My 1990 Trek 1000 came with a 7 speed Sachs freewheel and chain when I bought it on ebay. The gearing went from 13 to 28 teeth. This has been great when doing various rides outside. Especially for the 50 miler that I did with the hills.
But I have been doing a lot of spinning to Spinervals DVDs. The gearing they use in the videos seemed to be different than what was on my bike. First, I think the bikes they use have more speeds. Something like 12,13,14,15,16,19,21,23,25. But the Sachs freewheel on my bike was something like 13,14,17,19,21,23,28. Often on the DVD, the have you go to the middle cog on the freewheel which is a 15, but it was 19 tooth cog on my bike. I wanted to get a freewheel that more closely resembled the configuration used in the DVDs. I watched ebay. I figured it would be easier to find a Shimano freewheel and replace the chain. But I found a few Sachs freewheels and for whatever reason, they were the ones to most closely matched the gearing used in the DVDs. I bought one. The gearing is 13, 14, 15, 16, 17, 19, 21. So now when I go to the middle gear, I am using a 16 tooth cog, which is closer to the 15 in the video. The smallest gear is still only a 13. But this is generally pretty challenging for me anyway. Maybe after my legs get stronger, and I get used to the gearing, maybe I will hunt down another freewheel that goes down to 12 teeth. Or may be I will buy a new bike that actually has more gears. But for now, my 1990 Trek 1000 is great for the cross training I am doing. And if I do a long ride this summer, such as the 50 miler, or maybe a full century, I will be in much better shape.