Not that far out. Really. All it takes is a lack of proficiency on the old bike, with DT shifters and gearing that is far from optimum for his best cadence. Weight's got nothing to do with it.

Bicycle Guide did a test around 1989, and found that position changes on the bike, 4 per mile, equaled about 4 minutes over a 25-mile ride. They were actually testing aero bars and found that shifting body position on the bike made a bigger difference than riding position. (Still, they found drops almost equal to aero; both were better than the hoods)

The gearing difference and modern components on a bike that perhaps fits better could be 5 minutes over 21 miles. I doubt if weight was a factor, unless there's a lot of climbing involved. If you went out and bought a modern bike that fits properly, with modern components, you should be faster if you're better at shifting STI's than DT shifters.

I could never ride my workout loop at 20mph on my DT-shifted Ironman. I tried. I just wasn't that proficient at anticipation and shifting smoothly, and I had to change position to shift (or miss shifts). I also had problems keeping with a 20mph pace line with DT shifters. My best tri leg was using Sora's mounted low, in the drops.

That's why I find guys like norskagent, who can friction DT shift as fast as I can STI-shift, so amazing. I tended to try to make up for my deficiencies in DT shifting by ignoring shifts and mashing like crazy.

When I upgraded it to STI's, I could run that route at 20mph repeatedly, because I never left the drops, and missed far fewer shifts. I ride it now on both an 18-lb Kestrel and a 23-lb Cinelli. I doubt my time is more than 2 minutes different on any given day, for any wind condition, regardless of the bike. I may gain some climbing speed on the Kestrel, but it's so light, I tend to mash more on the smaller cogs and ignore smooth shifting. I suppose if I really, really focused on a high-speed run, maybe I could gain some by climbing in the right gear, but I doubt more than a minute.

I honestly believe that if I focused on every aspect of my riding on the Kestrel, I'd probably be close to 5 minutes faster over the same 21-mile course as my best shot at it with my DT-shifted Ironman. The difference would be my ability to shift efficiently, nothing else.

Vote for me. Free century rides for everyone.