Using GPS shows that the speedo in my work truck is off by 3% at 70 with just a tire change and no wheel size increase. I'd check with the dealer to see what they charge since it involves nothing more than an update to the PCM. Hypertech works too, but it may be more money than you want to spend.
If i'm not mistaken, the general rule with tires is that you are safe to go +/-3% before you have to have the speedo reprogrammed. in the case of mine, my old tire (now winter tires) are about 6% over, but i just leave it because i know how far they are out. [not sure why a tire shop installed tires this much over size despite knowing the +/-3% thing]. If you do need it reprogrammed, id check with the dealer (unless you plan on buying a programmer - it would give you a reason to get it sooner:great.
I would just muy a programmer so you can change it at will, and even though you wont notice a huge power increase, it is worth the MPG and the better shift points and firming up the shifts for the tranny. I picked up a new Hypertech III for 205 shipped from summit and craigslist in my area is usually loaded with ones for sell as cheap as $75. For me having a suburban, I got a 1.5mpg increase so after three tanks the programmer was payed for, and it also can read codes!!
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