well ahead so you can change lanes without having to loose speed. I've tucked in well behind a lorry before today too, when I was low on fuel which I'm sure helped but it wasn't a nice experience...

Benni.

Yas89

2002 focus

This is the first time Im actually calculating lol, but basically Ive done 200 miles or so in half a tank, with half those miles from the motorway and half from city driving. From my dodgy calculations, this equates to about 36mpg (i think)...which is shite!!! Ill wait though till the tank nears empty for a more accurate assessment. i have an aftermarket stereo however, which im usually blasting for the duration of my motorway driving, which could be affecting things

RichieST

iTrader: (2)

Time taken is irrelevant, as its MILES per GALLON, not hours per gallon.

RichieST

iTrader: (2)

Hello benni you old goat! Not seen you on here for a while!

phil_focus

if you think along the lines of i'm running out of fuel so i'll boot it at 100mph to the petrol station you are going to run out sooner.

my dad found from his 2l tdci mondo that about 55-60mph was best and he got over 60mpg but i think thats due to it having 6 gears. the focus isnt as good, i know my mum's got a 1.8 tdci focus mk1.5

Yas89

haha

good point man

just drove around 100 miles today, losing less than a quarter of a tank, so prob 40mpg! I was driving at pretty much 70-75, which aint heart stoppingly fast, but its a cruising speed

edit: no im not just driving hundreds of miles just to test the mpg lolol

Christian and Beccy

iTrader: (14)

Our 1.4 16v Clio is at its most efficient at around 50mph. If you sit at 70-80mph, it's hard to get it above 32mpg.

Oddly, the Megane can manage 32mpg at 80mph.

So, in real life, it's no worse on fuel really than the Clio!

st3v3

the car companys usually quote a car doing x amount of miles at xxmph this is usally not too far away from 54mph.

But then as im sure you all know mpg is all about rpm the less rpm you can do in 5th gear will be better, unless youre putting unnesasary (sp) strain on the engine, so doing 1000rpm in 5th gear could probably give worse mpg than at 2000 rpm in 5th gear,

S1

The most efficient speed is as fast as the car will go before wind resistance comes in to play. After that the faster you go more energy is used to push an ever-increasing amount of air as well as the car. It's usually between 50-60mph that it's most efficient.

jammerrs

a true 56mph is what companys test their cars at i beleive.

tuck right up behind a lorry, windows up, no aircon, and no electricals on for maximum mpg

i wonder how much taping up the front of the car would effect mpg?

dojj

iTrader: (3)

did a test the other day and the best economy i get, with the cruise on a flat bit of motorway, was at 62 mph

i tried it at various levels between 50 and 70 mph and that was the best, i did have 270 motorway miles to do though so i did 20 miles at various speeds to get a build up, but at certain speeds the trip computer was dropping fairly quickly, and at others it was going up fairly quickly too

that said, the trip was about mpg out on the way there and about 1 mpg out on the way back, both times it was worse on the computer than my brim it till it's full method

st3v3

And thats the only way to be sure of what you will be getting in mpg

iansoutham

Along with the above, it also depends on what the gearing is on your car.

My 4be is happiest sitting around 55-65mph where it will return around 40mpg. If I drive it around 70-75mph, it will average around 33mpg. 80mph+ and you are into the mid-20s.

However, my cabriolet on its twin-choke Weber will return about 35-40mpg around 70-75mph (as long as the second choke is not on when it returns about 15mpg regardless of speed and the roof is not down) but will return around 20mpg @ 55mph.

The difference is that the 4be is doing 3k rpm @ 70mph, but the cab is doing 2,200rpm @ 70mph due to the gearing of the diff. If I sat the cabby @ 55mph, I would have to either change down or use the throttle more every time I came to an incline, which in turn uses more fuel compared to sitting @ a higher speed where you do not need to drop a gear.

If you take a Sierra DOHC and do side-by-side comparisons (literally 2 cars next to each other) with a 3.92 diff and a 3.62 diff, the 3.62 diff will be doing 5mpg better @ 70mph, but the 3.92 will be better around 55-60mph.

Something my father told me years ago as well was that if you can drive around 500rpm below max torque, you should be getting the best fuel economy. Seems to work for me.

MAD Ade

iTrader: (1)

DazS1Turbo

How does heaters and radio use up mpg??? Doesn't that run off the battery which in turn is charged at a constant rate via the alternator??

Educate me!

furry

Manufacters used to base there figures at 58mph, they may well still do.

My octavia vrs sits at around 55mpg at 55mph on the motorway.
I'm gunna try and do Swindon to nurburg in may on 1 tank as it usally does 380 miles to a combined tank and tom tom is telling me 426 mile journey, gunna be close!!

S1

The alternator requires energy to drive it. If you've ever had an alternator belt off, switched on the ignition and tried to turn the aternator pulley, you willl find it difficult if not impossible to turn. The more current you draw by switching on heaters and radios then the harder it is to turn, which as it's turned by the engine increases fuel comsumption.

dojj

iTrader: (3)

with the twinkly, i came back from jims to mine, ok, so it was late and all that, slipstreaming a lorry all the way home and, after about 130 miels, i only put in 8 litres of fuel
no one raelly accepted that it did so well, other than the week later when i went back to jims after filling up at my house and he said he'd top me up when it was all done and dusted
480 miles later, albeit on the motorway for all but about 10 miles, we brimed it with 38 litres, and this was traveling at about 75 mph most of the day other than picking jim up, then getting the mk1 4x4, then going back to jims

but i did have a 3.14:1 diff in the twinkly, and it surprised jims twinky, with it's go faster bits and 20 brake more power and 20 lb/ft more torque, along the start finish straight at silverstone when i went from 2nd to 3rd to pass him, while he went from 2nd to 3rd to 4th

electrical drain from the heaters tends to be a fair amount, but it's also cooling down the coolant, and engines work more efficiently the hotter they are
the radio won't make a lot of difference really, unless you have big subs and shit in the boot

test indicate that doing the same journey with the windows down uses up more fuel in the form of drag that doing that trip with the windows up but the air con running

Yas89

maybe driving behind lorries is the future.....

DMR

might aswell get ya tow rope out won't use any fuel then

Yas89

lol i might as well not even take the car - ill just get the lorry driver to give me a ride

steves24V's

iTrader: (7)

I get better mph out of my range rover td at 56 mph towing a 2 ton (ish) trailer than driving at 80mph

this prop doesn't effect any one but me but there you go.

BlairM

Sure I've heard that 56mph is the optimum speed for best MPG, as wind resistance squares with increases in speed so that is the best point.

My Fez TDCi normally manages anywhere between 50 and 55mpg. Got over 55 last tank due to doing 320 miles in motorway journeys over the weekend. At 70mph it's doing 2600rpm, I wish it had a 6th gear to drop down to nearer 2000, at a guess I'd say it could be 2200rpm.

Is it right the RPM you get peak torque at is the best for efficiency as well? Think thats like 1800rpm in the Fez.

S3an

i have a cdti 150 astra and its only more economical in 6th gear if your travelling above 85mph on a motorway. the engine labours far to much below 60 in 6th. no way on this earth its economical unless your travelling above 85 and on a motorway. get about 45 mpg pottering about 50+ on a decent journey. my e46 M3 averaged 34 mpg+ on the trip to germany and back.

cjwood555

The amount of energy used (Work done) = Engine effort (Force) x journey length (distance) - time doesn't come into it directly.

The faster you go the more drag there is, so the greater the Force required to maintain speed. Internal combustion engines are buggers though so, for example in my mondy st24, 75mph = 2800rpm(ish), below secondary-opening point. 80mph = 8100rpm(ish), above secondary opening point. Unless you know the VE of the engine at the cruising speed you're interested in, it's anyones guess (or a trip computer can tell you, of course)

Chris

DazS1Turbo

Fuck me im going to be sitting in silence and freezing my tits off in the car from now on then ha ha

RichieST

iTrader: (2)

If I press the max demist button in my car, the heated front and rear screens come on, the heaters go to full blast and red hot, and the air con comes on, all at the same time. Can certainly hear the engine bog down then come back up to idle speed quite dramatically, so it must be putting a huge load on the alternator to almost stop the engine!

S1

One of my many Mini's (Classic shape not BMW) had four spot lights with 110W bulbs plus headlights were 110W as well. If I put all my lights on at top speed it used to knock about 5mph of the speed of the car!

timrud

I never knew that as the aternator is working harder its tougher to turn. Learn something new everyday

Chip

iTrader: (3)

Fuel gets used for a variety of reasons when driving, the main ones are:
overcoming air resistance
moving weight up gradients
overcoming the resistance of tyres


The 1st one is massively effected by speed, and for that reason generally the slower you go the less fuel you will use, within the confines of where your engine is efficiently geared for of course.

Most cars at 40mph in top gear will return very good economy figures.

In your examples, the slower one would be better for 99.9% of cars at least.

Chip

iTrader: (3)

The law of conservation of energy dictates that has to be the case.

timrud

I never listened in science!

jhot80

The quality of the fuel you fill up with can have a affect on the MPG. Plus the smallest side wind will kill fuel economy.

Turbosystems

iTrader: (5)

my poous has ranged from 40 mpg average per tank full to 69 mpg on a long motorway run

timrud

Would of thought it would of done way more than 40mpg Tony on a tank. Were you constantly late for work?