Hmmm don't even remember what stock fender edge to edge measurement is.
Measure your fender edge to fender edge distance.
Then get the Offset measure for your wheels (easier than Backspacing) in this regard.
JK Axle WMS to WMS is 65.5" >>> X 25.4 = 1663.7mm lets round that off to 1664mm
Take your wheel offset and multiply by 2, if the offset is negative you add the result to the axle width, if the offset is positive you subtract the result from the axle width.
Now you add the tire width to the above result and compare it to your fender measurement.
So lets take your example with 8" width wheels that have 4.5" backspace and assume they have 0mm offset (normally they do) plus your 245/75R17 skinny tires
axle width ± offsetx2 + tire width (/25.4 for inches)
1664 + 0 x 2 + 245 = 1909mm (75.16") outside tire to tire width
Hope that helps
2010 JKU Sahara
It's all about smiles per gallon, not miles per gallon