What is the most fuel efficient speed to drive at?

[luap]

I was really really really low on petrol. Warning light was warning me, gauge was off the gauge downward, 20 miles from home, nearest petrol station 15 miles away along motorway and large duel carriageway.
I drove really gently, very very slow acceleration, built up speed to about 45 mph and turned on the cruise control to avoid minor speed up / down.


If you drive at 15 mph you do xxx mpg but it takes yyy longer to get there, the engine is running longer therefore you use more fuel. If you drive at 75 mph you are travelling for a shorter time but you are not travelling at a speed/load/engine rev that is the most efficient. All cars will be different but I’m fascinated to know what is typically the most fuel efficient speed to drive at, assuming straight-ish roads, fairly level with minor inclines and declines .


Any ideas?

BTW. I did make it

 

[L_Plates]

0 mph

 

[Insane]

for some reason the saying is stuck in my head...

"fifty's thrifty"

but using the accelerator lightly works just as well.

or buy a diesel* car

(* even tho diesel now costs the same as petrol in most garages... )

 

[TedTheDog]

I wonder if a petrol delivery tanker has ever run out of fuel (they run on diesel after all).

Oh, I have no idea as to your questions, but wonder if the cruise control idea was the most efficient. Cruise control tends to rev going up hills and use engine braking going down. I'd guess the most efficient way to tackle hills would be to "take a run up" at them (by accelerating gently well in advance) and hope to coast up and then down the other side.

Damn it, you made me think. I can usually avoid that here.....

 

[Sibanac]

most efficient speed should be between 50 and 60 mph

This is basicly the speed your car goes in the highest gear, without giving much throtle,
the speed you change from 4th to 5th gear

since at that speed you have the lowest rpm per mph

 

[luap]

You might be right but wonder if a higher RPM is more efficient, most power is delivered around 3000-3500 rpm.

 

[xane]

I'd guess 55 mph because most advertised fuel consumption is calculated at that speed, so I'd presume manufacturers would respond by making the engine tuned that way, although I doubt that applies to larger capacity engines.

You should really carry a can of petrol in the back, even if it's a couple of litres.

The other thing I heard was it can damage the engine if you run out of fuel, as all the shit at the bottom of the tank starts going up the pipe.

 

[GDW]

I recall reading somewhere that most modern engines will be efficient at 57 mph, though I cant remember were. Apparently all cars are geared differently to acommodate weight engine power output, aerodynamics etc

 

[Any]

Im pretty sure that its 55mph.
Isnt that why the speed limit is 55 in the US?

 

[Insane]

In belfast, its more economical to get out and push

especially that westlink every day!

 

[GDW]

SHHHHHHHush.... we have horses and carts here remember. There might be Americans reading this and it might spoil their image of us

 

[Tom]

Think about it in the same way when you're riding a push bike. The best gear to be in is not necessarily the one that makes you do a greater or lower cadence (number of pedal revolutions per minute). It is the one that your body, and your fuel supply, can sustain for the longest period.

Its about 55mph in most cars, and that doesn't mean you should stay in top gear. If you're going up a steep incline, and your engine isn't powerful enough to keep up its speed in top gear, then you should change down, or you use more fuel (ie steep hill in 10th gear on your bike - no). Change back when you're on a flat section of road.

Or you could buy a car with a trip computer, like mine

 

[Swift^]

Depends on the car and engine...

but it's around 40-50. More than 50 is draining lots o'fuel.

 

[Deadmanwalking]

Originally posted by Any
Im pretty sure that its 55mph.
Isnt that why the speed limit is 55 in the US?

I dont know if you are right about the lmit.. but what you said implaies that

a) the yanks care about saving fuel

and

b) they THINK of something like that

 

[Any]

Its just something in the back of my mind, i dont know where from.
But wasnt there some sort of fuel shortage in the 70's? So the limits were set to the most efficient speed.
But this could be complete rubbish.

 

[Insane]

Originally posted by GDW
SHHHHHHHush.... we have horses and carts here remember. There might be Americans reading this and it might spoil their image of us

who do you think i was referring about when pushing???

and its 55 miles per Haybale

 

[Deadmanwalking]

I also have no idea, but Never pass up a chance to yank bash

 

[Deebs]

Originally posted by Swift^
Depends on the car and engine...

but it's around 40-50. More than 50 is draining lots o'fuel.

Spose I should stop doing 100mph up the M1 when I visit my sister then. Wondered why my fuel didn't last that long

 

[Deebs]

Originally posted by Tom

Or you could buy a car with a trip computer, like mine

Hehe, I had an Audi A4 1.8 Turbo Sport with one of them things. I had more fun seeing how low I could get the thing to go, you know, how many miles per gallon you can expect. Got it down to about 10 but then I was caning the shit out of the motor.

Best part was doing over 140 on the A21 down to Hastings the same day I got the car, forgot you had to run the thing in for the 1st thousand miles.

/me dons the wings of steel as the lurkers come out and slag me off for driving fast - go tell someone who might listen....

 

[Cdr]

Originally posted by DBs
Best part was doing over 140 on the A21 down to Hastings the same day I got the car, forgot you had to run the thing in for the 1st thousand miles.

fsr I thought you put you'd driving so fast you'd gone back before you bought the car, Back to the Future stylee (coffee has yet to kick in).

I remember reading some automotive magazine in which it says driving at the lower speeds but in a higher gear - I think they were quoted as saying something along the lines of 40mph in 5th gear will give you a better fuel consumption. But 55 generally sounds about right.

 

[throdgrain]

Its 56 mph. This is conjecture, its the answer. Thread closed .

 

[xane]

Every time I've been to America (Florida and Calif-arnie-a) its been 65mph limit.

 

[Any]

Yeah, a few years ago states were given the right to change maximum speed limits.

 

[Sibanac]

Originally posted by xane
Every time I've been to America (Florida and Calif-arnie-a) its been 65mph limit.

just came back from a road trip in the USA and it varies lowes it normal(no road works ect) limit on intersates and highways is 55mph highest i've seen was 75 mph

 

[Jonaldo]

Doesn't USA use kph?

 

[throdgrain]

No they don't.
Btw, I meant to say, this is not conjecture in my earler post !
56mph IS the correct speed to drive at to save petrol , according to petrol and car companys.

 

[evilmonkeh]

Originally posted by Jonaldo
Doesn't USA use kph?

doesnt mean that there isnt an equivilant speed in miles...

 

[Rubber Bullets]

Originally posted by xane

You should really carry a can of petrol in the back, even if it's a couple of litres.

The other thing I heard was it can damage the engine if you run out of fuel, as all the shit at the bottom of the tank starts going up the pipe.

The RAC and the like do recomend carrying spare petrol, at least on long journeys, but make sure it is in a suitable container. There are nasty safety implications in the event of a crash else.

As Xane says running out of petrol can be bad for the engine. It is potentially a lot worse for the catalytic converter, when you run out the engine starts to misfire and unburned fuel can go down the exhaust. This can kill the CAT and be very expensive.

RB

 

[leggy]

Originally posted by throdgrain
according to petrol

I find petrol to be over opinionated.

Anyway. Quoting one speed as the most suitable for every car is really quite comical though.

Surely torque, gear ratio, tyres, size of car all have a big effect on fuel consmption.

 

[CAC]

im pretty sure its 57 mph

allthought saying that in the sooty mobile im still convinced its 90 mph