Tyre size / speedo/distance ? What tha??

[Oldtoy74]

I've been searching on mud to try find a formula on how to calculate the variables if your cruiser has non standard rubber size, and what percentage do you add or subtract in relation to odometer reading, I have been going on the speedo and gps speed is 4% diff! thus amending my distance covered by 4%, Am I on correct here or am I off the mark.?. Is there a standard formula, or .............not ??
I have 32" tread I think bj74 are 30 off the floor,

 

[GTSSportCoupe]

I've been searching on mud to try find a formula on how to calculate the variables if your cruiser has non standard rubber size, and what percentage do you add or subtract in relation to odometer reading, I have been going on the speedo and gps speed is 4% diff! thus amending my distance covered by 4%, Am I on correct here or am I off the mark.?. Is there a standard formula, or .............not ??
I have 32" tread I think bj74 are 30 off the floor,

I've used this one with great success over the years:

http://www.1010tires.com/Tools/Tire-Size-Calculator

Just put your stock tire size in the bottom of the green box, enter the tire sizes you want to compare to in the white box, and enter the little code in the yellow box. Click 'Compare Tire Sizes' and it'll give you all the info.

 

[NGray]

My bj73 is stock with I believe 28" or 29. I'll send link to stock sizes when I get home. I'm around 12% change with my 32"

 

[NGray]

I swear this info has changed since I last looked, or I'm losing my mind, but here. http://www.toyota.com.kw/english/to...ser/history/html/history/models/spec_m70.html

215sr15...from what I gathered, that is a 28" wheel....according to what I googled

edit* according to http://www.1010tires.com/Tools/Tire-Size-Calculator I'm at over a 14% change...When I tracked out all of my driving and plotted it, I was around 12% change from what google maps was telling me and for whatever reason when I did a tire calculation earlier it was telling me 12%....but maybe my old tires were clocking on more then the stock.