Do you suppose that the people getting bad mileage with 19's could be because the spedometer is not calibrated? I don't know what the difference in overall circumference is between 19's and stock 15's with fatty tires but I'd imaging it a few inches difference. Whats the concensus here.

I always try to remind people of that when they complain about mileage.
It is impossible to account for every tire and wheel combination compared to stock wheels.Only thing I can tell you is to put your old wheels against your new ones and compare.
If your wheels and tires are bigger than stock,then it will look like you are getting poopy mileage.If they are smaller,it will look like you are getting good mileage.
I just fill her up and drive.When the gas gauge gets low,I fill her back up!
There was a question of whether the speedo is electronic or cable.
I believe it is electric and not cable.Have no clue how to calibrate it though.

my sentiments exactly! But what if you had you 19's from the dealer. Do they install somthing to correct for the difference. I am grabbing for straws with my 23 mpg.

Most dealerships, if you bring your car back and tell them what size tires you put on, will recalibrate your computer for that size tire. I had to get that done when I had my dodge. It came with 30" tires on it and I was getting 18 mpg, then I put on 33" tire and it said I was getting 13 mpg. I went to the dealership and they recalibrated my computer and then I was getting 16 mpg. You do have to remember that the extra weight of the new rims and tires plays a big factor in it too.

if this is the case, then we purchased 15 inch enkei rims...with i believe the stock size tires as well (wanna say 185 60 15 maybe??)..this should not have an adverse effect should it?

Go to one of the tire comparison site, such as

http://www.1010tires.com/TireSizeCalculator.asp

to see how much slower or faster your tires are compared to stock. If your tires are larger (which is probably the case), you are actually going further than what is indicated on your odometer. Incidentally your speedometer will indicate a slower speed than what you are actually going (so watch out for those speed traps).

So let's say the tire calculator indicates you are going 6% too slow. Just divide your gas mileage by 94% (100% - 6%) and you should get your true gas mileage.

Incidentally if your tires are listed at TireRack, they have more precise dimensions for your tires on their specification charts. I noticed that the Kumho ASX tires I purchased have a slightly different diameter that what is indicated on the tire calculator sites.