Riddle me this batman...odd mpg

Ok so this is before Christmas, I changed my long neglected air filter and drove from Missouri to Colorado. On the drive got my typical 15-16 mpg. Once in Colorado I filled up four times at different gas stations and got 18, 18.6, 19.3 mpg...these are the best numbers I've ever seen in my tacoma in two years of owning it! However my fourth fill up I was back down to 17, and now I'm back to my typical 15-16.

What the heck happend??? I want the 19 again! No big changes in driving style, weather, or anything like that...

v6 DC manual 4x4

 

Dunno for sure, but do you think altitude may have played a factor? Anyone with knowledge on altitude and it's effects on air/fuel combustion feel free to chime in.

 

Elevation change could affect mpg's. Driving style and conditions most likely the biggest contributer. I do better when I go to upstate NY vs Boston, less traffic and mostly back roads where you hold 50 - 60 mph which is great for mpg's. Maybe similar situation for you?

 

Isnt gas different in different states too?

 

I would say it was a mixture of the elevation and less traffic.
I know my trip through higher elevation a few years ago netted me big time gain in mpg's in my wife's old car.

 

First posting, but been lurking for awhile...

I live in Colorado and went on a trip to Missouri for New Years and had the same (but opposite) results. MPG is way better in CO for some reason (17-19 mixed) and in MO it was 14 - 16 mixed . I'm filling up with 85 Octane in CO and I used 87 in MO.

I've read on other MPG posts that the altitude can really make a difference. Guess I'm lucky living here. I used to live in Nevada and when we moved to CO, our MPG increased on our other cars as well but I don't think it was as big a difference as on the Tacoma.

2011 DC 4X4 Manual

 

I know here in Oregon the gas is formulated differently in the winter, more oxygen. I lose about 2 miles per gallon in the winter over the summer months. I'd guess that's what you are running into.

 

If anything you would get worse mileage at higher elevation. Less oxegyn in the air will cause the computer to put more fuel to the engine to get the correct A/F ratio

 

Opposite is actually true. The higher the elevation, the better the gas mileage. Less oxygen in the air should cause the computer to use less fuel to keep the air/fuel mix correct. But that's actually irrelevant.

The biggest factor that kills gas mileage is air density. That's why increasing your speed 5 mph from 65 to 70 will result in less of a loss of mpgs than going from 70 to 75. Resistance from air increases exponentially. With the higher elevations, you'll always get better gas mileage. Depending on the elevation difference, you can actually still see a huge increase in MPG going up in elevation even if the net gain in elevation is a climb of 4K feet (you'd think you wouldn't get as good of an mpg going uphill, but the thinner air makes up for it still giving you a net gain).

I've posted info like this in the past. Driving on a flat freeway near sea level will kill your gas mileage compared to driving uphill in the higher elevations. I've taken trips up to Bodie State Park which is over 8K foot elevation and I live at around the 4K foot mark. On those trips up, I've gotten the best mileage of anywhere I've ever gone (even traveling to the ocean downhill). Last time I went up there was with the BFG AT KO tires, and I still got between 22-23 mpg (that's in a 4x4 DC LB). With family living up in the mountains (above 6K feet), I learned a long time ago that no matter what vehicle we drove we always got the best gas mileage. Even my 1972 Rx2 I had, which averaged around 16 mpg, would get 18 mph on the flat highways near sea level, but I would see 20 mpg going up into the mountains.

 

^

I like your explanation on air resistance. I was thinking about that too. Explains why I've been getting better MPG when going skiing up I-70.

 

Why would less oxgen in the air cause the ECM to put more fuel to correct A/F ratio?

More likely explanation is that less oxygen in the air means the engine makes less power at any given RPM, so you step on the throttle a bit more to compensate and rev higher.

Temperature differences play a role. The gas pumps I've seen typically have a sticker that says "volume corrected to xxxx deg". On a cold day, it means the volume of gasoline that went into the tank is different than what the pump meter says.

 

There's another case in point. You lose power because the air is thinner and you're getting less fuel. Anyone that's a pilot and flies to higher elevations knows that you lose power at higher elevations. You either need a turbo or an engine with enough power to fly up to those higher elevations. This is usually taught in your first flying class (it was in mine anyway).

 

Only thing with the elevation change is that my last tank (17 mpg) was in Colorado. And when I used to live there I never saw anything above 17mpg. It almost seems to me that after I drove out here the computer corrected for new filter, then after awhile in Colorado it corrected again and gave me worse mileage?

Could the computer correct for the change in elevation (air density) to give me worse mileage? Does the computer even correct such things?

 

What's your current driving situation where you're getting the 17 now? Are you starting it up in the morning, driving to work, leaving work to go home, repeating the process until you need to fill up again? If that's the case, you're probably driving with the engine cold multiple times per tank.

I would say you probably got good mileage on your trip out because of the elevation gain and the fact that the truck was probably being warmed up at most once a day and then driven until low on fuel? You use a lot of gas when the engine is cold. The more you drive it on a single tank while the engine is warm (with only one cold start) will likely result in better MPGs as a rule of thumb whatever elevation you're at. My worst MPGs are in the winter because of the cold starts (we're on 10% ethanol year round so that has nothing to do with it here).

 

Actually while I was driving out there (highway) I was getting 15-16. But it was a mix of highway and town where I got the 18-19. Short trips as well as longer.

 

Then I'm stumped. Other than knowing that the faster you drive, the less MPGs you get. So if highway driving was greater than 60 mph, then the MPGs begin to decline the faster you go. Same goes for a lot of stop and go or just plain city driving. Now if you're doing back roads or highway in the 40-55 mph range, then that should give you the best mileage. Beyond that, I can't see anything else to affect your MPGs. Maybe someone else will have some other ideas.