Driving 120kph/80mph on Motorway - Fuel Efficiency

friendface

Was driving down the motorway this evening and got to thinking about my car's fuel efficiency. I drive at 320d and in 5th gear at 120kph on the motorway, I get approximately 38-40mpg on average, whereas if I drop down to around 100kph, this improves to around 50mpg. I was thinking what saving I would make by driving at a lower speed every time I use the motorway.

Say, for example, if I drive 50 motorway miles every week;

Motorway Fuel Usage
@60mph (100kph) - 50mpg => 1 gal (4.54L) => Time: 0.83hrs
@80mph (120kph) - 40mpg => 1.25 gal (5.68L) => Time: 0.63hrs

Difference: 1.14L @ 120c/L => €1.37/week and time lost => 12mins







Then I thought screw the recession, screw the environment and screw fuel efficiency and shoved the foot down. :pac:

So, does anyone else drop their speed on the motorway or main roads to save fuel? I seem to come across a lot of cars doing around 55mph on the new motorways so thought that might be the reason behind it.

tuxy

I find a lot of micras doing 50 km in 100 zones. I doubt it has anything to do with trying to save fuel.

Anan1

I might if I thought I was going to run out.

Heroditas

friendface wrote: »

So, does anyone else drop their speed on the motorway or main roads to save fuel?

Not a chance! I'll always be at the speed limit where possible.

Funnily enough, I get my best fuel economy on the Dublin - Galway run rather than the Dublin - Cork or Dublin - Belfast runs.

I usually get 37-38mpg on the Galway run but it drops to 34-35mpg on the other two routes.

I know the other two routes aren't full motorways but even if there is no traffic on any of the routes, the Galway one still gives me the best fuel economy!

af_thefragile

I do tend to drive in 5th gear a lot and at around 80kmph in 100kmph zones.
Partly because it saves fuel. Partly because my car vibrates significantly over 100kmph... need to get that fixed sometime soon, probably needs a new front suspension.

But sometimes I say screw the recession and I like to put my foot down because I don't own a micra!
I'ld rather spend my money on petrol than on food and partys!

Anan1

In the words of George Best: I spent a lot of money on booze, birds and fast cars. The rest I just squandered

BrianD3

If fuel prices keep going up then I might slow down My car only gets around 34 mpg at a constant indicated 130 km/h.

I wonder what sort of fuel economy the new 8 speed auto BMW 5 series gives at a constant 120-130 km/h . Apparently the car reaches its top speed in 6th gear.

AFAIK economy figures at constant speeds are no longer given for cars. The old system gave fuel consumption at constant 56 and 75 mph, now an "extra urban" measurement which includes accleleration and gearchanging is used.

unkel

friendface wrote: »

Difference: 1.14L @ 120c/L => €1.37/week and time lost => 12mins

That values your time at less than the minimum wage. Driving slowly would make economic sense if you are unemployed. If you are employed, floor it!

hrcbob

BrianD3 wrote: »

If fuel prices keep going up then I might slow down My car only gets around 34 mpg at a constant indicated 130 km/h.

I wonder what sort of fuel economy the new 8 speed auto BMW 5 series gives at a constant 120-130 km/h . Apparently the car reaches its top speed in 6th gear.

AFAIK economy figures at constant speeds are no longer given for cars. The old system gave fuel consumption at constant 56 and 75 mph, now an "extra urban" measurement which includes accleleration and gearchanging is used.

Im sure it reaches top speed in a lower gear!! Havent all the germans got a "gentlemans agreement" regarding top speeds so they are limited to somethin silly???

Anan1

hrcbob wrote: »

Im sure it reaches top speed in a lower gear!! Havent all the germans got a "gentlemans agreement" regarding top speeds so they are limited to somethin silly???

Not all, but many are limited to 250km/h.

Fishtits

Way back when fishtits was studying, the crossover between aerodynamic efficiency and rolling friction (most efficient cruise) was ~56 MPH, most manufacturers (British) designed their cars to have max torque (efficiency) at that point in top gear.

I have no idea what the target is now but I'd imagine the principle is the same. Check your Torque curve.

BrianD3

hrcbob wrote: »

Im sure it reaches top speed in a lower gear!! Havent all the germans got a "gentlemans agreement" regarding top speeds so they are limited to somethin silly???

155 mph. For powerful cars that are limited to 155 mph but could go much faster if delimited then yeah, you could have a scenario where the car reaches 155 mph in 6th, 7th, and 8th.

But for less powerful cars you might get the car reaching 145 mph in 6th but 135mph in 8th. This would indicate that 8th is very tall and will give good economy at "normal" motorway speeds.

Fishtits

hrcbob wrote: »

Havent all the germans got a "gentlemans agreement" regarding top speeds so they are limited to somethin silly???

Limited to 155MPH

German insurance somehow creates a 193 bhp limit.

Mate of mine, VAG insider & motorport guy, tested a few VAG 193bhp models on a rolling road... lets just say that not one failed to make 193bhp. :rolleyes: I'd presume the same applies to all the other 193 BHP Germans...

Anan1

Fishtits wrote: »

German insurance somehow creates a 193 bhp limit.

There's no 193bhp overall limit, but it does equate to 140KW, which is a nice round number for insurance groupings.

DirkVoodoo

Heroditas wrote: »

I usually get 37-38mpg on the Galway run but it drops to 34-35mpg on the other two routes.

Ugh, I got about 32.7 mpg from Galway to Dublin on Monday afternoon at about 120-130 km/hr. I think the ridiculous headwind had a part in the poor return though.

Zubeneschamali

Heroditas wrote: »

I'll always be at the speed limit where possible

I do a long motorway commute, I'm hardly ever at the speed limit.

:pac:

[edit]Sorry, fuel efficiency: 35-40 mpg out of a 2.0 TDCi.

-Corkie-

I still get reasonably good economy and i do 140/160 on the motorway

R.O.R

I get better ecomony when there is a bit more traffic around - busy but moving, which keeps the speed on the motorway between 80-100km/h. If there is nothing around (like the last 2 weeks) it drops off when I'm doing 120km/h.

I had the Passat 1.6Tdi Bluemotion averaging 4.7 l/100km for my journeys home when it was kept to 100/110 - that's around 60 mpg. Coming in had a bit more traffic so some stop / start which brought economy up to 5.0 - which is still over 55mpg.

Berty

I had a Bioethanol Volvo S40 recently and I was very concious of fuel efficieny because they guzzle fuel.

I would normally drive at 80kph and up to 100kph on a motorway to conserve fuel.

When Im back in action with a Diesel I would see anything above 3000rpm to be guzzling fuel and anything below to be adequate subject to using the correct gear of course.

My current car, 04 Nissan Micra Petrol hits 3000rpm at 80kph so thank god the car is light so its not so bad on fuel.

EPM

120 km/h equates to over 3500 rpm in my car...losing battle going over 100 to be honest.

Cork to Galway I can easily get 6.5l/100km (c.44 mpg) if I switch off the air con and averages 80-100km/k. When I used to do Cork to Clonmelhell I'd be doing well to get that below 9l/100km (c31 mpg) because of the motorway (120-130km/h). If I carried onto Dublin it was worse again:rolleyes:

cormie

ROR, do you notice many of your vans drop efficiency? My 02 Transit which I bough with 70K on the clock used to get about 400-450 miles from a full tank, the last time I checked though it seemed to be getting between the 350-400 mile mark, that with the increase in fuel prices isn't too good

On topic, I'd always keep at the limit too and wouldn't worry too much about the extra I'd save going slower. You never know what delays you could avoid in that extra bit of time you save going that bit faster. If you go easy, you may not catch up with the ultra slow person who'll delay you by a LOT more when the Motorway becomes single carriage/country road. Also, if you're paying someone by the hour it kinda balances out, quicker the journey is, the less you have to pay

R.O.R

cormie wrote: »

ROR, do you notice many of your vans drop efficiency?

Sorry - don't look after the fuel for any of the vans so don't know their consumption.

keithclancy

I drive based on the traffic pattern,

I know at a certain time there'll be a god almighty traffic queue off the motorway i'll slow down to 110 or 100.

I'll just be sitting in the queue up the road anyway.

kdouglas

Definitely notice a drop in efficiency at >100km/h, most efficient speed seems to be around the 95-100km/h. This is in a new model Mondeo 1.8Tdci with 5 Speed gearbox.

Had a 2.0Tdci 6 speed '04 Mondeo previous to this and got much better fuel efficiency out of that at motorway speed.

Seweryn

kdouglas wrote: »

most efficient speed seems to be around the 95-100km/h.

Not exactly. For almost any car (with an internal combustion engine I should say) the most economical speed is in around 60 km/h (or 40 mph) at most suitable gear (4th or 5th).
For an electric car in general - the lower the speed the more economical the car is.