MPG Normalization - Gas Savers

maximilian · 03-22-2009, 12:48 PM

I'm trying to make an estimate of how I'm doing that's a little more accurate. I almost never drive in towns or cities, so the EPA combined rating is a little off. That part's pretty easy to account for (just keep track of the fraction I spend at highway speeds), but the big hills here aren't. Can anyone suggest a non-arbitrary way to adjust for hilly terrain? What sorts of hill impact does the EPA even assume in their estimates? Adjusting the EPA rating may make more sense than trying to shift mine as their assumed driving style is known, right? Thanks.

**Jay2TheRescue** · 03-22-2009, 01:56 PM

Unless its part of the revised EPA formula, I don't think hills are considered. I think the EPA testing is all done on a dyno now.

-Jay

Dalez0r · 03-22-2009, 02:59 PM

Keep in mind that under some circumstances, hills can boost your fuel economy due to loading the engine properly then coasting down the other side burning no or little fuel...

maximilian · 03-22-2009, 03:03 PM

Good point! I meant specifically for trips I do both ways on.

**theholycow** · 03-22-2009, 03:54 PM

I would call it even and not worry about the hills. The EPA combined estimate ought to be fine; you're not making it a target, you're comparing to it.

Do the best you can, compare it to the EPA estimate, and enjoy.

maximilian · 03-22-2009, 04:24 PM

As good as my inflated numbers feel...I'm trying to get a feel for how close to the point of diminishing returns I am. I can at least do the city/highway adjustment, so that's something. No point in making a adjustment for the hills if it's just going to be arbitrary, that's for sure.

maximilian · 03-22-2009, 04:54 PM

Wait a minute, I've got that backwards! Hills would probably hurt the EPA estimates, since a normal driver doesn't coast in neutral or take other such actions.

**theholycow** · 03-22-2009, 05:00 PM

On modern cars, hills probably wouldn't change EPA estimates that much. At least, modern cars with good DFCO behavior...

maximilian · 03-22-2009, 05:06 PM

Really? Anybody ever get to test their cars on a track or other long flat surface? That's the only way I can think that we'd have any actual data.

Since it'd err on the side of being conservative, I feel better about neglecting it anyway.

maximilian · 03-22-2009, 05:23 PM

Searching around I saw a new (to me) driving tip:

gradually build up some momentum before going up a hill

**theholycow** · 03-22-2009, 05:26 PM

That's worth a try. The opposite is also worth a try. Both can be backed by theory and real world data.

maximilian · 03-22-2009, 05:31 PM

My intuition's no use for these multi-input tradeoffs. Just have to try 'em for long enough and keep track. I just removed my passenger mirror so I can't try anything until I establish a new baseline. Besides, summer tire season's almost here (we only got 4" of snow today)!

maximilian · 03-26-2009, 06:28 AM

The EPA combined mpg estimates appear to assume 60% city driving and 40% highway (worked it out from the ratings of my car). I just analyzed the routes that comprise over 90% of my driving and the reverse is true: 60% highway (two lane state highways with a speed limit of 50, but that's what I drive on the big highways anyway), 40% city. When I compute a new combined mpg with those fractions I get 30 mpg. In fact, since the "city" driving is really just small town driving so there are almost no stops, it probably should be a little higher than that. I figure it's close enough to the adjustment for all the hills around here that I can neglect it.

Any objections to my updating my combined reference number? It makes my % over EPA score worse (by 3.3%).

**theholycow** · 03-26-2009, 07:12 AM

fueleconomy.gov defines combined as 45% highway, 55% city. I believe by "highway" they mean freeway, not 50mph two lane state highways.

It lists your 2008 Accent manual as 27 city, 29 combined, 32 highway. Those are the numbers that you should have in your "EPA Values" section of the garage entry to fairly compare against other drivers.

FWIW, my driving works out to about 55% freeway, 45% town/two-lane state highway and I compare to my EPA "Combined" rating. My choice of where to live and work (and therefore my commute) are reasonably part of my hypermiling, IMO. Anyway, traffic lights and stop signs definitely mean you're not driving in what the EPA considers "highway", though those Vermont two-lanes do have long runs of no lights or stop signs. EPA combined rating is a fine baseline for you.

maximilian · 03-26-2009, 07:19 AM

Quote:

Originally Posted by theholycow

Those are the numbers that you should have in your "EPA Values" section of the garage entry to fairly compare against other drivers.

Your idea of "fairly" and mine differ (gives me an inherent advantage), but you're the moderator!

I guess unless everybody worked out their combined it wouldn't help in total. I'll just make a note in my description.

.55*27 + .45*32 = 29.25, so the rounding is what gets it to 29. That means the decrease from changing to 30 would be down to 2.5%, which is starting to get pretty small anyway.