i put on bfg ats 0n my 96 4x4 the are 31x10.5 r15
and the old ones were 235/75 r15 how do i find the difference in size

 

You can go on places like tirerack.com and search for each tire. They will normally tell you the width, overall hieght and on what rims they can be mounted.

I got board so here you go:

31X10.5QR15 235/70SR15

speed rating 109Q 102S
max load wgt 2250lbs. 1896lbs.
max psi 50psi 35psi
tread depth 19/32nds 12/32nds
fits these rims 7-9" 6-8.5"
selected rim width 8.5" 7"
section width 10.4" 9.3"
tread width 8.4" 6.7"
overall height 30.4" 28"
rev's per mile 685 762

These are just estamites for bridgestone tire's you would need to look up your exact tire info to be exact.

 

im lookin for the conversion from metric to sae (standard american English)

 

The difference is minimal between the 2 tires. If you really wanna know its about a 29/9.5 We did this size change on my dads 91 F150 and no noticable difference.

 

thanks guys