C8 wheels on a C5

[C2 VET]

I'm just in the process of making C8 wheels fit my '99 FRC.
The wheels are at the machine shop having the centre bores relieved to C5 spec, the rears should be fine if I've done my sums right, 20x11 e64 is just about the same outwards as a z06 wheel @ 18X10.5 e58 and I know it will clear inboard as I already have 19x10 e79 with 4mm spacer carrying a 295/35/19.
I had a 305/35/19 set up that rubbed ever so slightly on those wheels.
The question is, has anyone done this previously as I'm thinking 295/30/20 but would like 305/30/20 if they fit both inner and outer parameters.
I know about the PCD, going to go wobbly nuts even though I don't think it's warranted.

 

[Chevrolet]

Is this any help?

 

[teamzr1]

You will have to use like a GPS after this conversion to see if the speedo on dash is matching what the GPS says as to MPH
and not matching you could have ABS problems and would require the GM PCM calibration be tuned to replace the stock
wheel and tire sizes to have correct VSS to the PCM and EBTCM

Just a point for my 1999 Corvette

I replaced the front 17" to 18" and went with 275/35/18 tires, rims are close to 10" wide
Back went also 18" but 12 inch wide and have 335/30/18s
But I did have to do inner tubbing to get the clearance

My Rears




Inner Tubbing

 

[Dorsetsteve]

Does this help?

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.

www.willtheyfit.com

 

[teamzr1]

Just for info
Here is how your 99 FRC is programmed in the PCM related to VSS signal, tires, wheels and gear end gear

And IF after your mod changes speedo is off versus what a GPs reports then values would be to be tuned to
correct

 

[C2 VET]

The wheels are on !
Back bored to 70.3mm, wobbly nuts. No spacers, 295/30/20 rear, 245/35/19 front.