What uses more Fuel?

[transltr]

Me and a mate have been arguing the past few days as to what will use more fuel, driving for two hours at 80mph or driving at 40mph for two hours. I say the faster you go the more fuel you use, he disagrees.

Can anyone end our dispute?

[transltr]

Also, what would use more fuel if you were driving at 40 to anywhere, or 80 to anywhere, (ie where there is a time difference in getting to a place e.g. 1 hour going 80 but 2 hours going 40?)

[clancyblob]

Dunno why I'm trying to answer this as I'm totally rubbish at maths, but it intrigued me! Here goes!
Lets say the car at 40mph does 40 miles to the gallon. So in 2 hours it will use 2 gallons.
If it goes 80mph and still gets 40 to the gallon, it will cover 160 miles and so use 4 gallons in 2 hours.

You won't get twice as far for nothing!

What does anyone else reckon?

[d0gsb0dy]

I'm no expert, but as far as I know, on average, a speed of about 55 mph returns the best value as far as mpg go. Consumption increases with higher speeds. Another point is that, at higher speeds, tyre wear increases.

[boink]

Yes, 55 is the most economical speed to drive at. and to answer the question you will use more travelling at 80 mph. and to answer your second part of the question, in my experiance, i can travel around at about 60 mph @ 3000rpm, where as to go 80 id probably be doing around 4500 0r 5000 rpm which mathematically would mean the ration of rpm to speed is increasing therefore increasing fuel consumption.

That ration would change depending on your car and the load.

i think thats right?

[boink]

Btw ration should be ratio, i have no idea how i got that wrong twice!

[Agent-47]

So if you have a V8 range rover and a 1 litre corsa 55-56 is still economical?

[Catso]

To answer the Q, all other things being equal you would use more fuel by going faster. It takes more energy (& hence fuel) to keep the car going at 80, than 40, due to increased drag.

Obviously it would use more fuel, as you have you foot further down on the acelerator.

All this 55/56 mph stuff is rubbish. That's just the standard figure used for comparison pruposes.

The most fuel efficient speed depends on lots of things: the car, the engine, the gearbox, the wind speed and direction, the tyres, the road.

Assuming you stuck with the car you have, the next most important is the engine. The most efficient engine speed is the area where the torque is maximised. The road speed this equates to depends upon what gear you're in.

[kepla]

Yes it depends on the car, not just the engine but the aerodynamics of the car as well. Most petrol engines are at their most efficient at somewher between half and two-third of their maximum revs, so for your 'average' car this is going to be around 2500 to 3000 rpm. Depending on the gearing, this could be anywhere between 50 mph and 90 mph. So a small engine might only be pulling the car at about 50 - 55 mph at its most efficient rev range, but a big engine might be running the car at 80 -90 mph to hit its efficiency 'sweet spot'. The other factor is the aerodynamics of the car body. A low, sleek sports car is going to be much more 'slippery' than a square box type of contraption and thus have a lower drag factor. this will mean that a boxy-shaped car will loose fuel efficiency more as speed increases, and thus be economical at lower speeds.

[boink]

Hi, you guys are comparing all the 55 mph theory based on a straight road with no obsticles, and therefore saying about engines having a different peak spot, but traveling at 55 on most roads, not motorways etc, theres no reason to break, or atleast very rarely, therefore making less wastage of energy and making any car more economical at 55, or 56 as someone said.

[Catso]

No, I stick by what I said. Even on a motorway, or dual carriageway, with traffic, I would say that the optimum target speed would not necessarily be 55 or 56mph.

For my own car, being rather high and chunky, I think the optimum is probably 45-ish, 50 tops. Getting up towards 60, it uses noticeably more fuel. Over 70 it just frightens me the rate the needle drops!

[homer75]

Going back to the original question posed by transltr. The faster vehicle's engine will be running at double the rpm of the other one assuming the same gear ratio over the period of time. The faster vehicle's engine is also having to overcome an increased drag factor due to to air resistance. The distance covered is obviously doubled in the faster vehicle but the fuel used question is hardly rocket science ?

[Toureman]

Yes I can.

To answer your origonal question............Your friend is a plonker!, the faster you go the more fuel you waste.

[transltr]

yeh, he understood that the faster you go the more fuel you used, we just got a lil mis understood. My second question was an attempt to clear this up, he says that going at 40 from cardiff to swansea (just an example) would use less petrol then going at 80, as going at 40 the rpm is say 3000 and at 80 5000ish, so going at lower speed means u take longer to get where you're going therefore more revs overall. (eg 40 for 2 hours has less revs than going 80 for 1 hour) I disagreed, though it does make sense.

[kramer81]

I would have thought low to mid revs in 5th gear would be the most efficient. In a normal car I'd say 40mph would be more efficient than 80mph by a decent amount. A powerful car could be better at 80mph.