Best MPG for cruising @ around 100-120mph

[anonymous-user]

TheEnd said:

Patrick 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.

Really?
Ooops..

Hook, line and sinker...

Woops

[CraigyMc]

MSTRBKR said:

Sorry, I don't believe you. Your fuel consumption at 120mph WILL be higher than at 70mph, no doubt about it. True, acceleration does kill MPG but so does top speed because the drag on your car is higher at that speed. No practical experience required, that's theoretical fact.

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?

Thanks for writing this. I was going to get into that discussion but I remembered an old quote:
"Never argue with an idiot, they will drag you down to their level, then beat you with experience"
C

[bobt]

andoverben said:

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.

Edited by andoverben on Tuesday 14th September 22:51

What he said. Or close at least. Engine efficiency or (specific brake fuel consumption) varies with load and speed, and if you can find the min SBFC then that is your ideal cruising speed (ignoring speed limits)

[GingerWizard]

anonymous said:

[redacted]

toxicnerve... got sold on that one did'nt you, stop being so small minded and governed by "the man" she could live any where in the world.....

[NoelWatson]

bobt said:

andoverben said:

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.

Edited by andoverben on Tuesday 14th September 22:51

What he said. Or close at least. Engine efficiency or (specific brake fuel consumption) varies with load and speed, and if you can find the min SBFC then that is your ideal cruising speed (ignoring speed limits)

Does this take into account aerodynamic drag?

[NoelWatson]

andoverben said:

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.

Edited by andoverben on Tuesday 14th September 22:51

150mph is 2.5 mile a minute, so a gallon used every ~10 mins. I would estimate that travelling at 150 requires around 150-200bhp - say 175bhp. This equates to 130550J, which is 7.83*10^7J - a gallon of diesel is around 18.5*10^7J, which makes your drivetrain over 40% efficient. I think my calcs must be wrong!

[jokeruk]

Just thought i'd mention that (in the past) i've beaten you all on the whole 55-60mph consumption. Over 200 miles, my VW Boring TDI (90bhp version) managed 87 point something mpg

I'll get my coat

Leigh

[CraigyMc]

NoelWatson said:

andoverben said:

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.

Edited by andoverben on Tuesday 14th September 22:51

150mph is 2.5 mile a minute, so a gallon used every ~10 mins. I would estimate that travelling at 150 requires around 150-200bhp - say 175bhp. This equates to 130550J, which is 7.83*10^7J - a gallon of diesel is around 18.5*10^7J, which makes your drivetrain over 40% efficient. I think my calcs must be wrong!

For the record, my TT probably can't quite manage 150mph (maybe over 145mph though) and it has 197bhp.
C

[McSam]

NoelWatson said:

andoverben said:

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.

Edited by andoverben on Tuesday 14th September 22:51

150mph is 2.5 mile a minute, so a gallon used every ~10 mins. I would estimate that travelling at 150 requires around 150-200bhp - say 175bhp. This equates to 130550J, which is 7.83*10^7J - a gallon of diesel is around 18.5*10^7J, which makes your drivetrain over 40% efficient. I think my calcs must be wrong!

No, your numbers look about right - I would expect the diesel engine itself to be 35-40% efficient at its best. Taking drivetrain losses into account, the 42% efficiency your calculations give looks too good to be true, but bear in mind that the car was "showing" 24mpg, which I presume means from trip computer. Not unreasonable for that to be 10-15% wrong, so say it's 22mpg, and we get a gallon every 8.8 minutes, 6.89*10^7J, 37.2% efficient. Throw away three per cent for drivetrain inefficiency (that's 8% of the power the engine actually develops, which sounds reasonable?) and we have 40%, about what you could expect from a good modern diesel engine

So, astonishingly, it seems the car actually could do about 22mpg at 150mph, assuming it can maintain that with 175bhp.

[allgonepetetong]

anonymous said:

[redacted]

While Pistonheads, I am sure does not condeone speeding, I think you might be on the wrong website.

[frosted]

Actually all engines are most frugal at 40 mph . Doesn't matter if you drive a Clio or a momaro

[Trommel]

frosted said:

Actually all engines are most frugal at 40 mph . Doesn't matter if you drive a Clio or a momaro

[TuxRacer]

frosted said:

Actually all engines are most frugal at 0 mph . Doesn't matter if you drive a Clio or a momaro

EFA. Though I've no idea what a momaro is.

[McSam]

inspects 0mph claim for deliberate failage

Edited by McSam on Wednesday 15th September 14:15

[CraigyMc]

CraigyMc said:

...I was reading the other day about the top speed of the porsche 917 at le mans in the 70's. Porsche had a problem where it was designed for 250mph, and it would only pull to 243mph or something (I forget the exact number offhand). The German engineers were not pleased.

It turned out that the tyres were expanding at that speed (centripetal force), which meant the car was very slightly taller than it was expected/supposed to be (more draggy due to the height), and very slightly longer geared (due to the bigger circumference).

Lesson:
Anything with slippery aero at 30mph *might* be just as good at 100mph or more, but if it generates a lot of lift, it might actually get less slippery due to a taller ride height. That depends on the lift characteristics.

C

bobt said:

I reckon that even 40 years ag othe Porsche engineers would be well aware that a tyre would increase in diameter at 250 mph.

I'm pretty sure I read this in "an unfair advantage" (the book by Mark Donohue, who was central in the development of the 917). I'm not at home this week to be able to look it up to check for you. It's obvious when you think about it, but in 1971 they really went a LOT faster than they had been doing up until that year.

C

[frosted]

TuxRacer said:

frosted said:

Actually all engines are most frugal at 0 mph . Doesn't matter if you drive a Clio or a momaro

EFA. Though I've no idea what a momaro is.

The bread and butter of ph , if you drive a momaroo you can travel at 150 mph in 6 th gear and only do 900 rpm

[frosted]

TuxRacer said:

frosted said:

Actually all engines are most frugal at 0 mph . Doesn't matter if you drive a Clio or a momaro

EFA. Though I've no idea what a momaro is.

The bread and butter of ph , if you drive a momaroo you can travel at 150 mph in 6 th gear and only do 900 rpm

Iirc , 70 kph is

[busta]

MSTRBKR said:

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.

Edited by andoverben on Tuesday 14th September 22:51

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.

Nothing to do with rate of acceleration as the car is still accelerating and the load on the engine is still increasing. MPG is rising because the engine is using a constant amount of fuel at WOT from 130mph onwards, but the speed and therefore milage covered is increasing.

E.g gallons/minute stays the same, miles/minute increases. Therefore miles/gallon increases.

[NoelWatson]

busta said:

MSTRBKR said:

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.

Edited by andoverben on Tuesday 14th September 22:51

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.

Nothing to do with rate of acceleration as the car is still accelerating and the load on the engine is still increasing. MPG is rising because the engine is using a constant amount of fuel at WOT from 130mph onwards, but the speed and therefore milage covered is increasing.

E.g gallons/minute stays the same, miles/minute increases. Therefore miles/gallon increases.

On a treadmill, or the real world?

[busta]

NoelWatson said:

busta said:

MSTRBKR said:

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.

Edited by andoverben on Tuesday 14th September 22:51

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.

Nothing to do with rate of acceleration as the car is still accelerating and the load on the engine is still increasing. MPG is rising because the engine is using a constant amount of fuel at WOT from 130mph onwards, but the speed and therefore milage covered is increasing.

E.g gallons/minute stays the same, miles/minute increases. Therefore miles/gallon increases.

On a treadmill, or the real world?

Treadmill, going in the opposite direction, with indestructible wheel bearings.