So after reading threads here and researching the E0/E10 thing, I decided the only way to REALLY know the MPG difference is to test it myself. The EPA and other sources claim a mere 3% MPG loss with E10. Yet I've read threads here and other sources claim close to 10%.

So, I decided to track (and post) the ongoing results of my experiment. I'll update as the fuel burns and the miles roll.

METHOD
I will track and compare 5 tanks of Regular (only) E0 and E10. I see no advantage to a higher grade given these bikes simply aren't equipped to take advantage of higher anti-knock (octane) ratings.

My preferred retailer by far is QT as they are (and were one of the first) "Top Tier" fuel partners. They also have a very nice "Guaranteed Gasoline" policy. Read more about both here:

https://www.toptiergas.com/
https://www.quiktrip.com/Gasoline

Also, my son works for QT in Austin, so...

My secondary source is Spinx, a regional retailer. Note they are not a Top Tier provider and offer only LAC fuels.

Looking for Pure Gas/E0?
https://www.pure-gas.org/

Although my rides won't be carbon copies, all will include my reasonable (to me) riding style and (legal) speeds. The bike is equipped with an OEM windshield and aftermarket saddlebags. Some tanks will include 2-up miles with adjusted rear tire pressure. All rides use the recommended tire pressure for loading (1 or 2-up). All rides will include a mix of city highway driving at no more than 60 MPH (where legal).

EXPECTATIONS
I highly question the 3% narrative given by the EPA, major automotive makers, fuel companies, and most of all, the main stream media. I expect to see an approximate 10% difference in mileage, perhaps more, as well as increased drivability and/or throttle response.

COST PER MILE
Sadly, the norm here is an increase of $0.50 per gallon for E0 although I have found a few "unbranded" retailers only charge $0.20 more. I guesstimate the breakeven point to be around a $0.25 difference.

The first tank really doesn't count as the OEM petcock was leaking unmetered fuel to the intake. This was also a mixed tank of E0 and E10. I provide it only for general comparison of a poor running v. great running (petcock fixed) engine. Note this was with a fresh spark plug and may've included a full can of Seafoam.

100 miles @ 2.4 gallons = 41.6 MPG

Today's fill was the first reliable MPG on E0. I had to drain the tank (twice) on the previous fill to bypass Suzuki's engineering and get their vacuum petcock to work while waiting on the Raptor valve. This includes new and properly inflated IRC's, fresh Red Line 10w-40, a K&N drop-in, and a cleaned NGK Iridium. About half of these miles were 2-up.

Pure Gas (E0)

1. 108 miles @ 1.9 gallons = 56.8 MPG
Although a huge increase of 36%, this is really my baseline.

Not bad considering Wiki lists the average as 50 MPG cited from 2006 Motorcycle Consumer News tests: http://www.mcnews.com/mcn/technical/JAN06Pindx3.pdf]https://web.archive.org/w...[/url]

Also note MCN reports the wet weight as 390 pounds. My Owner's Manual reports 352 if I remember correctly.  

I also found this 2008 LA Times article interesting regarding motorcycle MPG and the EPA. Scroll to the sixth paragraph and you'll see Suzuki claimed 54.8 MPG for the S40. Interesting article. I'm curious how they got their figure and what fuel, tire pressure, loading, etc. was used.

https://www.latimes.com/news/la-hy-throttle2apr02-story.html

Regardless, I beat their "official" figure.  

2. 87 miles @ 1.5 gallons = 58.0 MPG
No 2-up miles; essentially the same combination of city/rural riding.

3. 122 miles @ 2.5 gallons = 48.8  MPG
No 2-up miles; higher speeds with about half the miles on rural 4-lanes or interstate. Cut this one kinda close.  

4. 73 miles @ 0.9 gallons = 81.1 MPG
Whoa! WTH? And most of those miles were 1-up @ 55-60. Guess I'll be filling up at that station every time I'm close.  

FWIW, I considered tossing the high and low figures, then using the average of the remaining tanks. I guess I'll be doing so as I really have no explanation for 80+ MPG.  

5. 97 miles @ 1.7 gallons = 57.0 MPG

Did some experimenting on this tank by removing the airbox door which brought back the "surge" with a vengeance. Tried tuning the idle mix to no avail and reinstalled the door. Guessing the combination of increased flow is too much without re-jetting.

6. 108 miles @ 1.7 gallons = 63.5

Did an extra tank because of my monkeying. Interesting to note this tank had 1/3 to 1/2 miles 2-up. This is also my transition fill to SG (aka "nuts gas, aka "E10")

Raw average MPG (all tanks) = 60.8

Adjusted average MPG (minus high and low) = 58.8

E10

1. 104 miles @ 1.7 gallons = 61.1 MPG

Interesting. The first full tank of E10 starts out with a definite bang. Looking like Japan (or someone) used jets favorable to E10.

2. 93 miles @ 1.9 gallons = 48.9 MPG

And more interesting. I planned to go further but had to switch to reserve within a half mile of my driveway. Mix of higher and lower speed miles.

Note: I went to 3 turns out on the idle mix (as listed on another thread as OEM setting) and "the surge" has disappeared. Also, nearly all decel pops are gone.

3. 83 miles @ 1.6 gallons = 51.8 MPG

The average so far is 53.9, in support of my hypothesis. I admit I was hoping for a bigger difference. Makes me wonder if OEM jetting and idle mix exacerbate the lean condition so many complain of, especially if running E10.  

4. 76 miles @ 1.5 gallons = 50.6 MPG

5. 95 miles @ 1.9 gallons = 50.0 MPG

Combined average is 52.4 MPG. Average of the 3 "middle" tanks is 50.8 MPG.

I actually filled today with E10 to give it a fair shot, and out of convenience. The results may be lower as I made a quick run up to Brevard, NC for lunch. A bit of a shakedown for the curves, more of rider than machine. I was also 2-up. The MPG may well be down.

In conclusion, after this tank, the S40 will see only pure gas, along with my old truck. Although I saw no real MPG change in my car, I'll begin half-filling and alternating with pure gas and E10, making my own E5.  

Lol. Funny story. I've heard of similar.

I considered measuring precise amounts of fuel etc. But then again, the EPA testing throws in a lot of driving variables in hopes of obtaining a true "average." I'll control what variables I can reasonably, mainly my riding style, loading, and area.

I considered the jetting issue but guessing most won't re-jet if they change fuels. My manual says E10 is okay, so I'm guessing it was jetted originally "in the middle," so to speak. I'm sure (good) tuning would improve MPG for both.

For now, I'll leave it be. I will add I noted the cap is missing for the idle mix screw. Apparently someone did a little tinkering. I will, too afterword.

In a way, this is all somewhat moot. Unless E10 somehow manages close enough performance, I plan on continuing to run pure gas in the bike. I guess my main purpose is to try and dispel (or confirm) the 3% narrative. The data will hopefully ease my mind at the pump and perhaps  useful to others.

I'm doing the same in my car, a 2011 Mazda6. Although the miles per tank are much higher, my driving varies substantially as I also use it for my side gig (Amazon Flex). The routes vary widely and often involve idling.

So far with the car, although I've seen no real MPG difference, it does idle slightly lower, seems a tad more quiet, and seems to run slightly smoother. However, as E0 is so much higher than E10, I'll probably go back to the darned ethanol blend. Bah.

I also have an old truck, 1994 Chev 1500 2WD Ext. w/350. It's driven very little and is really the best candidate for pure gas. Going forward, this is what she'll burn.

I did wanna add this link. Lot's of good information and resources for finding local E0 stations. There's even an app.

https://www.pure-gas.org/about