.29 drag coefficient + average speed

[AlcyoneDaze]

I've read that driving between 55-65mph will net the best fuel consumption out of most passenger vehicles, since speeds above 65mph increase drag exponentially...

now, does the fact that our car is so 'slippery' do anything to bump that number up, perhaps making the best average crusing speed for gas mileage any higher, say; 70+mph?

[It's Just Eric]

good question.

Figure they probably conducted that study when they were setting the highway speed limit (40's, 50's?) and cars drove MUCH diffrently back then. Everything from final drive ratio to the previously mentioned drag coefficiant...to how much air goes in the tires...has an effect on the corelation between gas mileage and distance covered compared to speed. I would imagine the only way to figure out exactly what speed you're getting the best gas mileage is to have an awd dyno, an extreamly accurate trip odometer, cruise controll, and some very effective way of measuting fuel consumption. You'd need to put the whole thing in a wind tunnel to mimick airflow while moving

this whole experiment would only yeild one result....Miles per gallon is such a vauge fuel consumption measurement...its retarted. I honestly think the only true way to measure fuel efficiancy is to measure it by liters/hour@X RPM under full load (Including properly inflated tires on a rolling road, and for practicality sake 200 additional pounds in the car representing a driver and misclanious stuff) Of course, every vehicle would be tested at multiple RPM's to actually graph fuel consumption per RPM...and it is with THIS raw evidence that you can actually figure out what speed your convering the most distance per ounce of fuel consumed

Now, becasue I've overthought this WELL into the point where everyone reading fell asleep,( except a few choice engineers, perhaps)Ill just go ahead and say **** counting your drops of gasoline and just drive whatver speed you and your car are comfortable with.

[AlcyoneDaze]

yea... but my tank goes down too fast @ 140

[dcarrb]

I routinely get fuel mileage in the high 20s at 70+MPH (in Atlanta that's just going with the flow and not getting blown into the weeds), but I still believe the MPG numbers would be a bit better at 55 to 65. Trouble is, I can't make myself drive at those speeds to test that belief.

dcb

[benebob]

I've gotten close to 40mpg in my XT6 (same drag as the SVX). Best is still around 50-55mph on the highway. Without caring about mileage I was getting around 30mpg driving the same routes around the same times in the same weather with the same gas (very important as the brand of gas can effect your mileage by about 10% and each of my cars does better with a different kind of gas!). I'm sure if I went 40mph I could've topped 40mpg. Drag is drag no matter what the vehicle is. Sure it might not decrease your mileage 20% going from 55 to 75 but it'll still decrease it a very large percent. AC is horrible in town, better or equal to having the windows open once you reach about 40. Better still if you can drive without it.

I currently get around 28ish or so in my 6 as I'm now sporting a 3.9 geared 5 speed vs a 3.7 auto. Auto killed the 5 in mileage!

[RSVX]

Mythbusters recently had an episode where they tested MPG with and without AC... they found that at both high and low speed, you are better off with the AC on...

[SSSVX]

Any formula to calculate how much/percent hp the AC taking from your engine?

On the small car/engine, I can really feel the AC taking quite some power on going up hills. On the bigger engine, it's less obvious.

[benebob]

Originally Posted by RSVX

Mythbusters recently had an episode where they tested MPG with and without AC... they found that at both high and low speed, you are better off with the AC on...

Didn't see it Chris as TV bores me but since you didn't finish you're sentence. While in an urban setting its idiotic to think that adding yet another strain to an idling engine is better than not having it on. Also, despite you're incomplete thought it of course is better to not have it on and drive with the windows shut at any speed.

[RSVX]

Yeah, I didnt remember all of the details, so I didnt want to misquote anything.

[XT6Wagon]

depends alot on the car and the gearing. My SHO with auto got the best milage at 77 indicated. About 200rpm above the low rpm torque peak.

My truck gets its best milage at 50mph.

Most of the subaru's I've had were geared so low 55mph is about the peak. The STi would hold its 26mpg till 80 or so, but there is like 0 varience in that range. I think its the turbocharger removing pumping losses as the load (drag) increases that makes it not lose much/any in that range. Well and moving to a more efficent rpm.

never drove the SVX that much, but it seemed dead stuck on 20-21mpg regardless of how/where I drove it. Its likely not representative though as its 180,000 miles old and it got its factory origional fuel filter changed when I got it at said 180K miles.

[benebob]

No gearing isn't the biggest factor. Drag is. I highly doubt that you got better mileage in anything at 77 vs 55. Doesn't matter if it was a vette or a semi. Drag is the same regardless. That same STi would get 34mpg at a constant 55mph.

Remember there are more than one gear and you are correct about efficiency in gearing. Truth is at 55mph had you downshifted that SHO you would've gotten about 20% better mileage then you did in a higher gear at 77mph. Never drove a Yamaford or don't know what kind of gears it has but I'm guessing they are SI Ford.

Originally Posted by XT6Wagon

depends alot on the car and the gearing. My SHO with auto got the best milage at 77 indicated. About 200rpm above the low rpm torque peak.

My truck gets its best milage at 50mph.

Most of the subaru's I've had were geared so low 55mph is about the peak. The STi would hold its 26mpg till 80 or so, but there is like 0 varience in that range. I think its the turbocharger removing pumping losses as the load (drag) increases that makes it not lose much/any in that range. Well and moving to a more efficent rpm.

never drove the SVX that much, but it seemed dead stuck on 20-21mpg regardless of how/where I drove it. Its likely not representative though as its 180,000 miles old and it got its factory origional fuel filter changed when I got it at said 180K miles.

[XT6Wagon]

nope, 55mph while *almost* as good as 77, wasn't quite as good. Why? While there is lower fuel burn per unit of time, there is also less miles per unit of time. The engine is also more efficent per HP at 2,800-> 3K than it is lower rpm. So its a buch of factors getting somewhat better all at the same time that offsets the higher HP needed to maintain speed.

Even funnier rolling hills is worth 1-2mpg MORE in that car. If you got hills that would pull the car down till just BEFORE it unlocked the torque converter, it would get the best milage as it turns off the injectors on the downhill, and the near WOT hill climbs are still in lean burn and at the absolute peak of the burn efficency of the motor. if the torque conver unlocks? you just lost a huge ammount of MPG as it switches to rich fueling, loses 15% of the power or so in the torque converter, and the RPMs shoot up right into the torque valley of inefficency. 20-30 seconds of that could pull a solid 1mpg off a 350+ mile highway run.

Anyhoo, using the 77mph speed I'd get 28-29mpg compared to EPA list at 26mpg. 75mph would produce 26mpg. I never tracked what 95mph was but it was low enough that it was a bit too low on the MPG front to make my standard trip unless I left with a absolutely full tank.

Likely did 40 or 50 runs in that car on the same route over a couple of years. On my trips to Vegas across AZ, I would track the MPG for entertainment value. I ALMOST broke 30mpg the trip my A/C died in August on I-10 in the south part of AZ. IIRC it was 29.8 or something.

[dcarrb]

I think the "Mythbusters" finding was that AC-on was more fuel efficient than having it off with the windows down, at highway speeds. (Wasn't this the show where each of them drove "identical" vehicles around a race track 'til they ran out of fuel? There are too many variables in individual driving styles to draw any concrete conclusions from such a test anyway, I think.)

There's a "sweet spot" at which power, gearing, and the ability to overcome aerodynamic drag combine for optimum efficiency, beyond which increasingly more power is required to move the mass of the car. I'd guess with most any street car, that's somewhere between 55 and 65 miles per hour. It takes considerable energy to overcome drag, friction, and gravity and move 3400 pounds at a constant 70 MPH, much more so than 55, and demanding that energy from the engine consumes additional fuel.

dcb

[JaySVX]

Logically speaking, when highway cruising, the ac will be more efficient... Stop and go, around town, the windows would be. Hopefully next week when i drive to texas my AC will be working and i can get increased mpg, as it's gonna be a long and crappy drive if the ac doesn't work, plus i'm doing everything i can to squeese every bit of fuel efficiency out of it. I'm calculating right now at about $550 round trip in gas.

With the weight of hte svx though, you have momentum, so if there's sudden gusts of wind, bumps, etc, your speed and mpg are less likely to be effected. Once you hit cruising speed you shouldn't have to make much in the way of adjustments due to resistance, since the momentum of the car will keep it moving.

[odepaj]

Originally Posted by JaySVX

Logically speaking, when highway cruising, the ac will be more efficient... Stop and go, around town, the windows would be. Hopefully next week when i drive to texas my AC will be working and i can get increased mpg, as it's gonna be a long and crappy drive if the ac doesn't work, plus i'm doing everything i can to squeese every bit of fuel efficiency out of it. I'm calculating right now at about $550 round trip in gas.

With the weight of hte svx though, you have momentum, so if there's sudden gusts of wind, bumps, etc, your speed and mpg are less likely to be effected. Once you hit cruising speed you shouldn't have to make much in the way of adjustments due to resistance, since the momentum of the car will keep it moving.

[Hijack]
Where in the big ol' country of Texas are you gonna be?
[/Hijack]

To add to all this, when I bought my '95 I drove it from Tulsa OK, back here and averaged just under 30mph with the cruise set at 80+mph with acouple short bursts to higher speeds (come on! it was a new car so it was mandatory )


Dustin