WHEELS AND BACKSPACING

The above excel sheet helps calculate offset for custom wheels using a known fitment. An example would be starting with the stock GTM wheels and using those measurements to determine the offset when wider or narrower wheels are chosen. Different manufactured Non-Custom wheels are made to fit several different cars and so their offset or backspacing may not serve the same purpose. By beginning with a known set of wheels that fit the GTM, this calculator will tell you what you need to have custom wheels made or to shop for wheels that closely match.

The below drawing shows what happens when swapping wheels. This was done beginning with known GTM wheels offered by Factory Five. The front are the 50mm 18x9" wheels while the rear are the 51mm 18x12's. I drew both as 19" to keep things straight forward. Taller wheels will need to be considered for interference but offset can be calculated. In the example, swapping a 9" wide wheel for one that is 9.5 results in the need for an offset increase. When swapping from a 12" wheel to an 11" wheel offset will need to decrease.

WITH REAR BACKSPACING AT +30, THE FACE OF THE WHEEL IS FLUSH WITH THE UPPER FENDER EDGE. A SLIGHT BUILDUP TO CREATE A FLARE AROUND THE WHEEL OPENING COULD BE ADDED BUT IT'S NOT NECESSARY. WITH THE CUSTOM WHEEL MACHINED TO +30 BACKSPACING, NO WHEEL SPACER IS NEEDED. I AVERAGED THE 19" WHEEL BACKSPACING OF WHEELS FROM GTM'S POSTED ON THE GTM FFCARS FORUM BUT WENT LOWER TO BRING THE EDGE OUT TO THE FENDER AS YOU SEE IN THE ABOVE PHOTO. I'M HOPING THIS WILL BE GOOD AS IS BUT MAY ADD A SLIGHT LIP FOR A FLARE APPEARANCE OF ABOUT 1/4 - 1/2 INCH. I STUCK MY NECK OUT ON THE LOWER BACKSPACING NUMBER AND IT WORKED PERFECTLY. I THINK 30 IS ABOUT AS LOW AS YOU WANT TO GO ON A CUSTOM 19X11 WHEEL FOR A GTM. ILL BE USING MICHELIN PILOT SUPER SPORT TIRES - 305/30/19'S