The calculator says I'm 2.4 F higher, but it would have been more accurate if it had taken more accurate coordinates instead of only whole degrees.

1 degree sampling is enough. It is a small correction after all. Probably between here (MI) and there (IL) it is all between 2.4 and 2.6, so what's the point of going finer?

Hmmm, I'm three degree warmer ... maybe I should do peaches, after all.

Should you want a higher degree of accuracy on your map try going to NOAA, which will pinpoint your area more precisely- here's the link to my town, from which you can easy move to other towns:

http://forecast.weather.gov/MapClick.php?lat=46.87990702860919&lon=-113.98590087890625&site=mso&smap=1&unit=0amp;lg=en&FcstType=text

The home page link is below.

Here is a link that might be useful: NOAA home page

It was irritating that I had entered the coordinates to 4 decimals, had to keep deleting them until the program would work. A matter of programming.

It worked for me with 3 and 4 decimals respectively. Maybe your period is not recognized by the page.

I am 2.8 degrees higher. Earlier apricots?

I'm not sure if I'm buying into the accuracy of this correction factor, and perhaps not even the current USDA map itself. I still see temperatures of -20 F here about 2 or 3 years out of 5, and yet the USDA says I am at 5b and the correction factor is 2.7 F which would bring me part way closer to 6a. That's definitely not true. I conservatively consider my home as being in 5a and a half(!), which apparently is not far different from 1990. I don't think the new USDA map is too far off, but to add more degrees on top of it might be a farce. And a lot of this is also probably due to my experiences in my own local microclimate. Make sure you understand your particular microclimate. As inputs, these tools like USDA and this corrective factor are fine. But take care not to discredit your own experiences of course. Perhaps I am just conservative by nature. I've been thinking about planting a peach for a couple years, but I haven't taken the leap quite yet. Maybe in 2014. Don't get me wrong -- I do believe in global warming. But I just don't know if it's accelerating to the point where my climate is going to jump a whole zone between say 3 years ago and 3 years from now.

Dmtaylor, I planted a McKay peach this spring (grafted from a seedling tree in Waterloo, WI). I'll let you know how it does. Currently it has bacterial spot (I think), it came from the nursery with lesions on its trunk, so I'll have to call them one of these days. It's not happy in my suburban clay loam soil, I'm planning on digging it up in the spring and putting in a 4' wide by 1' high mound of soil like veteran peach growers recommend.
I also generally like to be conservative with my plant choices, since it's just too expensive and frustrating to lose trees to the odd bad winter.

I won't plant a tree on the border temperature. The global warm is more accurately called climate change... the temperature can be warmer or ever colder...
Fruitmaven, I can't believe the nursary told you your McKey is not happy with the clay soil. I thought it native grow in WI and clay soil in WI had never been changed for thousands/millions years regardlss of global warming or not. I actually thinking about planting one of Mckey. But the tree's disease resistance is very important in selecting a tree.

I honestly haven't talked to the nursery yet. McKay peach is native to WI, but some areas of WI have very sandy soil. Waterloo, WI may be one of those areas, my WI geology is not very strong. My parents live 150 miles north of me, and their soil is completely different from mine, and very sandy.
I have seen McKay peach on a list as a disease-resistant variety, but it's not working out that way for me. I do have rather compacted soil, my house is 10 years old and before this spring was only growing grass. It was developed on farmland, but of course they scrape off all the good soil and only leave about 6 inches to grow grass. (I'm not too bitter, I actually think that's better than my aunt's house which was built 50 years ago. She has 8 feet of black topsoil, and what good is it doing her in a residential neighborhood?)
In any case, I'm going to give it a bit more time. I'll contact the nursery and let them know I'm unhappy that it came with lesions on the trunk, and my current disease issues. I'll replant it in the spring so it gets better drainage (mound or raised bed), and use copper (or something) to knock down the disease in the spring. I didn't know enough about peach diseases this spring to treat it with anything, I just pruned off the top half of the tree and thought it'd take care of it. Apparently not!