Experimenting with E85

Road tripping to High Performance Lubricants open house in Illinois.

August 5 fill-up.
7.3 gal E85 @ $3.10 (Hooray!)
2.8 gal 87@$4.10
Weighted cost per gallon = $3.53
Residual in tank was 54% E85. (3.0 gallons)
Total E85 in tank: 10.3 gal
Proportion E85=66%
Fuel economy of previous fill (54%) was 280.5 miles/10.1 gal = 27.8 mpg
Cost per mile = 15.3 cents

Fuel economy was close to the last tank.

August 7 morning fill-up.
7.3 gal E85 @ $2.81 (WOW!)
9.4 gal 87@$3.88
Weighted cost per gallon = $3.40
Residual in tank was 33% E85. (.66 gallons)
Total E85 in tank: 7.96 gal
Proportion E85=51%
Fuel economy of previous fill (66%) was 386.5 miles/16.7 gal = 23.1 mpg
Cost per mile = 14.7 cents

This tankful got messed up when I had to splash some gas in to get to a station with E85.
Fuel economy was horrible because of a lot of idling time.

August 7 evening fill-up.
7.1 gal E85 @ $3.52
3.7 gal 87@$4.02
Weighted cost per gallon = $3.60
Residual in tank was 51% E85. (2.45 gallons)
Total E85 in tank: 9.55 gal
Proportion E85=61%
Fuel economy of previous fill (51%) was 317.5 miles/10.8 gal = 29.4 mpg
Cost per mile = 11.3 cents

Good fuel economy to finish off the weekend. Back to commuting tomorrow.
 
Updated fuel economy chart:
1660094622337.jpg


Updated fuel cost per mile chart:
1660094829325.jpg


The sweet spot for low cost per mile is from 40-55%.
 
August 10 fill-up.
8.7 gal E85 @ $3.40
5.3 gal 87@$3.90
Weighted cost per gallon = $3.59
Residual in tank was 61% E85. (1.0 gallons)
Total E85 in tank: 9.70 gal
Proportion E85=62%
Fuel economy of previous fill (61%) was 369.5 miles/14.0 gal = 26.4 mpg
Cost per mile = 15.4 cents

Filling in the curve between 60 and 70% E85. Next fill probably in the 70-80% range.
 
August 13 fill-up.
7.5 gal E85 @ $3.30
6.1 gal 87@$3.80
Weighted cost per gallon = $3.53
Residual in tank was 62% E85. (1.24 gallons)
Total E85 in tank: 8.74 gal
Proportion E85=56%
Fuel economy of previous fill (62%) was 368.6 miles/13.6 gal = 27.1 mpg
Cost per mile = 15.3 cents

I was intending to do a fill at ~75%, but my cheapskate gene kicked in, and I decided to do a fill in the 50% range instead.
 
Filled up again today, this time with 9 gallons of regular at $3.599 for $32.39 and 13.9 gallons of E85 at $2.869 for $39.88, total of $72.27. Would have been better to just do a full tank of E85 since the price of premium is around $5.109 right now, which is about a 37% difference in price. Still, I wanted to get a tank with 45-55% alcohol and see how it does. For mileage on the last tank 233.9 miles, with the GM MPG calculator claiming 11.4 MPG. Since I filled up a total of 22.9 gallons, that calculates to 10.21 MPG, so the truck's off by about 1.2 MPG. Since my last fill up with just E85 was 8.70 MPG, that is an improvement of 1.51 MPG at 63% alcohol.

Current amount of alcohol according to the sensor on the way home was 54.9%. Left over of 2.1 at 63% (1.32), 9 gallons of 10% regular (.9), and 13.9 gallons of E85 at 80% (11.12) gives a total of 52.9%, so only a 2% difference between the sensor and the calculation. Curious to see if there will be any improvement in mileage this time, although if E85 keeps dropping in price while premium remains higher I'll probably just go back to running straight E85 for convenience.
 
LDM said:
Filled up again today, this time with 9 gallons of regular at $3.599 for $32.39 and 13.9 gallons of E85 at $2.869 for $39.88, total of $72.27. Would have been better to just do a full tank of E85 since the price of premium is around $5.109 right now, which is about a 37% difference in price. Still, I wanted to get a tank with 45-55% alcohol and see how it does. For mileage on the last tank 233.9 miles, with the GM MPG calculator claiming 11.4 MPG. Since I filled up a total of 22.9 gallons, that calculates to 10.21 MPG, so the truck's off by about 1.2 MPG. Since my last fill up with just E85 was 8.70 MPG, that is an improvement of 1.51 MPG at 63% alcohol.

Current amount of alcohol according to the sensor on the way home was 54.9%. Left over of 2.1 at 63% (1.32), 9 gallons of 10% regular (.9), and 13.9 gallons of E85 at 80% (11.12) gives a total of 52.9%, so only a 2% difference between the sensor and the calculation. Curious to see if there will be any improvement in mileage this time, although if E85 keeps dropping in price while premium remains higher I'll probably just go back to running straight E85 for convenience.
Click to expand...
That increase of 1.5 mpg is huge on a percentage basis: 17%.
Based on my experience with the Buick, the sweet spot for E85 content is ~50%, so your new tank should be better again, but by not as much.
 
August 18 fill-up.
7.0 gal E85 @ $3.24
7.3 gal 87@$3.74
Weighted cost per gallon = $3.50
Residual in tank was 56% E85. (.73 gallons)
Total E85 in tank: 7.73 gal
Proportion E85=50%
Fuel economy of previous fill (52%) was 383 miles/14.3 gal = 26.8 mpg
Cost per mile = 15.5 cents

I'm settling down now at 50% E85 as my "go-to" fill to minimize fuel cost per mile. I should do a couple of fills at 75% in the future, and some Premium-only fills to check my assumed baseline.
 
August 22 fill-up.
5.7 gal E85 @ $3.14
6.5 gal 87@$3.64
Weighted cost per gallon = $3.43
Residual in tank was 50% E85. (1.75 gallons)
Total E85 in tank: 7.45 gal
Proportion E85=47%
Fuel economy of previous fill (50%) was 322.4 miles/12.2 gal = 26.3 mpg
Cost per mile = 15.7 cents

Worst fuel economy I've gotten at 50% E85. My car had been running intermittently good and bad. Sometimes it wouldn't build boost, other times it would. No check engine lights were lit, but I checked it with my code scanner, and it reported a cam sync sensor error. I cleared it, and the car ran fine after that. Will look out for that in the future.
 
August 25 fill-up.
6.5 gal E85 @ $3.26
7.25 gal 87@$3.76
Weighted cost per gallon = $3.51
Residual in tank was 47% E85. (.88 gallons)
Total E85 in tank: 7.38 gal
Proportion E85=47%
Fuel economy of previous fill (47%) was 373.9 miles/13.75 gal = 27.2 mpg
Adjusted Cost per mile = 15.2 cents
 
August 29 fill-up.
9.4 gal E85 @ $3.40
2.6 gal 87@$3.90
Weighted cost per gallon = $3.51
Residual in tank was 47% E85. (1.7 gallons)
Total E85 in tank: 11.1 gal
Proportion E85=71%
Fuel economy of previous fill (47%) was 320 miles/12.0 gal = 26.7 mpg
Adjusted Cost per mile = 15.4 cents

Going to do a couple tanks in the 70% range.
 
September 1 fill-up.
9.4 gal E85 @ $3.36
2.6 gal 87@$3.86
Weighted cost per gallon = $3.48
Residual in tank was 71% E85. (2.56 gallons)
Total E85 in tank: 12.0 gal
Proportion E85=77%
Fuel economy of previous fill (71%) was 354.4 miles/12.0 gal = 29.5 mpg
Adjusted Cost per mile = 13.5 cents

This tankful at 71% returned incredible fuel economy. Far beyond what I expected. It's the third-best reading since I started running E85 blends. 29.5 mpg is about 10% better than my previous tank at 47%. The previous tankful at 75% was 25.0 mpg, and the previous tankful at 68% was 26.4 mpg.
But the car is not doing as well on the new tank at 77%. It has given fault codes for lean O2 sensor and MAP sensor out of range, and I have had a couple of occurrences of hesitation and surging. Every time I get close to 80% E85, I get lean O2 readings.
 
Last edited:
Ethanol has vapor pressure inflection points when mixed with gasoline.
This are somewhat dependent on temperature.

When you are near an inflection point your economy goes up dramatically above what you would expect
 
Rmay635703 said:
Ethanol has vapor pressure inflection points when mixed with gasoline.
This are somewhat dependent on temperature.

When you are near an inflection point your economy goes up dramatically above what you would expect
Click to expand...
How applicable do you think that is in a direct-injection engine?
 
September 2 fill-up.
6.0 gal E85 @ $2.79
3.5 gal 87@$3.79
Weighted cost per gallon = $3.28
Residual in tank was 77% E85. (4.7 gallons)
Total E85 in tank: 10.7 gal
Proportion E85=68%
Fuel economy of previous fill (77%) was 221.4 miles/9.5 gal = 23.3 mpg
Adjusted Cost per mile = 16.9 cents
The 77% tank fill was plagued with engine fault codes from the start. Intake cam position, lean O2 sensor, and MAP sensor out of range faults all appeared on my scanner, but the fault light never illuminated on the dash. Drivability was horrible as the engine had a noticeable vibration at idle, and throttle hesitations, and no turbo boost. I bought a new cam position sensor and installed it, and the fault codes have not returned. I attribute the poor fuel economy to the cam sensor failure that colored 160 of the 220 miles driven on the tank fill, as well as the high E85 content.

Sept 3 fill-up.
5.7 gal E85 @ $3.60
5.5 gal 87@$3.96
Weighted cost per gallon = $3.64
Residual in tank was 68% E85. (3.0 gallons)
Total E85 in tank: 8.7 gal
Proportion E85=56%
Fuel economy of previous fill (68%) was 290 miles/11.2 gal = 25.9 mpg
Adjusted Cost per mile = 12.4 cents
Engine is running fine now. Filling up with cheap fuel in East Lansing really helped the Adjusted cost per mile! Couldn't find cheap fuel in Indianapolis, though.

Sept. 4 fill-up.
5.0 gal E85 @ $2.90
3.6 gal 87@$3.30
Weighted cost per gallon = $3.32
Residual in tank was 56% E85. (3.9 gallons)
Total E85 in tank: 8.9 gal
Proportion E85=57%
Fuel economy of previous fill (56%) was 242.1 miles/8.6 gal = 28.2 mpg
Adjusted Cost per mile = 14.9 cents
Fuel economy was good for this tank, but the high fuel price in Indianapolis increased Adjusted cost per mile.
 
Interesting thread, glad to see other folks experimenting to see how varying levels of ethanol affect their cars.

This is 5 years of running varying amounts of ethanol in my FlexFuel 5.3 engine.

1662397249749.png


Sunoco classifies their E85R as 99 octane, so that is what I use when making octane rating calculations which closely coincides with the octane ratings assigned to E15, E25, E30, etc. Such is always changing, or was the last time I looked.
 
swathdiver said:
Someone mentioned earlier that E85 only goes up to 83% alcohol content. The other 2% is to make the fuel inedible. I forget the name of the additives.
Click to expand...
I read somewhere else that they put in 2% regular unleaded gasoline to denature the pure ethanol before blending to make E85.
 
A_Harman said:
I read somewhere else that they put in 2% regular unleaded gasoline to denature the pure ethanol before blending to make E85.
Click to expand...

Page 3 or the Introduction explains it, you are correct in most instances. E85 made at the refinery is usually blended with 85 octane gasoline. At my local Sunoco stations, they blend E85 with the 87 octane tank(s) (If they remember to turn on the pumps!) to make E30 and other blends.
 
You must log in or register to reply here.

Similar threads

TiGeo
93 vs. E blend (E22-ish) - logs
Replies
19
Views
1K
shortyb
Revving2Redline
Just the right amount of octane & ethanol
Replies
2
Views
427
Revving2Redline
D
Shell vs Mobil vs Chevron vs ARCO/Costco/76 etc Which gas is “best” or clearly better?
Replies
7
Views
10K
DraganMitic
L
Suggestion for DI cleaner for e85 fuel.
Replies
5
Views
2K
Kalle.J
UpNorthSooner
Traveling and can't always fill up with Top Tier Fuel
2 3 4
Replies
74
Views
5K
nomas
Back
Top