Help with MPG Calculator - Fuel Economy, Hypermiling, EcoModding News and Forum

redtracer · 07-17-2014, 01:34 PM

Hey guys, I'm trying to recreate a Chrylser 300 in this web sites MPG calculator so I can see how a Cd change affects mileage.

My problem is even though I don't have engine or drivetrain efficiency for this car I'm having a really hard time recreating the real world gas mileage I've gotten with this car. I frequently get ~30 MPG at 75mp/h, but to get the calculator even close to that I need to do crazy thing to the numbers, the biggest one being bumping engine efficiency up to a whopping 30%

Here are the numbers I was using (in order from top to bottom)

4200
0.0125
0.36 - Known accurate value
25.8 - Known accurate value
33557
0.25 - Guessed Value
0.85 - Guessed Value
0
1.225

With these numbers I get 19.27MPG @ 75mph. That is so far off what I get the real world that some of these inputs are way off, or the calculator doesn't work how I think it does. I've tweaked with the numbers but to get it close to 30 I need to do some crazy things that are just flat out wrong.

So my question is this: Does anyone know if there is a "baseline" or "standard" they used to make this calculator? Some vehicle where you put the values and it comes out correct? I want to use this information for a report, but as of right now I just can't trust it because it's giving me values that are just bananas and make no sense.

Thanks guys.

ksa8907 · 07-17-2014, 08:39 PM

The 30mpg at 75mph, is that hand calculated or are you getting that from the onboard computer? What year is the 300?

Frank Lee · 07-18-2014, 03:58 AM

I think several of OP's numbers and expectations are skewed.

I doubt the 300 returns about 30 mpg @ 75 mph. Fuel economy.gov says 31 highway which may be where OP got that number BUT EPA highway is NOT 75 mph cruise; it is a 10.6 mile dyno run with average speed of 48 mph and never exceeds 60 mph. Or he has a dashboard economy gauge that says 30 @ 75 but... that is likely a temporary value at optimal conditions like a slight downhill grade and/or tailwind. I wouldn't put too much emphasis on that gauge reading; I'd use hand-calculated tank-to-tank values for a reality check.

Furthermore I doubt the drivetrain is only operating at 85% efficiency at cruise speed, nor the engine at only the default value of 22%. Bumping the engine efficiency to 25% and the drivetrain efficiency to 90% results in estimated 31 mpg @ 55 mph and 20 mpg @ 75 mph which seems reasonable to me (although I have no personal experience with the Chrysler 300).

Being a little more generous with Crr- to .010- gives 33 mpg @ 55 and 22 mpg @75; .010 Crr is entirely (PUN!) plausible and the predicted results seem to be as well.

There are only seven 300 owners on fuelecon.gov sharing their mpgs but it looks like 26 mpg is a reasonable highway mpg expectation. We don't know how fast they drive though. One user reports 31 highway and if he cruises at 55 mph that fits nicely with the calculator's estimates.