Big increase in mpg since switching to ethanol-free gas

Only explanation I have for a large (more than, say, 10%) change in mpgs is that there’s some component in your system sensitive to ethanol causing the problem. It’s not related to how it burns, it’s causing some malfunction in your engine’s operation. E10 has about 3% less energy than E0.

Did you replace anything on the vehicle when your mileage improved?
This is from you other thread.

"Here’s what I’ve done/changed recently (i.e. within the past 5k miles):

Fuel pump, OEM installed by dealer.
Fuel filter.
Ignition control module, BWD.
Ignition coil, BWD.
Crankshaft position sensor, GM OEM.
Engine coolant temp sensor, BWD.
Oxygen sensors (all four), GM OEM.
Catalytic converter, replaced one of the two.
Plug wires, BWD.
Plugs, Delco OEM.
Distributor cap and rotor, BWD.
Cleaned ground connection (bolts to frame) for fuel pump.
Alternator, new not reman.
PCV valve.

Hey texases,

Whatever it is, no more E anything is going into the tank. With such a huge drop in mpg with the E-10 in it, something tells me it can’t be that good for my Burb. Thanks for the input.

Hey Nevada 545,

The change in mpg began with the first tank of the eth-free. Because I’m a dork and to make sure the increased mpg wasn’t due to the things I was in the process of replacing, I ran a tank of E-10 through it a while back to make sure I wasn’t misleading myself. MPG dropped to 10 again. Next tank was eth-free and mpg went right back to 14. I can’t explain why. It just is, as crazy as it might be.

Maybe someone will read this at some point so I figured I’d jot it down. It might be the cause behind the oddly huge increase in my mpg when using eth-free gas. I’m going to find out.

Increasingly sluggish acceleration due to what sounded like too rich of a mixture under heavy acceleration led me to swapping out the MAP (manifold absolute pressure) sensor on my Burb. That was the problem with the acceleration. The Burb gets up and boogies now. Plus the cold air intake sounds right again (I love that throaty sound).

And now I’m wondering if the MAP sensor going out (though it never caused an engine code to be stored) could be behind the eth-free vs. e-10 difference.

I’m going to try another tank of e-10 to see what mpg I get. My hunch is that with the new MAP sensor in there, my mpg will see at least a slight increase regardless of which of the two fuels I put in there. Whether the Burb will still get 40% better mpg on the eth-free is what remains to be seen. And I’m going to find out. Then I’ll post the result in case it matters to anyone down the road.

Like I said before, I’m still convinced that there’s some other reason, aside from the E10 itself, that the mpg dropped so badly. I’ll be interested to hear your results with the new MAP sensor.

Okay, so I have had a chance to run several tanks of both the eth-free and the E-10 through again (after I’ve replaced several of the very weathered sensors that were OE). Without fail, the eth-free gives me 14 mpg EVERY tank, but the difference between the two fuels is now MUCH less.

The E-10 best has been 13.2 mpg, arguably a very minor difference. But my average over 6 tanks of E-10 has been 11.7. The average over 5 tanks of eth-free has been 13.95 mpg (it truly hangs right at 14 for every tank).

Without a doubt, one or more of the sensors I replaced was key to a 40% difference in mileage between the two fuels.

Based on my math and the extra I pay for eth-free around here, running the eth-free nets right at a 10% savings. But then, factor in that I seriously doubt that the eth-free has all the nice cleaners that the Shell E-10 I’ve used when not running eth-free, so I wind up picking up some bottles of Techron when it’s on sale, I’d bet it’s more of a wash between the two.

I’ve stuck with the eth-free, only because it doesn’t look like I have a garden hose pouring water from my muffler when I’m running it. Maybe it doesn’t hurt anything, but something about water pouring from my tailpipe all the time just doesn’t sit well with me. It’s an exhaust pipe, not a water feature.

For what it’s all worth, at least for my '96 Burb, running E-10 or eth-free costs roughly the same.

Maybe this will be interesting to others who are just as dorky as me.

Water from the tailpipe has nothing to do with ethanol in the gas, unless it somehow makes the exhaust much cooler.
The primary products of burning gasoline or ethanol are water vapor and carbon dioxide.
Every gallon of gas burned produces about a gallon of water.

Hey circuitsmith,

Thanks for chiming in. Ethanol doesn’t produce more water vapor than ethanol-free gas?

I’m not being a smart-aleck. I was under the impression that ethanol produces more water, but I’m no gas expert. I’m actually quite ignorant on the subject of gas.

If you don’t mind me asking, it seems impossible to burn a gallon of gas and wind up with a gallon of water. How can that be? If you start with a gallon of gas and you end up with a gallon of water, it seems nothing could be consumed by the combustion process for that to be true.

Again, not being a smart-aleck. Just curious.


Is your Suburban a flex fuel vehicle? I believe this might contribute to the difference.

Flex fuel vehicles are designed to run on any ethanol mixture between E-85 and straight gasoline. Being capable of running on a wide variety of fuels, I’ve often wondered if this means they run more efficiently on these various mixes or if making an engine that can adjust to different fuel mixtures requires sacrifices in performance.

It could be that the loss of mileage on ethanol is due to the burn characteristics of higher octane/slower burning ethanol.
Ideally, the peak of the explosion in the cylinder should occur when the piston is right around TDC. With slower burning ethanol the peak may be occurring as the piston is well on its way down on the power stroke and that’s why the loss of mileage.

Programming some additional timing advance into the ECM could offset that by moving the peak back.
Of course that brings up the issue of possible pinging on low octane regular gasoline if you revert back to that. Pinging could be even more prevalent if the EGR system is not functioning as it should.

Well nothing is actually done away with in the combustion process ,a lot of the the exhaust products are a result of the fuel air mix ,combusting and the hydrogen combing with atmospheric oxygen to produce H2O ,amongst other combustion byproducts.The engine consumes a lot more air by volume then fuel-Kevin

Well let’s see…

Complete combustion of ethanol:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O
Using molar weights:
46 grams of ethanol + 96g of oxygen produce 88g of carbon dioxide + 54g of water
That’s 1.17 lb of water per lb of ethanol
Accounting for ethanol’s density (79% of water) it’s .92 gal water per gal of ethanol.

Complete combustion of octane (for simplicity’s sake):
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O
228g of octane + 800g of oxygen produce 704g of carbon dioxide + 324g of water
That’s 1.42 lb of water per lb of octane
Accounting for octane’s density (70% of water) it’s .99 gal water per gal of octane.

It’s a tie, IMO.

It’s actually more like 1-1/2 gallons of water per gallon of gas.

Think of it this way: gasoline is a hydrocarbon, roughly 1/2 hydrogen atoms bonded to roughly 1/2 carbon atoms. Air is roughly 22% oxygen. During cumbustion, th ehydrogen and the carbon atoms in the gas are torn apart. The carbon atoms bond to the oxygen atoms and form mostly carbon dioxide and carbon monoxide. The hydrogen atoms largely bond to oxygen atoms and form H2O…water (in vapor form).

So, for every gallon of gas, 1/2 of it (hydrogen) bonds to twice its own volume of oxygen (from the air) and the combination comes out as water.

Remember that the water comes not from the gasoline, but from the combination of the gasoline and the air. While one gallon of gas cannot by itself produce more than a gallon of water, 16 gallons total of GASOLINE PLUS AIR can and does produce more than a gallon of water.

Does it make more sense now?

“It’s actually more like 1-1/2 gallons of water per gallon of gas.”

Maybe if you take into account the moisture content of the incoming air, which I did not.

Good job mountainbike

Thanks Joseph.

Circuitsmith, my attempt was to present the explanation in a way easily understandable to those less versed in science. Getting a gallon or more of water out of a gallon of gasoline seems at first blush to defy the laws of physics. It’s only when one realizes that the gallon of water comes from a conbination of the gasoline AND the air, and that each gallon of gasoline is combiined with roughly 15 gallons of air, that it starts to seem feisable.

The moisture content of air, the density of the air, the impurities and the additives in the gas, etc. all matter to the exact amounts, they all matter to the scientist, but they can confuse those without the proper background.

I just changed gas STATIONS and get better mpg !

The average mpg messege screen had been indicating a consitant 13.7 or so mpg over the past years and the math proves that out when I do that once in a while.
– ( no stations in this town are known to sell ethenol free gas. ) –
I always fill up the the same station or two.
As the prices fluctuate you can see the delay from station to station and if you play your cards right you can catch the lower price before they raise it too.

Then one day my stations were a whole ten cents higher that this other one I had been avoiding for its assumed reputation for being the el-cheapo place and putting anything in their tanks from anyone who had a partial load to sell.
— 14.6 indicated mpg on the first tank !..??? could that be right ?
so I reset the trip odometer next tank, bought at the same place and did the math.
YES, I’m getting better mpg from this station.
The math on last weekend’s Albuquerque shopping trip shows 15.29 mpg with the messege center average showing 14.8 now.

I wonder if, even though a label on the pumps says up to 10% ethenol, are they selling more pure gasoline ?


I can appreciate you addressing yourself to the layman, but…
The first line of your post came across as dismissing my analysis with the wave of a hand.

If you really think I’m off by 50% then show your work!
Otherwise you could have left that first line out.

What I did was high school chemistry.
I bet most of us here didn’t sleep through that.

“I just changed gas STATIONS and get better mpg !”

I posted something to that effect here a few years ago and I was burned to the ground.
I’m really tired of fighting the dogma.