we have stock 215/55R16 tires on our GLE on stock wheels (i assume 7" wide), we're keeping the wheels, but need new tires. my friend recommended Potenza RE910, but they are not available in that size, the P225/50R16 is the closest match, but it's slightly smaller diameter and a little wider.
will this size work?
will it throw off the speedometer much?
what other sizes will work on stock wheels?

http://www.miata.net/garage/tirecalc.html
for speedometer,

www.tirerack.com
for good sizes of good tires

faq and search
for all other answers

Double check on the width of your wheels. I believe they're 6.5" wide, but I could be wrong about that. Either way, the minimum rim width for both the 215/55R16 and the 225/50R16 tires is 6.0", so you should be alright. The other thing to keep in mind is load rating. You want to select a tire with the same or higher load rating as that of your OEM tire.

Yeah I too am in the middle of picking out a new set of tires, so far if what Y2kse said about the GLE having 6.5" wide rims then I going to get a set of Dunlop SP sport Fm901'a in 225/50VR16 92V SL
sizes. They also have a load rating of 1389lbs, an improvement to the 1356lb load rating stock.
and they're $75 bucks, fairly cheap.