I've wondered the weight difference between original period wheels, tires (tubes?) compared to new aluminum wheels, tires. Is there much of a difference? Is there a comparison out there somewhere?
I haven't got the full data but this might give you some ideas. From my cars and weight on the same set of bathroom scales (!)
Elan, 13 x 4.5 steel rim, 155/80 tyre, 26lbs.
Europa, 13 x 5 Cosmic Alloy with 175/70 tyre, 28lbs
Europa, 13 x 5 Cosmic Alloy with 185/70 tyre, 30lbs (yes that's the numbers, I didn't believe it at first but they are different brands)
Europa, 15 x 6 MGF Alloy with 195/50 tyre, 36lbs.
I would imagine the Elan steel rim is going to be similar to the S1/S2 steel rims and the Cosmics, because they are slightly thinner, somewhere around the TC/TCS alloys. But the message is that if you're fitting 15" rims even without significantly wider tyres then there's a definite weight penalty compared with standard. When you look at the original steel wheels they weren't that bad at all, especially considering the 80 series tyres.
Brian