If your average temp is 80 then that is helping. Are you recording the true mean average temp or just guessing at it? We have been colder as heck the last two days, for this time of year the mean average temp for the last 3 days has been 54 °F / 12 °C, but I have been off work anyhow so it really hasn't affected my mpg.

For example of your area, for the last seven days the mean temps at Chicago Midway have been 82 °F, 70 °F, 65 °F. High, Avg, Low.

 

Hemisync,
The "Add a Tank" screen asks for temps in 10 degree gradients. Since I go through a tank of gas in 5 to 6 days it's a pretty accurate guess. Sometimes I drive home from work late at night where the temp might be 60 or lower, inbound commute is usually mid-day when the daily temp is highest. I just round it out to somewhere in the middle.

 

That's why I usually use the average mean temp, but CleanMPG allows you to enter exact temps and graphs them with your mpg so you can see the effect.

As for my first tank of mid-grade, it is not going well, so far I have not seen any increase and am fighting to maintain my 60 mpg that I got on the last tank of regular. We shall see where this goes, but so far it is a rocky start.

 

Finished 3 tank test with mid grade.
Last tank got 60.1 mpg for 553.8 miles.

Total mid grade average for 1875 miles was 59.2 mpg.

The tank is now filled with regular 87 octane. This will be the first of 3 tanks for comparison test.

 

Hemisync,
I just looked at your stats at Clean MPG.com and was surprised to see the tire pressure (60psi) you are running at! It seems you are raising it a little at a time.

What kind of tires are you using? What is the maximum PSI?
When are you going to explode?!

 

I am running OEM Bridgestones with max sidewall rating of 44 psi. I doubt they will ever explode at least not from inflating them to 60 psi. I would never recommend that any individual inflate their tires above the max sidewall. What I have done is a personal choice and if there are any risks they are my own. Now with that said, check out some of the following information:
Driving Under Pressure
What PSI are your tires? (Poll)

 

Mid grade seems to be working better for me. My mpg went from approx 46-48 to 52-53 with the 89 octane rating. Driving is mixed freeway/city with some A/C use. I still have about 1/4 tank left with 450mi on the trip odo. I have a 2005 HCH PZEV with the 11 gal tank. Looks like I will get my first 500 mile tank since they changed to reformulated gas here in MA.

I calculated approx $3 saving per fill-up even with the higher price of mid grade... (one gallon saved of regular = $4. Filling up with mid costs $1 more than filling with regular, so I save $3)

 

So I finished the first three tanks of mid and my mileage is up roughly 6%. (50 --> 53+).

I put the first of three tanks of regular in and will update when I am done.

I don't know if it is the weather (it has warmed up) or the gas...

 

Completed 3 tanks of regular 87 octane gas for a total average of 57.2 mpg. Very similar to results for mid grade test (59.2 mpg). Two miles per gallon doesn't quite justify the extra cost for mid grade.

However, near the beginning of the first 87 octane tank I raised the tire pressure to 45 psi. I think it's only fair to run 2 more tanks of mid grade 89 octane with this same tire pressure (and steady summer temps and weather conditions) to get good comparable results. I should have this completed by the end of July.

 

I finished the last 2 tanks of mid grade for an average of 59.45 mpg. My 3 tank average for regular was 57.23 mpg. This is with almost exact weather and driving conditions and 45 psi tire pressure for the past 5 tanks.

So, I guess the mid grade gets a little better mpg over regular.
With a 10 gallon fill-up I will get an extra 22 miles of distance for an extra dollar.

It's almost a wash when you consider the extra cost for mid grade. So let's calculate miles per dollar (mp$):

Regular fuel cost.... $4.259
Mid grade fuel cost. $4.359

Regular gas miles... 1490
Total fuel cost... ÷$111.16
Average mp$......... 13.4

Mid grade gas miles.. 1002
Total fuel cost....... ÷$73.23
Average mp$........... 13.68

Although fuel costs have varied slightly during these tests, the price difference between regular and mid grade has been consistently $0.10 at my filling station. The price per gallon I use in the calculations is an average of these prices. As fuel prices go up in the future, the difference in mp$ will become narrower.

So it looks like mid grade beats regular by a slight margin of .28 mp$. It isn't much - and this test certainly is not scientific - but it's enough for me to decide to stay with mid grade 89 octane for the foreseeable future!

Who else out there has tried this mpg comparison?
It would be great to hear from some Hybrid veterans who can compare their results of mp$ or mpg stats from the same time last year.