I took off the stock PYO's and added Eagle Alloys 17 x 8, with 265/70/17 tires. How does the new tires and wheels effect my speedo? Also will the MPG and miles be off when using the Driver Info Center on the dash.
How can I correct the speedo?

yes it will be off i'm not sure how much i'm no math wizard. a tuner can correct the spedo or the dealer might help you.

If you don't have a tuner as a resource there are 2 basic options:

1) Have the police radar you at highway or interstate speeds. (my spedo is fairly accurate until speeds above 60mph)

2) Sometime when your on the interstate, set your cruise at the speed limit and clock how long it takes you to get from one mile marker to the next. Then calculate your speed from that time.

E.G. - 65 seconds to go 1 mile between mile markers at a speed of 70mph
65 seconds / 60 seconds per min / 60 minutes per hour = .018 hours
70 miles per hour x .018 hour per mile = 1.26
70mph + 1.26 = 71.26 mph actual speed

I believe that's the formula. It's a lot easier to think through when your actually driving and not making up numbers for speeds and times. Hope that helps.

I am running 285/70R17s and according to my GPS the speedo is 2.5-3mi slow at 65MPH.

TB

yup I have 34's and my speedo is about 5mph off at 65 mph according to the GPS...tune will help...dealer will RAPE you when it comes to cost for readjusting the speedo...better off putitng the money into a tune.

According to my dealer, they can't reset the speedo for over stock size tires. They said GM has "locked" them out of fixing that with a tech2. They said to use a Hypertech tuner as most customers have good luck with them.