So I have been seeing a lot of people post about after they change tire size that they have to go into forscan and adjust it. I currently have 275-65-18. I'm going to put in a front leveling kit then change the tire to 285-70-18. Is there a tutorial for adjust this? I looked in the forscan thread and unless I missed it, I didnt see anything about it.

 

I always recomend checing the GPS speed after a tire size change, just to see if any adjustment is needed. Often speedometers are off from the factory or in your case the size change is minimal. Any smartphone can display GPS speed with a free app. KM

 

Ahh ok good deal. Thank you.

 

https://tiresize.com/calculator/

Quick and easy guide.

 

All that math is fine and dandy but the actual rolling radius of an inflated tire is not static. It can change with tread wear, tire pressure, and other external factors and this does not account for tire size variables between manufacturers. You can do all the calculation 100% correctly and still be off because of the variables. KM

 

Speed is allowed to be off by as much as 10% from actual. I knew that was the case years ago, but recently ran across the information that it hasn't changed. I've never had a vehicle with OEM tires that was significantly off from actual speed.

Regarding using the sidewall size cslculated math vs rolling radius comment, one can easily get around that by directly accessing the tire manufacturer's size and revs per mile information to get actual data. In my experience, using the calculated math gets you pretty close, so then you only need to make a few tweaks based on what the gps-indicated speed says.

 

There is a Forscan method to directly correct for GPS averaged speed that simplifies the correction process. I use that and am within 1/2 MPH from police speed gun as checked by a very co-operative county sheriff.KM

 

Thats conclusive even with stock/OEM tires. No matter what tires you put on there and make corrections, it'll always be off a bit by reasons of which you mentioned. It is what it is. The general point is just making we try to get the tires as close to what they 'should' be for frame of reference and keep that in consideration. They'll never be perfect the entire time, and I doubt manufacturers have put in algorithms to account for tread life & wear of time and mileage. The point is to try match as close as possible overall for speed and mileage, and transmission shift points. Thats MY take.