2007 Pontiac G6 Tires

The best thing to do would be to get a set of 16" steel rims.
Right now you have the 225/50/17, the standard size of the base G6 with 16" rims is a 215/60/16, which is a 1.15% difference in diameter compared to what you have now. I decided to go with 205/60/16's because the diameter difference is only 0.66%, they are also slightly cheaper.
With the 205/60/16's on, the GPS and the spedo are exactly the same to atleast 90mph (145kmh ish).
Either size would be good, most companies seem to reccomend staying under +/- 3% diameter difference. The larger +/- the size difference, the more the spedo will be off of how fast you are actually going.
Here's what I use to compare sizes

08 WP tC said:

May I ask why you want to go down to a 16" tire? I personally put Continental Extremecontact DWS 225-50-17's on my fiance's 08 GT. I am very happy with the grip and ride of the tire wet and dry, but the sidewall on the tires are a little on the soft side.

Click to expand...

I was able to get a set 4 of Michelin X-Ice with 2 seasons left on them with 4 steel rims for $250.
Looking at it now, prob should have gotten 17's so I have the option to get bigger rotors/calipers, but it was a great deal.

What size do you have now?
Everywhere I check, the 09 Vibe GT has 17's
Once you figure out what you have now, pop the size into
Tire Size Calculator and play with teh sizes, keeping in mind that you want to stay under +/- 3%, ideally under +/- 1%