Optimum Cruising Speed?

[Guest MATT]

hi,

 

after replacing the MAF sensor not so long ago, we have already noticed an increase of 6mpg round town.

 

now what i want to know - what is the optimum speed, on a 2001 ford galaxy, 5 speed auto 1.9TDI?

 

i did a search, and found only one topic, which didn't really come to an answer

 

anyone know?

 

MATT

[Grumpy]

Matt

 

What was your MPG before replacing the MAF sensor?

[seatkid]

Least fuel is used at 0 mph with engine off :D

[NikpV]

but not most mpg :D

[Guest MATT]

What was your MPG before replacing the MAF sensor?

before i got 30mpg round town.

new MAF sensor - and now I'm getting 36mpg - but that includes some caravan towing, air con, and the occasional use of sport mode.

 

if it was fully solo, and full economy, then i would think about 38mpg?

 

have yet to try a run the motorway, hence this topic.

 

MATT

[Guest MATT]

I'm guessing 65 mph ish is about right?

 

60 seems a bit slow, and 70 seems a bit fast (all in terms of mpg)

 

anyone agree?

 

MATT

[NikpV]

Just replaced our MAf on a run of about 7 miles - A6 50-60mph mpg has jumped from 37 to about 43, at motorway speeds its made less difference mind you the only run I've done is along the a14 and was plagued by the usual hgv syndrome of lorries waiting til I've pulled out to overtake when they decide that they can overtake the hgv ahead - just maybe - so slow down for 3 miles inching past tanker, then accelerating past until we get to next pair of hgv's - it happened about 5 times over the 30 mile stretch so it was difficult to stay on cruise.

[NikpV]

I'm guessing 65 mph ish is about right?

 

60 seems a bit slow, and 70 seems a bit fast (all in terms of mpg)

 

anyone agree?

 

MATT

atm - finding that 55 -60 is best but having trouble testing at 70mph see post above

[Davetheref]

I have a run down the A10 to Cheshunt every morning. I have tried all different speeds to get the optium and I have found around 55ish to be best but I only have a manual TDI

[Guest MATT]

mmmmmm, thanks for the input guys.

 

55mpg (ish) does seem to keep popping up.

 

i will have to experiment a bit (or try to)

 

MATT :D

[mumof4]

does dave not mean 55mph not 55 mpg???

 

thats the way i read it anyway!!

[Guest MATT]

lol

 

yeah ok - i got mpgs on my mind lol

 

55mph

 

MATT

[mumof4]

In my trusty handbook...it says..(auto 115)

 

1.9 TDI fuel consumption=28.5 urban

=43.5 non urban

=36.2 total

 

 

1.9TDI manual 115 =32.8 urban

=51.4 non urban

=42.8 total

[Davetheref]

Sorry, I did mean 55 mph and I cant even spell optium, optinmum, optinnum. Oh Ba***cks, you know what I mean :D :D

[mumof4]

ooohh..optimum....

 

 

:D like the time of posting!!!..couldnt do that agin if you tried!! :D

[Ogben Schmutzel]

I always precede these sort of things with

[Guest MATT jr]

1) being in the appropriate grea (ie top generally) and 2) being at the revs that yield the most power!

agree with 1st statement, but the 2nd, mmmmmm, most power is produced at 4000rpms isn't it? and most power, as power comes from fuel - must be consuming most fuel, when giving max power.

 

so would the opposite of this be where least power is given?

 

MJR

[r1ch4rd]

I think you will find that maximum efficiency is in and around the rev range of maximum torque ie on a tdi 1900 rpm.

So if you drive keeping the revs around this area you will get better fuel consumption.

For petrol engines it is around 2300-3400 depending on the engine.( check your manual)

I wonder how many people will now try this out for a week to see!!

I dare you!

 

:angry:

[NikpV]

I'm not driving for a week at 4k revs in my tdi :angry: :o

[Guest MATT jr]

I think you will find that maximum efficiency is in and around the rev range of maximum torque ie on a tdi 1900 rpm.

So if you drive keeping the revs around this area you will get better fuel consumption.

i will give that a try.

 

70mph gives (i think) 2,200 revs or slightly more. and i think 60mph gives just below 2000 revs, will check lol

 

MJR

[Ogben Schmutzel]

Yeh, agree that it can't be power can it!!! Diving at 90 odd mph is not conducive to economical motoring.

 

Efficiency sounds much more like it and 1900rpm too for the best mpg!!

 

Cheers.

Oggy.

[katman]

Isnt one of the "offical" fuel comsumption based on 56mph as this was decided to be the optimum speed for economy. I would expect that many vehicles were geared such that the engine was at its most efficient at this speed to give the best MPG figures.

[Guest MATT jr]

mmmmm, i dont know - you could be right.

 

but surely 70mph would have been a better optimum speed?

 

70mph at 1,900 revs? - can the 6 speed manuals do that?

 

btw - at 1,900 revs, you can do up to 60mph, once you go over 60, the revs go up and up.

 

MJR

[katman]

but surely 70mph would have been a better optimum speed?

 

Not really, as you increase speed so the effect of wind resistance increases and fuel economy starts to plummet.

 

On the roads round here (we dont have motorways in East Anglia :unsure: ) you are often lucky to acheive the magical 56mph on the single carriageways.

 

I used to travel from Great Yarmouth to Ipswich daily and the usual cruising speed was between 50-55mph on the bits with a 60 limit. I was getting a reliable 52-54mpg from my Pug 309 GLD. On the rare occasions when I was in other parts of the country and could do some faster driving, this would drop quite severely.

 

I think 56mph is probably the magical figure at which fuel economy then starts to drop as wind resistance takes over.

[NikpV]

Not really, as you increase speed so the effect of wind resistance increases and fuel economy starts to plummet

 

 

agreed, the air resistance increases as the cube of the car speed so doubling the speed will produce an increase of eight times in the air resistance, without even considering rolling resistance which also increases with increasing speed.

 

So why isn't the most economical speed 1mph then ??

 

consider the extreme case - standing still and engine idling, fuel is still being used , the friction in the engine has to be overcome, fuel pumps, alternator etc are still being driven.

 

So like many technological/engineering solutions we end up in a compromise - balance whilst I don't know if there is some fundamental reason that cars tend to be most economical at 56mph the fact that this is the figure that manufacturers have to quote comsumption figures leads them to select gearing etc to make this 'headline' figure as high as possible.

 

 

ps Mattjr - Its a bit like the maximum power theorem - most power is transferred when internal and external resistances are equal in a circuit (see also impedance matching) these are curves with a maxima.