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YACT: Sips gas at 75mpg, but guzzles it at 80mph?

JEDI

JEDI

Lifer
I have a 2002 Chevy PRism with 40k miles.

When i drive at 80mph (2600 rpm) my mpg is 36.

but when i drive at 75 mpg (2500 rpm), i get 39 mpg. (~10% increase!)

does anyone know why 2500 rpm is the magic spot?

THX

edit: Automatic
 
Originally posted by: MacBaine
Maybe wind drag increases exponentially once you get past a point around those speeds
Click to expand...

bingo! Most models use air resistance proportional to the velocity squared.
 
Yep, thats about right. Nearly all engines get the best gas milage at a fast idle, but before midrange.. 2500RPM is about right.

If you look at the fuel usage chart of an engine, it looks like a lopsided C laying on its back. At idle, fuel consumption will be slightly high.. as you climb in RPMs, it goes down to a certain point.. around a high idle, ~2500RPM where it then climbs steeply.

Edit: Oh, I read it as 2900RPM @ 80MPH, not 2600.

With only a 100RPM difference, the answer is definately wind resistance. The energy required to propel the vehicle forward increases exponentially with speed.

Try traveling 70MPH. What kind of milage do you get at that speed?
 
Originally posted by: JEDI
I have a 2002 Chevy PRism with 40k miles.

When i drive at 80mph (2600 rpm) my mpg is 36.

but when i drive at 75 mpg (2500 rpm), i get 39 mpg.

does anyone know why 2500 rpm is the magic spot?

THX
Click to expand...

What's wrong with you being so Un-American, you must drive the biggest SUV possible and suck down massive amounts of gas at 8 mpg or less.

 
Originally posted by: Eli
Yep, thats about right. Nearly all engines get the best gas milage at a fast idle, but before midrange.. 2500RPM is about right.

If you look at the fuel usage chart of an engine, it looks like a lopsided C laying on its back. At idle, fuel consumption will be slightly high.. as you climb in RPMs, it goes down to a certain point.. around a high idle, ~2500RPM where it then climbs steeply.

Edit: Oh, I read it as 2900RPM @ 80MPH, not 2600.

With only a 100RPM difference, the answer is definately wind resistance. The energy required to propel the vehicle forward increases exponentially with speed.

Try traveling 70MPH. What kind of milage do you get at that speed?
Click to expand...

i guess i will try it just for my own curiosity. but 70mph is tooooo slow hen the speed limit is 65.

also, it's a little bit above 2600rpm @80mph. so it's like 2650. i guess it's 150rpm difference.

do you know any websites that would show what the curve chart of my car would be?
 
Somewhere between 75 and 80MPH is probably a speed at which airflow on a major portion of the car no longer becomes a smooth laminar flow but a turbulant, cavitating air flow which increases drag alot. You probably also notice a noise difference at that speed.
 
I've always wondered... since I drive a turbo that spools very low (2400rpm for 15psi) what my fuel map looks like, esp for part throttle.

I know I get much better mileage on the highway but since my 5th gear is kinda short, I tend to run my engine at around 3100rpm+ but since I'm well past the spool point with any touch on the gas pedal = boost. Car with a Cd of 0.28

When I do my normal city driving (boosting during acceleration is a hard temptation to resist) I tend to get around 19-21mpg. I've heard others get up to 30mpg on the highway though they tend to run slightly bigger turbos.
 
There is more than 1 thing that factors in to mpg. Yes wind resistance and drag effects do increase as speed increases. Another thing is the engine itself. The limitations of the engine cause it to become more inefficient at higher rpms. many transmissions use an overdrive gear for the highest gear now to enhance fuel economy. If it is an automatic the cars electronics determine the optimal gear and shift points for the driving conditions. The faster the vehicle is going the more resistance the vehicle experiences thus more power is needed to maintain speed. The overdrive gear is actually for lack of a better term a positive torque conversion meaning in theory the output of the transmission is turning faster than the input from the engine crankshaft. The the other gears are a negative conversion meaning the tranny output is slower the the input ftom the engine. This is how an engine is able to produce the necessary torque to accelerate the vehicle. At high speeds the electronics may determine that the engine is unable to maintain speed at the high gear ratio of the overdrive gear so will force the vehicle to run at a lower gear thus higher rpms and higher fuel consumption enabling it to produce the torque via conversion to maintain the vehicle speed. Not very familiar with prisms but I doubt it has a barnburner engine so I would say you are hitting the practical limits of the engine and it is starting to become less efficient at those speeds. Used to be that 55-60mph was the sweet spot for fuel efficiency in general. I know our minivan gets about 27-30 at 60mph but drops to 22-24 at 70mph over 70 drops to the teens.
 
Heh, for a while in my Dakota I was commuting at 85, I found I got half a MPG better than at 75. My guess is that since the intake is tuned for a certain RPM (almost all are designed that way) that at 85 it's in the sweet spot, and you could hear it. The ram effect was in full force and was actually forcing more air into the cylinders. Mind you, Dakotas aren't aerodynamic marvels. They're compact pickups. This might also be happening to your engine, but at 75 and not 80.
 
yeah, i hear a continuous high pitch sound from the engine between 70-75mph. whenever it makes that sound, i get better gas milage. i guess that's what u are talking about Xanth?

 
Jedi: yep, that high pitched sound is the intake's "pulses" resonating to the point where air is actually forced into the cylinders. The speed at which this happens can be changed by modifying the intake runner length.
 
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