I just bought new tires that were 60 series instead of 55 series. THis threw off my speedo a few mph, according to my magellan sport track map GPS.
nothing major though. the Speedo reads a few mph slower than actual speed. 1-2mph off at 55 2-3 at 70mph.
If the overall tire height changes at all it will throw it off,
Where did you get this figure of an inch taller???
Those 2 tires are the exact same height. Go to tirerack.com pick a 245/45/17 and go to specs.. you will see they are both about 25.7" diameter.
If his original tires are bald as hell, it could be a little shorter than the new ones... but those two tire sizes have the same sidewall height. Don't worry about it.