ok so since im getting rims tm i went ahead and put on my new tune for my tires so the speedo will be adjusted. driving to work tho it didnt really appear that my speedo was off. normally i can gauge my speed by my rpm's in whatever gear im in, and it seemed pretty normal to me. with the bigger tires, should my speedo be reading high or low? for reference my new tires are 275/40/18 and my stock tires are the pony package ones at 235/55/17

 

I would think that if there was gonna be much of a difference your speedo would be reading low because your wheels are now larger good luck with getting that sorted out, I found out the hard way that my speedo was low by about 7mph by getting a ticket.

 

You will be going slower than what the speedo reads. But not by much. At highways speeds (65mph) you would actually be driving between 63 and 64 mph

Two good sites to try:

http://www.barretire.com/calculator.htm

http://www.powerdog.com/tiresize.cgi

 

275/40-18s are the almost same RevsPerMile as the stock 17" size.
So are 255/45-18s .

Your speedo is slower with larger diameter tires and faster with smaller tires.
Less revs per mile with bigger tires and more RevsPerMile with smaller tires.

Try here and see what I mean or do the math yourself...

275 wide times 40 equals sidewall height.
Double that and add the rim diameter.
Then times it by PI for circumfrence in inches.
And last, divide 63360 (inches per mile) by the circumfrence.
Thats the Revs Per Mile.

 

thanks for all the answers guys!
and the thorough explanation dB