My average MPG, New York to California

Hi All,

I just drove 2850 miles from New York to California.

My 2018 CX9 GT AWD (which now has 18K miles) was fully loaded with a family of five (and our gear). Also, Thule roof bars (Aero blades) installed (without any kind of rack / box).

Average mileage was 19.2 MPG. For a lot of the trip we were 10-15 over the speed limit, so cruise was set at 80-90 MPH for a lot of the trip west of Chicago.

I almost always used Shell 93 in New York. There were a few stops in the mid west and Wyoming where I could not find anything but 87 Octane gas (at one station, options were regular or ethanol free regular -- I tried the ethanol free). When I filled up with 87, milage per tank dropped to below 17MPG.

Overall, very impressed with the CX9. Only thing that is annoying is trying to use Apple CarPlay and XM Radio at the same time.
 
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Mazda CX-9 Signature
I think that's pretty good average mpg considering car was fully loaded and the speed you were traveling. I'm curious to see how dirty your car got after the trip lol
 
Yeah, not bad all things considered. Interesting to see the impact of the 93 vs 87 oct.

Bought a Thule Motion XT XL while in CA. Will be interesting to see how it impacts the MPG!
 
Interesting info. Hard to understand why the octane rating had such an impact on mpg. The higher octane only affects (increases) hp and torque above 4000ish rpm, so at highway speeds and rpm, the octane shouldn't affect the power and therefore mpg. An I missing something?
 
Interesting info. Hard to understand why the octane rating had such an impact on mpg. The higher octane only affects (increases) hp and torque above 4000ish rpm, so at highway speeds and rpm, the octane shouldn't affect the power and therefore mpg. An I missing something?

Agree. It could have also been the no-name, ethanol free gas. I'll try a couple of tanks of Shell 87 on the right back east to determine if it's the octane or the weird farmer gas. :)
 
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Ft Worth
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2021 CX9 Signature
Higher octane fuels typically make MPGs better. This is pure physics and typical across most engines. With higher octane fuel, each piston stroke receives more “push” as there’s is more energy in that explosion. As a result the engine has to work less to create the same hp. Thus, better MPG.
 
Sorry but I don't think that's correct. The energy content in gasoline is constant, whether 87 or 93 octane. Octane rating is just an indicator of resistance to knocking.
Car & Driver tested 87 to 93 on 4 vehicles and saw essentially no impact on mpg; in fact mpg went down (insignificantly) on one.
The FTC says "In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage, or run cleaner."
WRT the CX-9, the anti knock benefit is only seen above 4000 rpm, when the power and torque are increased with 93, but at highway speeds, the engine never gets anywhere near 4000 rpm.
Non ethanol fuel actually has a higher btu/lb than with E10 so one might expect a slightly better mpg without ethanol.
 
I did some checking.
Non ethanol gas: 114,000 btu/gal
E10 : 111,836 btu/gal
Also, various parts of the country use "winter" gasoline formulations that are even lower in btu/gal - maybe that's what the OP saw. Maybe NY gas was summer while the others were winter fuels?
WRT the energy content of 87 vs 93, I found many anecdotal references that stated that 93 actually has slightly less energy than 87 due to the antiknock additives in the 93. A tiny change but it confirmed to me that there should be no noticable difference in mpg with the octane rating, unless it's the winter fuel issue.
 
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2021 CX-5T AWD CE
Higher octane fuels typically make MPGs better. This is pure physics and typical across most engines. With higher octane fuel, each piston stroke receives more “push” as there’s is more energy in that explosion. As a result the engine has to work less to create the same hp. Thus, better MPG.
The available data on whether higher octane, by itself, improves fuel mileage is extremely murky and highly engine-tune dependent, but from a “pure physics” standpoint, this is completely wrong as higher octane fuels do NOT have “more energy”.

Any ethanol-laced fuel does have less energy than pure gasoline, in direct proportion to the percentage of ethanol. E10 for example has about 3% less energy than pure gas and everything else being equal, should get about 3% poorer fuel mileage. But this has nothing to do with octane.

- Mark