..more on fuel consumption..

[Phil Robson]

So as not to hijack the other threads on related subjects, my query is regarding speed & consumption.

Supposing you have a car that does 50mpg at 50mph. It would seem reasonable that if the same car does 25mpg at 100mph, then driving faster will not use any more fuel because the journey is over quicker.

However, this isn't the case:

Drive for 2 hours at 50mph - distance covered = 100 miles, fuel consumed = 2 gals.
Drive for 1 hour at 100mph - distance covered = 100 miles, fuel consumed = 4 gals.

Time must somehow be taken care of in the mpg calculation, but I don't see how :?

 

[rockdemon]

wind resistance, engine efficiency at a given rpm....

 

[quattro]

So as not to hijack the other threads on related subjects, my query is regarding speed & consumption.

Supposing you have a car that does 50mpg at 50mph. It would seem reasonable that if the same car does 25mpg at 100mph, then driving faster will not use any more fuel because the journey is over quicker.

However, this isn't the case:

Drive for 2 hours at 50mph - distance covered = 100 miles, fuel consumed = 2 gals.
Drive for 1 hour at 100mph - distance covered = 100 miles, fuel consumed = 4 gals.

Time must somehow be taken care of in the mpg calculation, but I don't see how :?

If you drive at 50mph for 100 miles at 50mpg then you will will indeed use 2 galls of fuel in 2 hours i.e. 1 gallon per hour.

If you drive at 100mph for 100 miles at 25mpg then you will indeed use 4 gallons of fuel in 1 hour i.e. 4 gallons per hour.

So twice the speed but 4 times the consumption

Richard

 

[Phil Robson]

So twice the speed but 4 times the consumption

Richard

Yes - I think that's the confusing point - because the quoted consumption at twice the speed is only 2 times, not 4.. :?

 

[richarduk]

wind resistance, engine efficiency at a given rpm....

I would say mostly win resistance as you say. It's not a linear scale. Stealing a snippet from Wiki
"The standard equation for drag is one half the coefficient of drag multiplied by the fluid mass density, the cross sectional area of the specified item, and the square of the velocity."

Notice the "square of velocity" at the end. I'm sure when Top Gear took the Bugatti Veyron to it's top speed they quoted how much of the power was needed for the last few miles an hour, I can't remeber what it was but something on the lines of most of it's 1000bhp is needed for the last few 10's of MPH.

As an old school friend used to wonder, if he's 50cc moped can do 30mph why can't a 500cc bike do 300mph.