Sharp MPG drop at higher speed

[DragBean]

Well I guess we can rule out bad gas since you got the better mpg when you slowed down..
If it wasn't the wind, then it must be something mechanical..

Or wait a second.. Did the mpg go back to normal when you went into the slow zone?

[cyclopathic]

Found this graph posted by Bob:

the Gen2 seems to be showing exactly the same pattern. Most likely the same combination of increased aerodrag due to increased speed and reduced engine efficiency at higher RPMs

[Hersbird]

The EPA could easily require automakers to pull steady state MPG numbers off their dynamic runs used to get sticker info. They could get them for 10 mph increments from 50-80 to help show people the advantages of slowing down and also what cars might be better suited for long distance, high speed interstate travel. Old Mechanic posted a paper from 1975 showing that info, with today's computer programs they could do it with every car with little extra effort.

[cyclopathic]

I'd expect car manufacturers would resist it on the grounds of making certification more expensive or in case of vw cheating cover-up. Interesting to see to what extent gearing/aerodynamics are optimized for epa test procedures, prius is no exception.

[cRiPpLe_rOoStEr]

Originally Posted by cyclopathic

Interesting to see to what extent gearing/aerodynamics are optimized for epa test procedures, prius is no exception.

That one-size-fits-all test standards are ridiculous. It seems to have been there to give automatic transmissions an advantage.

[cyclopathic]

Originally Posted by cRiPpLe_rOoStEr

That one-size-fits-all test standards are ridiculous. It seems to have been there to give automatic transmissions an advantage.

I don't think it was intended that way back in 70s when procedures were established, but current generation of drive by wire software controled cars takes advantage of it.

You could argue that CVT equipped car has taller final ratio and can be programmed to lag engine at low RPM/low intake vacuum to reduce friction and pumping losses which would help economy.

[ciderbarrel]

Originally Posted by cyclopathic

I punched the numbers into recommended calculator 2800lbs, ,28 Cd, .31 engine, .92 transmission efficiency, etc and calculator shows 53 at 65 and 42 at 75. Test snows 53 at 65 and 38 at 75 consumption. Calculated drop is 21% and actual is 28%, 1/3 worse.

MPG is not linear, and the actual rate of consumption over a set distance must be used. The calculated drops are much worse than you realize.

For example, a drop of 5 MPG is meaningless without knowing what the start and end MPGs are. A 5 MPG change from 25 MPG to 20 MPG is an increase of 1 gallon of gas per 100 miles (A change from 4 to 5 gallons = 25%). Another 5 MPG change from 10 MPG to 5 MPG is an increase of 10 gallons of gas per 100 miles (A change from 10 to 20 gallons = 100%).


For your numbers:
53 MPG drop to 42 MPG increases fuel consumption by 0.494 gallons per 100 miles (26%)
53 MPG drop to 38 MPG increases fuel consumption by 0.745 gallons per 100 miles (39%)

Gallons per 100 miles = 100 / MPG

53 MPG = 1.887g /100m
42 MPG = 2.381g /100m
38 MPG = 2.632g /100m

[cyclopathic]

So got to travel on the same road in very similar conditions (load, temperature, wind, etc). Once again using cruise control to take driver out of consideration.

Computer calculated MPG was 41.4 vs 38.7 last time, this is average over almost an hour (had to abandon test after ~50mi as we ran into construction zone with lower speed limit). 7% improvement.

Difference? A set of EVO vortex generators installed alone the roof trailing edge.

I am building the underbelly panels when it warms up.