So speaking to someone who doesnt know that much about the details (but is trying to). If i increase my rim size from a 17in to an 18in, what modifications must be made to make sure that everything is still tuned right? Do i have to buy a chip? How does it work? Thanks!
Posts: 2745
Joined: 6/8/2006 From: SF Bay Area Status: offline
No chip needed, but a couple thing to consider. First is clearance of tire in wheel well. For example: if you have a 17" wheel with a 225/50 series tire, generally speaking if you go with an 18" wheel with a 40 series tire and have approx. the same outter dimension so speedo is not affected (may not be perfect, but only 1 or 2 MPH off)
Next is rim width, ***HERE*** is a website to help you understand wheels, off-set (can be critical if you want to go WIDE tires/rims). Another example: my Saleen, when it was a base model Mustang, came with 15"x7" wide rims and most likely had 225/60/17"s. My Saleen now runs 275/35/18"s up front and 295/30/18"s out back (will go with 315's later this year) You will need to do a little research on tires ***HERE*** to understand how much wider my tires and rims are from stock. Keep in mind, my car has nearly a 2" drop (thanks to Griggs GR-40 suspension system ) and have ZERO rubbing on any tires.
You didn't mention the year car or your existing tire dimensions. If needed the speedo can be calibrated in a couple ways. My '85 GT Mustang needed a small gear replacement in tranny and took about 10 minutes. Later models (not sure when the change happened, but probaby '99+ models) you need to have speedo calibrated with a hand-held tuner by Ford or someone (not sure who does it, but can be done)
Do some research by calling Tire Rack (site sponsor) and they will hit you up with ideas based on your budget and what you like
Printable Version
Rim Size Question - 
) and have ZERO rubbing on any tires. 
