[hellion240sx]

old , new
Diameter 615mm(24.21), 651mm (25.62 inches)

Circumference 1932.1mm , 2045.2mm

Poke 42.6mm , 81.6mm

Inset 122.6mm , 121.6mm

Speedo error 0% , -5.53%

Reading at 30mph 30mph , 28.34mph

Reading at 60mph 60mph , 56.68mph

Ride height gain 0mm , 18mm

Arch gap loss 0mm , 18mm

shouldn't the diameter stay the same if the sidewall and rim size are the same? from 15x6.5 -> 15x8????

I went out and measured it to double check. I have on now, 195/60/15s but the calcu over at will they fit.com says the set up is 24 inches, but I measured it and its 23 1/4" = 584.2mm

so why is the diameter changing if it's the same size rim (height) and sidewall?

link-> Online Wheel & Tyre Fitment calculator. Offset, Tyre stretch and Rolling Radius calculator

I'm trying here yall...