Hi All- Newb here. If I am changing from 17'' to 18'' wheels, do I need to have the car "re-programmed" to allow the speedometer & odometer to work correctly with the larger diameter wheels? Thanks Vince D
Posts: 194
Joined: 11/22/2007 From: San Diego Status: offline
You dont HAVE to but you can if you want. Where are you located at? RPM'S over here in San Diego has the programer but its a couple hundred bucks. Not really worth the money in my opinion
If you get the right size tires, the outside diameter of the tires will essentially be the same, so you shouldn't have reprogram anything. For instance, a 235/45R17's OD (outside diameter) is 642mm, and a 235/40R18's OD is 645mm. To get really specific on ya, that means your wheels will turn 3.7 times (that's 9 1/2 feet) less per mile with the 235/40R18's...which means at 60mph your speedo will read slightly over 1/10 mph slow. Tire size: 235/40R18 - 235 is tread width, 40 is the aspect ratio which means the side wall is 40% of the tread width, and the 18 is, of course, your rim size. This link will make it easy to see what will fit your car and what the OD is on the tires you're looking at.
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Wheel size change question - 
