Rikki 95WRX

* I always presumed it would be better at lower speeds and used to cruise at around 60 when I was trying to be efficient.

* A while back I saw a number of posts from people saying that there's is more efficient cruising at about 70, even 80.

* I have also seen people say that lower speeds is better - 60ish.

* What are your views? Any consumption stats to back it up? Reasons why?

Rikki

PS Mods if you want to move this to a forum you feel more appropriate please do. I thought it may be OK for general?

Jolly Green Monster 2

Well there was research I believe and that 56mph is said to be the most efficent speed. But this was a general figure and not specifically for a particular car. I believe this is why alot of car brochours used to quote MPG at 56mph as well as in town etc..

The only time I have cruised at 60mph was when towing and it was more efficent than 70 + a number but less than when I have cruised at 70... but then I was towing so that obviously makes the comparison unfair.

Try it and see?

JGM

Mo

At a constant 60ish I get 34mpg - MY94 with 136,000 miles on it and a few mods

cockburnij

I was quite taken back last week as i managed to get approx. 349 miles out of a full tand. This was driving to Wales on the motorway and A/B roads. I didn't really go over 70-75mph on the M4 and needle hardly moved. Once at the destination I still had just under 3/4 of a tank.
However coming home averaging 90mph I managed to use 3/4 of a tank which was more like it.

mik

The "56mph" and "75mph" that are always quoted are simply 90 and 120kph ~ speeds that "someone" decided would be used for std consumption tests.

Every car will give better results at 56mph, which leads loads of folks to assume that "56mph is the most economical speed".

Surprisingly ~ it isn't.....how could that be true for every car?

Best speed for economy does depend to an extent on speed - since aerodynamic drag increases with speed-squared. Going twice as fast leads to 4x the aerodynamic drag, hence requires 4x the power to hold you at this speed.

(note: ignored drag from friction there - which is pretty linear).

Moving faster requires more power, but allows you to use higher gears ~ hence 50mph in top may be as economic as 5mph in 1st.

The "best" speed is largely goverened by your engine. I used to think that the optimum speed was that at which max torque is generated ('cos at this speed the engine is giving max "punch" for the fuel used).

In reality it's more complex, since you don't need max throttle to cruise at steady speed - so you're looking for an engine speed which gives efficient part throttle fuel use.

Which could be anywhere!...and also means that 60mph may be best on the flat, with no wind......but 65mph may be more efficient on a slight incline....with a headwind of 4.7342 mph.

My car has a fuel computer with instantaneous mpg, and even then I can't come up with "the best speed".

It's somewhere in the 50-70 zone. ish. probably.

Jay m A

iTrader: (2)

Rikki.

For your particular model (MY95 WRX) your ECU assumes WOT (read runs rich) at positive boost or any rpm over 3800, whichever comes first. So if you cruise at less than 3800 rpm in top your fuel economy will be much better than cruising at over 3800 rpm!

3800 rpm in top is around 80-85ish. whether you'll be more economical at 60 as opposed to 70 I have no idea

Justin

Sith

Jay,

Could explain my rather poor economy for Saturday......

P.

Rikki 95WRX

Thanks guys - interesting reading

anyone else?

droid42

Going twice as fast leads to 4x the aerodynamic drag, hence requires 4x the power to hold you at this speed.

You're right that going twice as fast leads to 4x the drag but you then need 8x the power to hold this speed.

(Drag is proportional to speed-squared, Power is proportional to drag times speed i.e. speed-cubed)

Ian.

Rikki 95WRX

so wouldn't that suggest that the slower you can go in any given gear would be most efficient?

Shimster

With my Legacy GTB, the first turbo starts cutting in a 2000rpm. This means that the ECU is virtually always running on the rich side and I'm always getting crap mpg.
From my last std turbo scooby, I found if I was crusing at a rpm so the turbo was not cutting in under gentle acceleration in 5th gear this gave the best compromise between not taking ages to get there and reasonable mpg.
ALEX

naturbo2000

Best economy I ever had was going to the Big One.

And I didn't get it out of 3rd all day !!!

( Sport - trying to keep up with turbos )

Only time I've seen the far side of 25mpg

Adam M

accoring to figures my clients (motor industry giants) use to map their cars and design their gearboxes, the most efficient speed although entirely car specific tends to be between 40 and 50mph.

It tails off very rapidly above 60.

mik

Droid,

From memory of my aerodynamics lectures...quite a while ago now(!)

Power (required) = 1/2(p*v(squared)*CdA)
where:-
p=air density
v= speed
CdA = drag coef x frontal area.

Power required is therefore directly proportional to speed-squared, so doubling speed does require 4x the power.

Like I said originaly, this doesn't take account of frictional drag which is pretty much directly proportional to speed.

Apologies if I sound like "Mr Logic"

droid42

Power (required) = 1/2(p*v(squared)*CdA)
where:-
p=air density
v= speed
CdA = drag coef x frontal area.

Power required is therefore directly proportional to speed-squared, so doubling speed does require 4x the power.

No, as I said in my first post, the formula you quoted above is for aerodynamic drag, not "power required".

Power is equivalent to force * velocity and, in this case, "force" is aerodynamic drag. So, if drag is proportional to speed-squared, then power is proportional to speed-cubed. Trust me, I've written a straight-line performance simulator myself so I know what I'm talking about.

Think about it another way ...

An average-shaped family saloon needs around 60-70bhp to get to 100mph. By your calculations, the same car would only need around 250bhp to reach 200mph (Hmmm). By my calculations, that car would need more like 480-500bhp to reach 200mph.

How many cars with around 250bhp do you know can reach 200mph? Of the cars you know that can reach 200mph, what kind of power outputs do their engines produce?



Ian.

BaldyMan

Drive to london - late friday night - no one on roads - 80-90 - gettin near 400 out of the tank

Usually get 330 ish

droid42

I've also managed to dig out a link in case you're still not convinced ...

Click here for an explanation of drag and power

Ian.

sempers

staying off boost makes a big improvement to the fuel economy.. as does keeping a steady speed.

Have on occasion borrowed various saabs owned by my father:
'fully blown' turbo - lag-bang variety, nothing below 3000. 36mpg
'ecopower' ****e(light) pressure turbo - boosts really early, but not a lot 32mpg
my old non-turbo 33-35mpg. Both eco & full have similar gear ratios, n/a is much shorter.

Old one was great - subject to a short wait, power was available in spades, but you cruised off boost. Big garret turbo.

Anyway, getting on topic again, I find the scoob starts to really suck fuel at about 3000rpm.

- Mark.

mik

Droid,

<munch munch>
(noise of eating ones hat)

I did say my memory was ropey (.....I think..did I?)

You have however managed to sound more like Mr Logic than I

Cheers, Mik

droid42

You have however managed to sound more like Mr Logic than I

Indeed Luckily I don't look like him. Much worse actually ...

Ian.

Nick L

400 to a tank - average 330
What have you got.........a metro.

i think the best i have ever got is 300 and the average is 240. Reckon I got a leak by your figures!!

Vate

best to stick to constant speed.
Steady 80 will be better than 60, 70 80 in spurts.
Most manufacturers print something like 10-15% pooer fuel econ @ 80 than 70. Obv. quite general but a reasonable guidline.

MattN

no no no

go as fast as possible you will use les fuel.

If you go at 60 mph it will take an hour to go 60 miles but go at 120 and you'll be there in half the time so if your car isn't in use as much!!!!!!