Hypothetical mpg question fo a 90 mile journey.

Norman_Castle

reeac wrote: »

No, of course it isn't unless she's a masochist. Your friend should have referred to her own historical useage or asked a few other people for theirs. I say useage [in kWh of gas or electricity or litres of oil] in order to allow for the inclusion of increased fuel prices. Alternatively read a few of the many recent newspaper articles on fuel costs which quote typical overall household fuel bills of £1300-1400 p.a.

I think £200 is far too much considering the exchange rate and high temperatures in Ethiopia.

Not a good tactic to ignore factors which aren't tightly specified.

Norman_Castle

vaio wrote: »

if that's true then it seems strange that (generally) fuel economy gets worse the faster you go and, during the fuel crises of the '70 & 80's most governments lowered speed limits to reduce fuel use

Lower driving speed normally leads lower engine speed which saves fuel. The ops question uses the same engine speed in a lower gear for a longer time.

westwood68

bsms1147 wrote: »

With the hypothetical scenario proposed, the car traveling at 30mph will use three times the fuel as the one travelling at 90mph, minus whatever things you want to factor in, such as air resistance.

I doubt the air resistance (and other unaccounted for factors) could treble the fuel-use of the 90mph option so that it matched the fuel use for the 30mph option.

I think there has been a major misunderstanding on this thread. Air resistance doesn't treble between 30mph and 90mph; it goes up by 9 times (the square of the speed)

http://phors.locost7.info/phors06.htm

That bottom table suggests a 26 times increase in power required between 30mph and 90mph. You'd have to seriously rev the bollox off the engine to get a lower mpg at 30 than you do at 90....

Joe_Horner

Norman_Castle wrote: »

Ignoring the variables of friction, load, wind resistance etc[STRIKE] which will have a limited effect, an engine running at 2000rpm will use the same amount of fuel at any speed. The distance and engine speed are fixed. The only variable is the time taken due to the gear used.[/STRIKE] neither scenario will use any fuel once you get up to speed, as long as you don't brake down hills so your momentum will carry you up the other side.

So 40mph will be more economical because of the shorter acceleration time at the start.

There you go, fixed that for you seeing as "ignoring" everything that will require fuel during the trip means you won't need any fuel for either

reeac

Norman_Castle wrote: »

I think £200 is far too much considering the exchange rate and high temperatures in Ethiopia.

Not a good tactic to ignore factors which aren't tightly specified.

You caught me with the Ethiopian friend move but I've not heard anything more re. city stop-start driving vs. open road. One all.

reeac

westwood68 wrote: »

I think there has been a major misunderstanding on this thread. Air resistance doesn't treble between 30mph and 90mph; it goes up by 9 times (the square of the speed)

http://phors.locost7.info/phors06.htm

That bottom table suggests a 26 times increase in power required between 30mph and 90mph. You'd have to seriously rev the bollox off the engine to get a lower mpg at 30 than you do at 90....

Just like I said in post #4. [For the purist, I said x27].

Norman_Castle

90 mph for 1 hour in 6th gear @ 2000rpm.
or


30 mph for 3 hours in 4th gear @ 2000rpm.


would there be a big difference in fuel consumption?

Nothing else is known but the above.
The 1 hour, 90 mph journey is in a higher gear but will be under greater engine load due to wind resistance which will depend on how aerodynamic the vehicle is.
The 3 hour journey is in a lower gear giving less engine load, but more engine revolutions per mile and with less wind resistance.
Both journeys are the same distance and at the same engine speed.

For the mpg figures to match, the 90mph journey would need to use three times the amount of fuel per mile compared to the 30mph journey while running at the same engine speed. This is possible, but I think vehicles designed to travel at 90mph tend to have decent aerodynamics which makes it unlikely.

Norman_Castle

Joe_Horner wrote: »

There you go, fixed that for you seeing as "ignoring" everything that will require fuel during the trip means you won't need any fuel for either

I've never suggested ignoring the effect of wind resistance etc. I said ignoring the variables. At 90 mph the effect of wind resistance will vary depending on how aerodynamic it is which in the original post is unknown.

Norman_Castle

reeac wrote: »

You caught me with the Ethiopian friend move but I've not heard anything more re. city stop-start driving vs. open road. One all.

Its an example of low speed consumption compared to high speed consumption.
One all?. I'm not interested in point scoring.

Ultrasonic

Norman_Castle wrote: »

Ultrasonic wrote:

If you were a newbie I'd assume you were trolling.

The difference in air resistance between 90 mph and 30 mph is a HUGE factor, you simply cannot ignore it.

I had a 1.4 Citroen which averaged 40- 45 mpg at 1500 - 2500 rpm in general city driving, 20-40mph, and 35-40 mpg at a constant 80-90 mph at 3500 rpm. It did not use three times as much fuel.
The difference air resistance makes depends on how aerodynamic the vehicle is.

What does the fact it didn't use three times as much fuel have to do with anything :huh:. But more importantly comparing the average speed of city driving with lots of acceleration and deceleration to constant high speed driving is ignoring another shed load of highly relevant confounding factors as well.