Impact of minor changes in elevation on fuel economy

[Bill Murray]

Here's a question for the big brains in the room.

Is there a demonstrable impact on fuel economy if there's a negligible change in elevation?

Scenarios:
a) Driving 1200 miles and there's a net increase in elevation of 250 feet. So for every mile driven you rise 0.208 feet.
b) Driving the same route but in the opposite direction, 1200 miles but there's a net decrease in elevation of 250 feet. So for every mile driven you fall 0.208 feet.


Scenario a:

Obviously if there's a marked increase in elevation as you drive your fuel economy is going to suffer but what about minor change?

 

[wiggins]

Here's a question for the big brains in the room.

Is there a demonstrable impact on fuel economy if there's a negligible change in elevation?

Scenarios:
a) Driving 1200 miles and there's a net increase in elevation of 250 feet. So for every mile driven you rise 0.208 feet.
b) Driving the same route but in the opposite direction, 1200 miles but there's a net decrease in elevation of 250 feet. So for every mile driven you fall 0.208 feet.


Scenario a:
View attachment 710491

Obviously if there's a marked increase in elevation as you drive your fuel economy is going to suffer but what about minor change?

the altitude differences are probably too small to make a meaningful difference and would be below the limits of accuracy.

extensive testing of aircraft engines at altitude in the 1930's and '40's and in particular the testing and use of turbo super chargers is where your answer probably lies...

 

[DeathMetalWife]

Yes.

 

[Prince 👑 Harming]

Yes.

your dad knows the answer to this question

 

[Grand Mal Caesar]

It would be roughly around 6.

 

[Gone crazy back soon]

There is a difference in fuel economy at altitude experienced by engines aspirated by a carburetor in which the fuel/air mixture is effected by barometric pressure. The difference occurs in a much greater change in altitude more like +/- 1,000 feet elevation. In fuel injected engines and engines using forced induction such as turbo chargers with or without intercoolers or super chargers there is no effect from barometric pressure because the intake air pressure is regulated.

 

[Bill Murray]

the altitude differences are probably too small to make a meaningful difference and would be below the limits of accuracy.

That was my original assumption. However one of my regular drives I'm convinced that I get better mileage on the way there than on the way home. There are a number of constants:
-Its a divided highway that I'm going to assume the blacktop is relatively similar in both directions. There are a few hills but they seem to balance out, meaning that on the whole you never feel like you've either gone higher or lower.
-Vehicle is going to be within 20lbs gross weight in either direction. Always start the trip in either direction with a full tank of gas (including the same grade of fuel)
-While wind can influence fuel economy I've tried to observe my results while there hasn't been a significant (i.e. >5mph) head, tail or side wind.

Trip is 200 miles in either direction with a net change in elevation of 2000', down on the outbound trip or up on the return trip. That means the net gradient of ~0.20% (I think I did ciphering correctly).

I'm disappointed in myself for not tracking this more closely given I've probably done this drive >1000 times over the past 30 years. I'm going to have to start.

Yes.

Are you being a smart ass or do you know of what you speak?

There is a difference in fuel economy at altitude experienced by engines aspirated by a carburetor in which the fuel/air mixture is effected by barometric pressure. The difference occurs in a much greater change in altitude more like +/- 1,000 feet elevation. In fuel injected engines and engines using forced induction such as turbo chargers with or without intercoolers or super chargers there is no effect from barometric pressure because the intake air pressure is regulated.

Altitude at home is ~ 4100', at destination is ~2000'. Its a fuel injected Ram 1500 (clean air filter, regular oil changes, no aftermarket mods).

 

[Gone crazy back soon]

With a fuel injected naturally aspirated engine altitude should not change your mileage because the engine's computer should be adjusting the engine on it's own. Any change you are noticing over a 2,000 foot elevation change is more likely to be accounted for by the engine needing to work harder to drive uphill than downhill.

 

[Bill Murray]

With a fuel injected naturally aspirated engine altitude should not change your mileage because the engine's computer should be adjusting the engine on it's own. Any change you are noticing over a 2,000 foot elevation change is more likely to be accounted for by the engine needing to work harder to drive uphill than downhill.

Sorry, I probably wasn't clear. I'm not so much concerned about the fuel economy while driving at a particular altitude vs another. Its that even though it doesn't feel like I'm going up or downhill there is a 2000' difference between the start & end point.

 

[Grand Mal Caesar]

If you happen to be traveling west, depart while the moon is post zenith to take advantage of its gravitational pull for a slight boost in fuel economy.

 

[mrln]

are you talking carburetor (2 or 4 barrel). because you are able to adjust the air fuel mixture to get maximum performance without interference.

 

[Bill Murray]

are you talking carburetor (2 or 4 barrel). because you are able to adjust the air fuel mixture to get maximum performance without interference.

Fuel injected. And again, I wasn't clear. Question wasn't about fuel economy at different elevations.

If I'm driving up a hill with a gradient of 10% for a distance of 20 miles clearly I'm going to use more fuel vs a flat road/0% gradient. But what if I'm driving over a long distance and there's a change in elevation but its a relatively low gradient (0.2%) does that impact mileage.

I think I need the mythbusters.

 

[mrln]

Fuel injected. And again, I wasn't clear. Question wasn't about fuel economy at different elevations.

If I'm driving up a hill with a gradient of 10% for a distance of 20 miles clearly I'm going to use more fuel vs a flat road/0% gradient. But what if I'm driving over a long distance and there's a change in elevation but its a relatively low gradient (0.2%) does that impact mileage.

I think I need the mythbusters.

i would tend to think so. even with cruise control engaged. your vehicle is fighting gravity id think.
are you driving against the wind or is it at your back. also,what are you driving? i ask only because if its lower to the ground or had ground effects it makes a dif. if your driving a truck,theres more resistance.
theres a lot to think about before you go on your trip.

 

[Chief Queef]

I had an idiot friend who insisted on speeding and then throwing his vehicle in neutral to coast for a bit to "conserve" gas. This stopped sometime later when we were coming back from a skate park and he accidently threw it into reverse going 80mph. Fun times...

 

[Bill Murray]

i would tend to think so. even with cruise control engaged. your vehicle is fighting gravity id think.
are you driving against the wind or is it at your back. also,what are you driving? i ask only because if its lower to the ground or had ground effects it makes a dif. if your driving a truck,theres more resistance.
theres a lot to think about before you go on your trip.

Wind, it depends on the trip.

Vehicle, Ram 1500 5.7l

I had an idiot friend who insisted on speeding and then throwing his vehicle in neutral to coast for a bit to "conserve" gas. This stopped sometime later when we were coming back from a skate park and he accidently threw it into reverse going 80mph. Fun times...

I went to uni with a muppet who did something similar but for different reasons. He'd drive at >30kmh over the speed limit for a while, then 10kmh under the speed limit for a while. He'd do this because he thought it was an effective tactic to avoid getting a speeding ticket. He was, I assume still is, just a stupid mother fucker.

 

[billiam]

My understanding is more about cars and trucks.

It is not until you get to around 10,000 feet which milage is effected. You then lose about 10% of fuel efficiency and 10% of towing capacity. This is due to the lack of oxygen which requires your engine to burn more fuel.

 

[mrln]

then again,the computer in your engine should make adjustments for it all

 

[btmboi]

all depends on engine load.