So I took it as this:
FT-86: 225/40/R19 ==> 662.6 mm
GT-R : 255/40/R20 ==>
688.0 mm (<== incorrect, pointed by Dimman)
Now, based on my own judgement on where to see the single pixel of the wheel (this will introduce error since this is not a precise measurement...), here is the revised comparison.
(Warning: image was not scaled correctly.)
You can see that now the back of the GT-R is within the FT-86 image. I guess this is probably off one or 2 inches...
This is not an exact science....