Best MPG for cruising @ around 100-120mph
Discussion
McSam said:
The cube law gives you 53.6% greater power requirement, so it would be interesting to know exactly how much more fuel the engine would use doing that. Certainly not 53.6% more, but just how much more efficient (in terms of fuel input/power output, rather than fuel input/miles covered) does it get?
That starts getting into engine efficiency at different speed, but it's not going to be too far out.At 70-ish, it's not going to be at full throttle, so before it starts compressing the cylinder, it's not going to be full, and only a small amount of petrol will be injected.
I suppose the easiest way to think of it is half full with half vacuum in it.
At a steady state, ie not accelerating, the throttle will be at a position to cut off the air going in so the amount of fuel is proportional to the load it is under not the engine speed.
You can quite easily be using more fuel at 1500rpms than at 4000rpms depending on how much power the engine is producing.
Obviously air resistance will have a major influence on fuel use and the power required to get a car to a certain speed, but isn't one of the major players here engine rpm? If I am at 30 in 2nd, I will be using more fuel than at 70 in 5th.
So isn't it going to be the car with the most efficient engine (I.e less mechanical and energy loss) combined with the lowest rpm needed at that speed?
So isn't it going to be the car with the most efficient engine (I.e less mechanical and energy loss) combined with the lowest rpm needed at that speed?
Just to add something to think about - an engines effeciency varies with speed.
Example - my car (MG ZT Diesel, same base engine as a 320d) has loads of pull, slowly tailing off until it hits 3000 RPM. At this point, the performance improvement RAPIDLY tails off, but the fuel used is obviously quite linear.
Crusing at 1.2 leptons pushes it (from memory...) to ~3,500 RPM, and so obvously the economy suffers massively.
What you really want is something with low drag, a 6th gear and a large powerful diesel. A 330d, for example...
Example - my car (MG ZT Diesel, same base engine as a 320d) has loads of pull, slowly tailing off until it hits 3000 RPM. At this point, the performance improvement RAPIDLY tails off, but the fuel used is obviously quite linear.
Crusing at 1.2 leptons pushes it (from memory...) to ~3,500 RPM, and so obvously the economy suffers massively.
What you really want is something with low drag, a 6th gear and a large powerful diesel. A 330d, for example...
CypherP said:
The question I wanted to ask is why your 'friend' would even need to travel at that constant speed at night?
To use less fuel whilst saving time?I've averaged >120mph over long distances in Germany and it's getting to speed that guzzles the gas to a much greater extent than maintaining cruise. My car does about 18mpg @ 120mph.
MSTRBKR said:
Even between 70mph and 90mph there is a noticeable difference in fuel economy on every car out there. You can't get around that so I'd love to see the proof of your car doing only 1mpg less at 120mph than 70mph 
Practical experience, not theoretical sums, I am afraid.I've been driving across Germany for the last twenty-odd years,
completing hundreds of thousands of miles.
Various cars over the years. Even the 1600cc MR2 I had
would do 135 mph indicated and 37 mpg, so not too shabby.
The current BMW 530 will do anything from 25-29 mpg,
cruising from 100 mph to 150 mph, depending on how keen
I get with the loud pedal.
As any fule know, it's acceleration that kills mpg, not top speed.
And anyway, even if I am talking a load of old nonsense, how
come 40 million German drivers seem to use 120 mph as a
reasonable cruise speed ?
According your less than completely detailed theoretical
analysis of the situation, it would be economically stupid
for them to do so.
Proof by counter example. QED.
Still wanna argue that what I've seen in practice didn't happen ?
TuxRacer said:
H_Kan said:
Regularly doing those speeds in the UK is likely to result in a ban sooner rather then later.
Tried it have you? I'm fairly sure it's not very likely.I feel 120mph is often safely achievable for short stretches even at peak times on parts of my commute (M5).
Focus ST seems to cruise at ~22mpg at those speeds. It doesn't get much more than that with normal driving on A roads.
Edited by TuxRacer on Tuesday 14th September 09:13
NoelWatson said:
TheEnd said:
dcb said:
Various cars over the years. Even the 1600cc MR2 I had would do 135 mph indicated and 37 mpg, so not too shabby.
Still wanna argue that what I've seen in practice didn't happen ?
I'd like to argue that one please.Still wanna argue that what I've seen in practice didn't happen ?
dcb said:
MSTRBKR said:
Even between 70mph and 90mph there is a noticeable difference in fuel economy on every car out there. You can't get around that so I'd love to see the proof of your car doing only 1mpg less at 120mph than 70mph
Practical experience, not theoretical sums, I am afraid.I've been driving across Germany for the last twenty-odd years,
completing hundreds of thousands of miles.
Various cars over the years. Even the 1600cc MR2 I had
would do 135 mph indicated and 37 mpg, so not too shabby.
The current BMW 530 will do anything from 25-29 mpg,
cruising from 100 mph to 150 mph, depending on how keen
I get with the loud pedal.
As any fule know, it's acceleration that kills mpg, not top speed.
And anyway, even if I am talking a load of old nonsense, how
come 40 million German drivers seem to use 120 mph as a
reasonable cruise speed ?
According your less than completely detailed theoretical
analysis of the situation, it would be economically stupid
for them to do so.
Proof by counter example. QED.
Still wanna argue that what I've seen in practice didn't happen ?
When Clarkson went to Edinburgh and back on a tank of diesel did he travel at 120mph? No he travelled at 65mph because it's the most economical speed for the car, and most other cars are more economical at around that speed. When Audi drove from London to Rome on a single tank in an A4 2.0 Tdi did they travel at 120mph? No.
Are you saying that it doesn't matter how fast you go as long you accelerate to that speed slowly? Why is it that a Veyron can empty its 100L tank in 12.5 minutes at 250mph then? Should he accelerate a bit slower?
Also, 40 million German drivers probably don't give a monkeys about using more fuel and just want to get where they are going faster.
From practical experience the most economical speed for my car is an average of 70mph. Want to dispute what I've seen in practice?
MSTRBKR said:
Even between 70mph and 90mph there is a noticeable difference in fuel economy .......
From practical experience the most economical speed for my car is an average of 70mph. Want to dispute what I've seen in practice?
I don't think anyone can realistically suggest that their car is more economical at 120mph there does seem to be a sweetspot where a car is most efficient for example mine seems to be genuinely more economical at 90 than it is at 70 but then it is also very susceptible to the turbo boosting or not. equally if I floor the car at 100mph the MPG needle will drop to about 16mpg while accelerating as I start going past about 130ish it actually starts to creep up until as I mentioned earlier at 150ish with my foot flat on the floor it is showing 24 mpg it is not using any more fuel but is traveling fast enough to counterbalance.From practical experience the most economical speed for my car is an average of 70mph. Want to dispute what I've seen in practice?
Edited by andoverben on Tuesday 14th September 22:51
beanbag said:
zx10ben said:
y2blade said:
Trommel said:
zx10ben said:
I will cruise regularly in my Jag XFS between 115-125mph and achieve about 30mpg
I think that's fairly impressive.
I'd like to see what mine does at 120mph on cruise
160kph on cruise and thereafter I take control.....
.
Try holding cruise on at 150 mph plus.
Even for the thrill,
It's not that difficult.
andoverben said:
I don't think anyone can realistically suggest that their car is more economical at 120mph there does seem to be a sweetspot where a car is most efficient for example mine seems to be genuinely more economical at 90 than it is at 70 but then it is also very susceptible to the turbo boosting or not. equally if I floor the car at 100mph the MPG needle will drop to about 16mpg while accelerating as I start going past about 130ish it actually starts to creep up until as I mentioned earlier at 150ish with my foot flat on the floor it is showing 24 mpg it is not using any more fuel but is traveling fast enough to counterbalance.
Oh yeah, I understand accelerating will use more fuel and then the mpg will rise as you begin to maintain a constant speed. But as above I don't believe 120mph is economical speed for a BMW 530.Edited by andoverben on Tuesday 14th September 22:51
8400rpm said:
This thread is brilliant. 
The OP is quite clearly taking the piss out of an earlier thread, with a similar question that was running about the same time this one started.
A lot of people in here have gotten carried away.
I thought it was pretty bloody obvious as well but apparently over half a dozen pages say otherwise. The OP is quite clearly taking the piss out of an earlier thread, with a similar question that was running about the same time this one started.
A lot of people in here have gotten carried away.
Although I'm sceptical about a few people suggesting there doesn't need to be a speed limit on some motorways at night, sitting at over 100mph in the pitch black is begging for an incident with a foreign object, full beam or not you can't see that far ahead of you at that sort of speed.
Edited by Patrick Bateman on Tuesday 14th September 23:02
ksPatrick Bateman said:
8400rpm said:
This thread is brilliant.
The OP is quite clearly taking the piss out of an earlier thread, with a similar question that was running about the same time this one started.
A lot of people in here have gotten carried away.
I thought it was pretty bloody obvious as well but apparently over half a dozen pages say otherwise. The OP is quite clearly taking the piss out of an earlier thread, with a similar question that was running about the same time this one started.
A lot of people in here have gotten carried away.
Ooops..
Hook, line and sinker...
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