Calculating City vs. Highway driving for fuelly.com

[ErikBEggs]

I posted this in the other thread. It works well for me. Thought I'd share.

I use 1 factor and 1 factor alone to determine my city / highway split: the average speed on the DIC. I reset it every time I add a fuel up. Now lets get into detail:

Average speed = Distance / Time. For the sake of this comparison.. time is what is important. How much time in the city vs. how much time on the highway.

I use the EPA testing cycle as a benchmark. The average speed of the city test is 21 mph. Therefore, I use 20 mph or lower average speed as a 100% city conditions. This has only happened on 1 of my fuel ups. The highway test cycle puts an average speed of 48 mph I believe, so I round that to 50 mph.

Now here is where I make my distinction. I factor in *true* speed of my highway driving. Back at home in Buffalo, NY drivers are less aggressive because the cops brutally enforce the 55 mph state speed limit on all the local highways. Therefore, I am able to safely travel at 55 mph cruise every time I get on the highway. For simplicity (and to counteract getting on and off ramps), I assume a 50 mph average speed or higher in this scenario to be 100% highway driving. I also realize that most people don't have the luxury of driving this speed. While I'm at school in Delaware, 55 mph on I-95 is not only unlikely - it is dangerous. So when I'm here, I bump the cruise up to 65 mph which is about what people can get away with on most highways. In that case, I use 60+ mph average speed as 100% highway driving.

So calcuating the split, the average speed basically tells you how far your car has traveled in the time it is on since you last filled up. I interpolate between the two values. An average speed of 40 mph is exactly halfway between 20 mph (100% city) and 60 mph (100%) highway. It will show up on my fuelly as 50% city / 50% highway. Back in Buffalo, 35 mph is directly between 20 mph (100% city) and 50 mph (100% highway). (Not) Coincidently, this really does accurately describe the driving split. For calculation simplicity (so that you don't need a calculator to do it), just take your average speed.. find out in multiples of 4 (for 60 mph highway) or 3 (for 50 mph highway) how far it is from either end and that is your split to the nearest 10%. So a 36 mph tank for me equates to 60% city / 40% highway in Delaware and 50% city / 50% highway in Buffalo. For the nearest 5% (what is used by fuelly), just use multiples of 2 or 1.5. And on the couple road trips I've had.. speeds of 64 mph and 62 mph have shown up. 100% highway. . Some road trips you get city like conditions like accidents. I've had one slow my average to 57 mph with a half hour of traffic. That was included in the entry as 10% city / 90% highway. Sound accurate?

By keeping this that detailed, my fuelly displays the exact amount of driving conditions I drive in. As of now it reads 53% highway / 47% city.

[70AARCUDA]

...EPA assumes 45% higway and 55% city, nearly the inverse of your driving pattern.

[XtremeRevolution]

Thanks for posting this. I'll definitely be using this.

...EPA assumes 45% higway and 55% city, nearly the inverse of your driving pattern.

That's only for the calculation of their "combined" fuel economy. That ratio is otherwise irrelevant.

[70AARCUDA]

...my point was that EPA "assumes" everybody's "average" driving habit will be 45%(H) and 55%(C), otherwise different ratios would be used.

[XtremeRevolution]

...my point was that EPA "assumes" everybody's "average" driving habit will be 45%(H) and 55%(C), otherwise different ratios would be used.

I'm not understanding what the point of that is. The purpose of his post was to describe a guideline for determining how to differentiate city/highway percentages. The thread in which it was posted was referring to how people enter data in fuelly.com.

[gman19]

Nice post Erik. Seems using your formula/method, my driving is an even 50/50 split based on my average speed. This will come in handy when entering Fuelly.com stats.

[gman19]

I came up with this using Erik's method...I think it works, haven't proven it completely yet though...gotta love Excel!

Mileage Split Calculator
City MPH = 20	Max Hwy MPH	Average Speed read from DIC	Percentage City Driving	Percentage Highway Driving
20	60	20	100	0
20	60	25	88	13
20	60	30	75	25
20	60	35	63	38
20	60	40	50	50
20	60	45	38	63
20	60	50	25	75
20	60	55	13	88
20	60	60	0	100
20	70	20	100	0
20	70	25	90	10
20	70	30	80	20
20	70	35	70	30
20	70	40	60	40
20	70	45	50	50
20	70	50	40	60
20	70	55	30	70
20	70	60	20	80
20	70	65	10	90
20	70	70	0	100
20	50	20	100	0
20	50	25	83	17
20	50	30	67	33
20	50	35	50	50
20	50	40	33	67
20	50	45	17	83
20	50	50	0	100
Free Form Entry
City MPH = 20	Max Hwy MPH	Average Speed read from DIC	Percentage City Driving	Percentage Highway Driving
20	50	25	83	17

[70AARCUDA]

ErikBEggs & gman19 -- hey, that's a pretty useful method for estimating the HWY/CITY "split" percentage of your individual driving style.

For those who wish to "roll their own" numbers (the 'free form entry' above?):

%(hwy) = 100*(DIC - CITY)/(HWY - CITY)

%(city) = 100*(1 - %(hwy))

...where:

_DIC = Average MPH since last reset.
CITY = Average minimum speed of 20 MPH, in city (assumed value).
HWY = Average maximum speed of 50, 55, 60, 65, 70, or 75 MPH, on highway(*).

(*) -- When some of your highway driving is at 55 MPH, and other is at 65 MPH, you 'split-the-difference' and use a HWY value of 60 MPH.




CREDIT RECOGNITION to:
• ErikBEggs for idea implementation
• gman19 for spreadsheet implementation

[ErikBEggs]

The table works nice, and the spreadsheet formula is even better. Sticky???

[Patman]

Found something new on Fuelly: I just added my Wife's car and now it asks percentage of City vs Hwy driving. Have not seen what it is doing with it yet. It does nothing for old data already entered. FYI for anyone interested. Xtreme you seem to be good at figuring these thing out maybe you can find out what the change is.