Hey Guys,

I have a 2013 SR5 with the 6 cylinder(100k miles) and have been living in the mountains of Colorado for the past 7 years. I usually reset my gas mileage once a year, or before extremely long road trips, etc. There is obviously not near as much stop and go in the mountains, but I typically would get ~21mpg over several months of usage at around 8000'. I just moved to sea level(rural) and and noticed my gas mileage get much worse(17mpg)and do not think it is just because there are more stop lights/accelerating around here, etc. Thinking that gas engines should typically be much more efficient at sea level than altitude, I checked the air pressure in the tires and waited until I took a road trip down for another comparison. Even with correct tire pressure and same weight in the 4runner, etc., I am only getting 18-19 on 200 mile interstate road trips when driving by myself and used to get 23-24 on 200 mile road trips. Anyone have any ideas? It really feels like it has changed outside of normal difference of highway/city drops, etc.

First thing I am going to suggest and ask is have you done all maintenance ? Air filter? Spark plugs? Oil change and filter ? Etc

Mountains = more coasting down hill compared to flat ground.
Also thinner air at 8000 feet so the truck calibrates how much fuel it delivers based on how much air the engine is sucking in.
But I bet your truck seems to have a good bit more pep at sea level.

Actually, all things being equal, gas engines with EFI theoretically operate slightly more efficiently at higher altitude because pumping losses are reduced. The engine makes less peak power, but for a given power output at part throttle, it operates at larger throttle openings. That means it doesn't have to work as hard to squeeze the air it's inhaling past a mostly closed throttle plate.

Also, because the air is thinner at high altitude, there is less wind resistance -- fewer air molecules means less force pushing against the truck at a given speed. This is probably a bigger factor for MPG than the pumping losses.

In any case, your sea-level fuel mileage is right in the normal ballpark. There are any number of MPG threads on this site that you can use for comparison purposes.

Google "do cars get better gas mileage at higher altitude" to find lots of hits and some conflicting information because many of the sources are anecdotal.

Two factors are in effect at higher altitudes, (1) throttle loss, and (2) aerodynamic drag. Both effects are reduced, which produces better fuel efficiency if the engine is in tune and the vehicle well maintained.

See MIT School of Engineering | >> Which engine is better at high altitude: diesel or gasoline?

See ResearchGate

The internal combustion engine is an air pump with an inlet air throttle. Higher altitude implies less dense air and lower power. Driver unconsciously compensates by depressing accelerator pedal for more power, which opens the throttle plate, reducing the inlet loss, thereby contributing to better air pumping (air through-flow) efficiency.

Lower air density at altitude reduces aerodynamic drag. A 4Runner has higher aerodynamic drag than most passenger vehicles, so it is likely to offer more improvement at altitude.

(I began to type a reply before yotantexas posted his reply. I was interrupted and finished my post after TEujunga's response. This explains why there is duplicate information in my reply.)

I have a 13 and I would get mpg in the low 14s on the Front Range.

very conflicting stuff..

at higher altitudes means thinner air which equals to less aerodynamic drag which is a plus for mpgs... however higher altitude also means less oxygen which means a less efficient combustion for our engines which should decrease mpgs

however when humidity comes into play at higher altitude the air "thickens" again and introduces turbulence which is the same idea of a shaky turbulent airplane flying through clouds rather than clear skies..

has the weather been wet and humid where you've been driving? Since you moved to a place with sea level, what material are the roads? concrete? blacktop? I've read that the type of surface can affect mpgs as well..

Sounds normal to me. I average 16-17 around town, and around 21 if I’m going 55-60. At actual highway speeds I’ll average ~17. That’s how it’s been since the day I drove it off the lot with 1 mile. I’ve never seen 23-24 in normal driving.

Nothing "conflicting"... just incorrect

All modern EFI engines with O2 sensors automatically compensate for changes in oxygen content due to altitude or temperature when operating under closed-loop conditions, such as at part-throttle cruise. Assuming the EFI system is working right, combustion efficiency remains the same under all conditions of air density, humidity, and altitude.

The stuff about humidity making air thicker and more turbulent is... how shall I say it... fake news?

Humid air actually has less oxygen content because the water vapor molecules displace air molecules. Cool, dry air is the densest and has the most oxygen available for burning more fuel and thus making more power, which is why racers love it.

Humidity doesn't directly cause turbulence. Airplanes flying through clouds encounter turbulence because of rising and falling air. Rising humid air condenses to form clouds. The amount of turbulence is dependent on the air movement, not the humidity.

Honestly, this is all pretty basic science easily available to anyone willing to learn it rather than make stuff up.

This is interesting information about MPG at altitude. I calculate my gas mileage for each tankful for comparison. It gives me a better idea of what’s going on. The onboard meter is an average between resets. It smooths out the wide swings you get from various driving conditions over time.

I’m wondering if there’s a difference in the OP’s fuel situation at the lower elevation. In some high elevation areas the fuel is 85 octane for regular, for example. Also if there’s a winter blend formulation change you just encountered that could affect gas mileage.

I have no idea how people see these numbers. I live in Dallas and at best get 15. During a trip to Moab I got 19.

This might happen more or less "on the fly" with the computer, but I would maybe try resetting the ECU for fuel/air mixtures. Something like disconnecting the battery for 20-30 minutes does it on most cars.

IMO: Above 21mpg Hwy on a stock vehicle needs perfect conditions. No headwind, moderate temps, light traffic, less than 75mph.
Below 20mpg Hwy usually means headwind, above 70-75 speeds, heavier traffic with lots of heavy throttle use to pass or keep up.
I can get around 23mph on back highways where traffic is light, temps are moderate, no headwind, 60-70mph
A/C use does make a small hit but I’ve really not been able to accurately calculate. Some of my best mileage has been with A/C on.

x2 on that! with one more addition, thinner air = less resistance, consequently lees fuel needed.

To OP: I leave in DFW (Texas) area, but have made many many trips to mountains (Colorado, New Mexico, Utah, etc.)... every time elevation goes up so does my MPG... on the average in the elevation of 5000-7000 feet my MPG is about 2-3 mpg better vs. my home (flat land)... and I have seeing very similar result with every 4Runner I had in the past (2nd, 3rd and 4th gen).

Higher elevation having less air pressure it takes a lower compression to ignite gasoline, lower octane(85) gas can be used which intern reduces the power of the motor engine needs to work harder but requires less gas.