Elevation mpg increase?
 

Are there are numbers for how much your mpg increases at higher altitude?

Like 2 percent every 1000 feet?

How much better mpg do you get at 7000 feet versus sea level? Wondering if there are any hard numbers

I can see how a Mass Airflow Sensor might compensate, but don't Bonneville (4,219ft) racers struggle when they go to the salt flat in Bolivia (~12,000ft)?

Too many variables, but it does seem like enough data would produce some rule of thumb.

The reduced air density means aerodynamic drag is reduced, and the reduced density is also like having a warm air intake, meaning the throttle will be open more for any given engine load, which reduces pumping losses.

My worst mpg ever was up around 7,000 to 9,000 feet.
Diesels definitely get worse mpg.

No turbo?

I don't know the MPG on 450cc dirt bikes with a carb at 14,000 ft, but power fell off a cliff.

Most modern vehicles will get better MPG though.

How and why?

Explained above. Throttle by wire combined with O2 sensors and MAP/MAF sensors means the car adjusts fueling rate to compensate for the reduced air density.

Carbs don't compensate. They are calibrated for a particular air density and do not change.

Diesels should get better fuel economy due to reduced aerodynamic drag so long as the turbo is feeding enough air to burn all the fuel. If it's rollin coal, then it's wasting fuel.

This part?

I mentioned airflow sensors. Carbs got turbochargers in WWII to compensate for altitude.

The rule of thumb would pit ICE efficiency against aerodynamic drag.

i was just asking how much drag is reduced due to air density. i know theres less oxygen too and that usually translates to less gas burned from random things ive read online too

I believe drag is proportional to density (someone correct me if I'm wrong).

This density calculator can give air density at any given elevation and temperature.

https://www.mide.com/air-pressure-at...ude-calculator

I used 1 ATM (sea level) and found that at 10,000ft density is 0.69 ATM.

I expect a 30% reduction in aerodynamic drag at 10,000 ft.

Regarding gasoline consumption in a modern vehicle; it's proportional to the amount of power needed. It takes the same amount of fuel to produce 50 horsepower regardless of elevation. The only thing the elevation does is require a wider throttle opening for any given horsepower compared with sea level, but the fuel consumed for that given horsepower is the same. The only time elevation becomes a limiting factor is at wide open throttle. You will generate more peak power at sea level than at elevation.

maybe that could explain why when im driving thru central utah at 6k feet, i get 44-46 mpg with cruise control set to 89/90mph...

This explains why all my examples involved salt flats. :)

Less air is no good for diesels. I get less boost at altitude. At sea level I hit over 20psi a few times, at 9,000ft I had trouble hitting 13psi.

Drag drops proportionally to air density, throttle losses on most engines are something like 0.8 * vacuum in BMEP, so if you have like 0.5 bar vacuum at cruise and go to 5000 feet where the vacuum drops to about 0.3 bar, you're saving 0.2 bar of specific torque.

If that used to be say 4 bar specific torque, 1 bar worth of friction and throttle loss (say half and half), and 1/4 of the energy went to rolling resistance now you only need 3.4 bar to maintain the same speed, friction is still about 0.5, but throttle now consumes only ~0.4ish, so you're looking at 10%ish less fuel.

For a shorter geared cruising gear, you might see a bigger % of throttle loss, but also a bigger % of mechanical friction loss to pair with it, so it could come out around the same.

Even with a turbo, it may eventually take longer to overcome turbo-lag at some extreme altitudes. No wonder some cars still have naturally-aspirated engines for Mexico and Bolivia even when smaller turbocharged engines are often the only ones available in other places. Even the Volkswagen T-Cross, which had always been available with the 1.0 and 1.4 TSI engines in Brazil, used to be sent to most regional export markets with the 1.6 MSI instead.

'numbers'
 

There are lapse rate 'relationships' between air density and elevation, other than 'standard air' criteria, but you shouldn't use them, except for 'qualitative' work.
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During road tests, actual meteorological conditions in the vicinity should be monitored and documented for post-test data reduction ( lots of mathematics ).
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You really need someone riding shotgun with you, so you can focus on 'driving', while the 'geek in the right seat' searches for, and records the pertinent data. Otherwise, you risk becoming the 'dead' road tester, as well as taking some innocent people down with you.
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Some online air density calculators are available, and they spell out the required data you'll need to supply in order to properly use them.
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* 'Raw' barometric pressure is a critical component ( some reported barometric pressures are 'modified' for the benefit of pilots, for altimeter calibration, conditions they'll be flying into, and are not necessarily what you need ).
* Many airports provide a phone number for the Automated Weather Observation System ( AWOS), but you better understand exactly what it is that they're actually reporting.
* Technically, you'd need the actual local acceleration due to gravity, as it varies with Earth's crust below you.
* A sling-psychrometer can provide wet-bulb and dry- bulb temperatures, which, along with a psychrometric chart can give air mass/unit volume, but you need the local acceleration due to gravity in order to do the division to produce (rho).
* NWS isobaric maps ( online in real time ) can show if you're 'connected' to any region of 'known' raw barometric pressure.
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If you ever get to Bonneville, you encounter Rick Gold's Racing Fuels, who maintains three onsite weather stations that he provides data from to the racers, which they use to calibrate their fuel systems and computerized telemetry for 'current' salt flats conditions.
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When CAR and DRIVER tested Spindletop at the Chrysler Proving Grounds they brought their own scientific-grade test equipment with them. The late Don Schroeder did all test driving while his assistant simultaneously recorded all the weather conditions. They do this wherever they do road testing, worldwide.

Unless the cars are still carburettor-fed, wouldn't the EFI be supposed to have the ability for self-adjustments in real time? Yet it's still good to know the weather conditions to have an accurate baseline to find out the impact of such variables.

EFI
 

Unless you make the mistake of throttle-stop testing, yes, the suite of sensor data to the CPU, and it's intended real-time interaction, will allow for allow for both fine adjustments for mixture and ignition timing, to keep the BSFC in it's most efficient island of efficiency.
At the Aerodynamics Forum, the other interest in local air density is, it's affect on air drag, power consumption, mpg ( mpg-e ), and range as we ecomod the body's shape.

Odd. I live at 7,700ft and would drive over 11,000ft or so passes. But I thought my non-turbo diesel got pretty good fuel mileage. I did adjust the max injection though to get as little black smoke as was practically possible. Yes, it was gutless, but I like it that way.

IIRC the governor of the injector pump had some mean to adjust fuel volume to the air density, yet not so accurately as an electronically-governed engine. BTW did you know Yanmar now makes naturally-aspirated Diesel engines fitted with common-rail injection for boats?