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elhigh-"Take your car to the local dump with a full tank and you aboard, across the scales, that'll give you the exact wet weight for your car + all your mods. Or a quick trip to your local truck stop. If it's info worth knowing, it's worth a few bucks.
---I never thought about this- I figured it would not be very accepted or welcomed if I pulled into a weigh station lol. I am soon taking a trip to NC, so I will have to make a stop by one when I see it. I wouldn't mind doing it more often, but that is too much extra getting to speed.
SentraSE-R-It's a lot more than just cD. It's also frontal area, gearing, tire RR and inflation pressure, ambient temperature, air density, etc. My 2012 Elantra with .25 cD will get ~44 mpg @ 60 mph. A Prius with the same .25 cD will get 60 mpg @ 60 mph.
---Absolutely. But I accounted for the rest of those options, not at 100% precision, of course, but I figured close enough to be in the ball park. Gearing doesn't seem like a variable I can put in, nor does inflation pressure (which seems like a part of rolling resistance).
NeilBlanchard-You can't get from MPG to Cd of your car. It is hard enough to do a coastdown test accurately - which is "just" the momentum/kinetic energy of your car vs speed over time - adding in the consumption of your drivetrain adds in waaaaay too many unknowns.
You need to do a coastdown test if you want to find the approximate Cd.
---I figured it would work. If I know a close enough (as reasonable for my equipment(public roads lol)) info, I could plug it in and would get a value. The tool works for putting in my CD and other values, to show mpg. If I know the other values, and mpg, why can't I get CD?
CD+other valuess =
2 + 2 =
x + other values = mpg
x = 2 = 4
I figured I could mix my plug ins, and get the result I was looking for.
aerohead-*You've got to have a baseline MPG for the stock vehicle determined at the same speed as future testing will be accomplished. (I had built up baseline performance (mpg) for 39,49,59,69 mph, but that testing location I used before is now 2 hours and several modifications away.)
*You need to be fully warmed up before you begin any testing.About 23 miles of steady 50 mph driving is the minimum. (It takes less than that to get me there, but the point is clear. Which emphasizes my point that my CD must be higher than .2, but even exaggerating my weakness (as in, this mileage I posted was not with a engine at operating temperature from the start)-it is not right.)
*Top off right next to the highway you're going to do your testing on,get on the road directly,and up to speed, and then hold that speed as constant as you can before you stop for fuel again.
*Or drive as far as 1/2-tank,then do a u-turn. (300 miles, +5 hours, not unless someone from Guinness is with me lol)
*Run the reverse leg and return to the same station where topped off,at the same pump,parked in the same direction.
*Top off the tank.
*Divide your miles by you gallons and see where you're at.
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When you have this baseline any new drag coefficient will be easy to reverse-engineer from the new mpg at the baseline speed.
---I was not doing this specifically to see CD, I just noted this information during a trip I was taking, and thought with such simple conditions would assist in this. If my goal was to calculate it, I would use a much easier technique and review the threads on calculating it.
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