DrRSwank - Member
Unless the efficiently of you engine improves massively in the range 70 - 85 mph I don't think it is possible.
Hmmm, mines a bit like this as well though.
65mph - 55mpg
75mph - 45mpg
85mph - 48mpg (which does surprise me)
So it's cheaper to go faster (or slower)......
Don't know why it does this - gear ratios, revs, dunno.
I think the computer is lying to you. Something in inaccurate.
Very roughly the drag on a object moving through the air is proportional to v^3 where v is the velocity.
So increasing the velocity by 13.33% (10/75) would result in 69 % more fuel being used.
If we let you engine be x_1% efficient at 75 miles an hour with a given fuel energy density d then the energy available to us at 75 mph and needed to move the car one mile
E(75)=(1/45)*d*(x_1/100)
now at 85 mph we will assume an efficiently of x_2% we have
E(85)=(1/48)*d*(x_2/100)
energy available to us, which also must equal the amount of energy to move one mile at 85 mph.
We know that the energy required to move at 85 mph is 68% more than at 75 mph so
E(75)*1.68=E(85)
(48/45)*1.68*x_1=x_2
1.79*x_1=x_2
So your cars efficiency would have to improve by 79% at 85 mph than at 75.mph.
E.g it was 31% efficient at 75 mph and it becomes 55.5% efficient at 85 mph.
To gain this must efficiently would be amazing, and for a manufacture to place that imprisonment in efficiency in the range 75-85 mph, when cars are sold on urban mpg and extra urban mpg of 62 mph would be very strange.*
*Unless I have made some very silly mistakes.