Hypothetical mpg question fo a 90 mile journey.

Joe_Horner

Norman_Castle wrote: »

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.

But, for the same vehicle, at 90mph it will always be approx 9 times the resistance of that vehicle at 30mph because that's how the physics work - for a given shape with given aerodynamic properties, multiply the speed by 3 and the resistance goes up 9 times (3^2).

That simple approach ignores effects like turbulent flow replacing laminar at various points, but those effects will all tend to increase he high speed drag even more.

Ultrasonic

This (detailed) calculator may be of interest to some:

http://ecomodder.com/forum/tool-aero-rolling-resistance.php

reeac

Norman_Castle wrote: »

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

No, no! It's the same distance and so if the mpg. figures match that means same amount of fuel PER MILE. At the higher speed that means three times the amount of fuel PER HOUR but for one third of the time.

jamesmorgan

Whilst increasing RPM does increase fuel usage, driving at a fixed RPM does not indicate that fuel economy is the same. This is because the amount of fuel usage will also depend on the load on the car. A simple example is that driving up a hill at 2000 RPM uses a lot more fuel than driving on the flat at 2000 RPM. Driving at 90 mph greatly increases the load on the car (maintly due to wind resistance) so will use a lot more fuel (per mile) than driving at 30 mph at the same revs.

The situation is slightly complicated by the fact some of the fuel usage is time related rather than distance. The most significant one being powering the air conditioning. In a typical car about 5% of fuel usage is used for air conditioning, so driving for 3 hours will add a premium of around 10% fuel over driving for 1 hour. I would still expect driving at 30 mph to use less fuel overall.

DUTR

jamesmorgan wrote: »

Whilst increasing RPM does increase fuel usage, driving at a fixed RPM does not indicate that fuel economy is the same. This is because the amount of fuel usage will also depend on the load on the car. A simple example is that driving up a hill at 2000 RPM uses a lot more fuel than driving on the flat at 2000 RPM. Driving at 90 mph greatly increases the load on the car (maintly due to wind resistance) so will use a lot more fuel (per mile) than driving at 30 mph at the same revs.

The situation is slightly complicated by the fact some of the fuel usage is time related rather than distance. The most significant one being powering the air conditioning. In a typical car about 5% of fuel usage is used for air conditioning, so driving for 3 hours will add a premium of around 10% fuel over driving for 1 hour. I would still expect driving at 30 mph to use less fuel overall.

On my car there is a convenience consumption meter as well as the usual mpg stuff, the max reading on the meter is 1/4 gallon per hour, things like heated mirrors, windscreen , aircon etc have effect on this meter reading.

almillar

So, we've established that drag squares with speed, meaning that going 3 times faster gives you 9 times the drag, this applies to F1 cars as much as lorries.
The engine will be working far harder, at the same RPM, but for 1/3 as long.
Do we have an answer then?

WTFH

almillar wrote: »

Do we have an answer then?

Yes, it's obvious. The answer is:
Yes, no and maybe.

I've contacted Erwin Schrodinger for help, but he's not replied. I think he might be burying his cat.

Ultrasonic

almillar wrote: »

So, we've established that drag squares with speed, meaning that going 3 times faster gives you 9 times the drag, this applies to F1 cars as much as lorries.
The engine will be working far harder, at the same RPM, but for 1/3 as long.
Do we have an answer then?

Time is virtually irrelevant, the only small effect being fixed loads like e.g. AC as mentioned above.

The answer was given at the start of the thread by several people 30 mph.

Norman_Castle

Time is virtually irrelevant

So first gear@2000 rpm and 10 mph for nine hours would be cheaper?.

almiller, the engine will be running for three times as long at 30mph and be in a lower gear.

Herzlos

Norman_Castle wrote: »

So first gear@2000 rpm and 10 mph for nine hours would be cheaper?.

Yup. You'd use a lot less fuel to move the car. You'd use more fuel for fixed items like A/C and the radio, but that shouldn't have much impact on the overall fuel economy.