40 MPG @ Highway Speedlimit - Cd?
 

Today, I drove 38.1 miles in one direction, getting 39.8 mpg w/ cruise control locked on at the speed limit. This did include getting started in a parking lot and waiting at a red light, hurting mpg.

2.9 miles @ 55mph
12.3 @ 65
22.9 @ 60

I later drove this backwards ( the opposite direction, return trip) and saw 40.2 mpg. Temperature was not noticeably different, one trip from 0930, the way back was at 1800.

Of course when I usually drive, I speed, and hypermile. So EOC, and cruising at 4-9 over, building on down hills, losing speed uphill- usually engine on performance gets me 35+, not including my EOC.

I originally paid attention to this because someone on here (couldn't remember who) brought up something along the lines of not knowing about a vehicle getting 30+ mpg at 70 mph. So I thought I would keep track, find my rough figure on the Cd chart, and look to see at 70 mph.

Anyway, knowing my vehicles info (wet/loaded weight, frontal area), general driving figures (drag from road, air density), I am trying to calculate my CD by looking at the tool for it. I was placing lower and lower to find, but putting in .2 for CD gets me 35 mpg at 60 mph. I know my CD is NOT that low, yet I was getting BETTER mpg.

Does anyone have any tips, help, or ideas on it?

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.

You could also start collecting CAT cards.

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.

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.

idea
 

*You've got to have a baseline MPG for the stock vehicle determined at the same speed as future testing will be accomplished.
*You need to be fully warmed up before you begin any testing.About 23 miles of steady 50 mph driving is the minimum.
*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.
*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.

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.

59 and 69
 

If you have mpg info at 59 mph,then any 10% drag reduction would equate to about a 5.5% improvement in mpg at constant speed.
At 69 mph,you'd pickup 6% mpg for any 10% drag reduction.
So with your new mpgs,you can reverse-engineer your new Cd.

But that was a different segment of road. True, I can make connections to them, but to calculate my actual improvements, I like to remain with that constant.

Frankly Ultra I don't think your car has any aero drag.

A Mustang getting 39mpg just doesn't happen. Who's pushing you down the road at 70mph?

:)

I wish I had someone to! I was trying to convince someone to pull it to Maryland, and imagine the mileage! (If only it would count lol). Normally driving that stretch is better, because I speed, draft and hypermile, not just cruise control at a set speed.

Eventually as I get skinnies on the front, lower it, change to a lighter driveshaft (rethinking after reading about the flywheel), increase gearing further, and lighten the car up, I hope to make it better.

I was hoping to keep 40+ tanks, but since the underbody was ripped off, it wouldn't quite happen. Maybe this tank, though. My battery died so a lot of my info was lost, I really only have my miles traveled left.