One of the the most common questions asked about rims is if FD3S rims can fit on an FC3S?  The stock FD rims are very light for it's size and relatively cheap on the used parts market.  They are a nice 5-spoke, star design.

In the rears, yes, with maybe a slight bending of the muffler heat shields.
In the front, not without the proper spacer.
How big of a spacer you ask?  Here's the nitty-gritty math...

Stock FC3S rims are 16" x 7", +40mm offset.
Stock FD3S rims are 16" x 8", +50mm offset.

7" rim converted to mm:  7" 25.4mm/inch = 177.8mm
Divide by half for "0" offset for 7" wide rim:  177.8mm 2 = 88.9mm
Add offset to calculate "backspacing":  88.9mm + 40mm = 128.9mm
Convert mm backspacing into inches:  128.9mm inch/25.4mm = ~5.0748"
Note:  if you're worried about stock rim backspacing, stock is ~5.0748" or 5".

8" rim converted to mm:  8" 25.4mm/inch = 203.2mm
Divide by half for "0" offset for 8" wide rim:  203.2mm 2 = 101.6mm
Add offset to calculate "backspacing":  101.6mm + 50mm = 151.6mm
Convert mm to inches for backspacing:  151.6mm inch/25.4mm = ~5.9685"

So, for to get identical backspacing on an 8" rim to match the 16"x7" stock rims, you need:
151.6mm - 128.9mm = 22.7mm spacer or 5.9685" - 5.0748" = 0.89" spacer

This is strictly speaking in terms of rim backspacing without regard to tire case width.

If we apply this to tire size and use the stock 205/55/16 size tire on a 16"x7" rim versus a 225/50/16 size tire on a 16"x8" rim, by comparing the tire nominal widths...

Width difference between 205 wide tire and 7" wide rim:  205mm - 177.8mm = 27.2mm
Divide by 2 to calculate tire overhang per side:  27.2mm 2 = 13.6mm

Width difference between 225 wide tire and 8" wide rim:  225mm - 203.2mm = 21.8mm
Divide by 2 to calculate tire overhang per side:  21.8mm 2 = 10.9mm

So the difference in tire overhang is:  13.6mm - 10.9mm = 2.7mm
Subtract this difference to the backspacing:  22.7mm - 2.7mm = 20mm

So bottom line, for a 225 wide tire on a 8" rim to fit, you need a 20mm spacer.
A 20mm spacer is approximately 3/4".

Side note:  these numbers are assuming the nominal tire widths are accurate.  When in reality, tire sidewall design varies widely from the nominal tire width spec.  With so much variance in measurements, these numbers are meant to be used as a guide.

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