I am the same as you are I use Fuelly.com it is a great and easy way to keep up with what fuel and the cost are over time and best of all it is a free service all you need to do is update each time via your computer.
Do they list all octanes?
Not sure I see the sense in going out of your way to save 3 or 4 cents a litre. Plus I will only use 1 brand of fuel anyway
For the OP, I'm pretty much getting the numbers you're expecting... On my commute into work (50+ mi, and always in a hurry), I'm on her a little, sometimes in sport mode, and my mpg drops just a tad. But in the evening when I'm pretty much mellowed out, tired from my day, I lay off of her a bit, using the Adaptive CC, then my mpg climbs steadily up near 19-20...not to mention if I keep it under 70mph (for the most part), then I'm seeing into the low-to-mid 20s easily (new ECO gas saver technology really helps out on the new 6.4). Might I add as a side note though, if you're THAT worried about saving on gas, you might not want to burden yourself. I've owned two large GM SUVs over the last six years and I'm getting better mpg than both of those on average... Tahoe #1 was 14-15, mixed driving; Tahoe #2 was a little better w/ the 6-speed @ about 16+. Hope that helps... Your call! But would love to see you join the club--good luck!
I see an average of 19 fir mixed driving, on long road trips I think we saw 23 on our Ohio to Florida trip. I drove both the r-spec and srt before buying my srt. They really are two different class of car. If I based it solely on comfort I'd get the r-spec. All around good fun, srt!
I have an OCD habbit of calculating mine after every fillup, and my 6.4 gets right at about 17 with 93 octane and 50/50 highway. On a loong road trip cruising at about 85 and gentle throttle I got 23.
BUT. Nobody buys this car for gas mileage