Hi Mark,
The correct Vauxhall FD / HB / HC Viva / Magnum / Firenza Rostyles are 13" rims, 4" x 4 PCD and 5" wide - not 5.5".
I think MGB Rostyles are a 14" rim, 4.5" x 4 PCD and 5.5" wide and they look similar. MG Midget Rostyles are 13" 4x4 PCD and will fit Viva - although with the wrong offset. Ford and Rootes also used a similar design.
For GM Rostyles we would need a picture of the wheels to see whether they are the (correct for Viva) Vauxhall imperial 4" (101.6mm) PCD or the incorrect Opel (Cavalier / Chevette) 100mm PCD. If they are stamped 5J - not 5.5 and look like the lower picture below they are probably OK.
Because the difference is not that apparent people often fit the wrong (metric Opel) wheels to their Vivas. This causes the stud holes to become oval and places lateral strain on the studs - not good!
The way to tell the difference visually between Opel and Vauxhall wheels is that the Opel metric wheels have four spokes that narrow from the outer rim inwards towards the wheel centre, whereas the imperial Vauxhall Rostyle spokes narrow from the centre outwards toward the outer rim.
Like the MGB Rostyles the correct imperial Vauxhall wheels also have an additional 'half spoke' pointing from the outer rim towards the studs. These are missing on the Opel metric wheels.
Here's the Opel metric rim (wrong for Viva)
- untitled wheel 2 JPG.jpg (29.03 KiB) Viewed 1691 times
And the correct Vauxhall imperial rim
- ROstyle%20wheel%20small.jpg (25.62 KiB) Viewed 1691 times
To confuse matters further the Opel rims are correct for four stud FE Victor VX series (but not VX 4/90 / Ventora and not FD) because the FE used metric hubs.
Hope this helps.