I’m seeking advice on a new cassette for my road/touring/bike-path bike. My current 7-speed 11-27 cassette doesn’t give me a low enough gear now that I’m an old man and doing some light touring. I’ve ordered a new, longer range rear derailleur that will handle up to a 34-tooth low gear Deore SGS RD-M591, since my current derailleur is about maxed out with the existing 27 low gear. I am changing to Vuelta Tourist wheelset, which the manufacturer says accepts 7,8,9 and 10 speed Shimano cassettes. This is a 1970s frame with downtube friction shifters, so I don’t need to worry about indexing compatibility, etc. When I first converted to 700c wheels, the wheelset I bought came with a 7-speed 11-27 cassette, and the shifting has been just fine – although the 27 isn’t low enough for some of the hills I frequently ride. I will be replacing my chain with whatever chain is recommended for the cassette I buy. I’m using the stock road double chainwheels of 39 & 53 and I don’t want to bother with changing to a triple or a compact double.

So, the question is, since I’m not racing, and since all I’m seeking to achieve is a really low climbing gear, and since I don’t demand all the gear options of a 10-speed cassette, if I find a 7 speed with a 32 or 34 low gear, should I go with that, or hunt around for an 8, 9, or 10 speed cassette with a 32 or 34 low gear? And is the quality difference between a $50 cassette and a $250 cassette of much importance to an aging recreational cyclist who rides only about 100 miles/week? Will my downtube friction shifters have enough precision to handle the tight spacing of a cassette with more than 7 cogs?

Any recommendations on a Shimano cassette with 32 or 34 on the low gear?

7s and 8s spacing is the same so if you're comfortable shifting your 7s now, 8s will be the same. And since the wheel is already spaced around 8s you might as well go that route. Personally I'm not a fan of super wide cassettes, since the extra range comes at the expense of wider spacing in the middle gears where you do 95% of the riding.

I prefer to go the route of a triple crank when setting up wide range gearing for touring or just hilly areas. Combined with a medium range cassette (12-26 or 28) this gives excellent gear selection in the mid range, while allowing super low gears, usually lower than what can be achieved with a double and a 34t rear. If you do go the triple route, consider your needs at the high end. Most people have little use for the top end 53/11 or 53/12 or typical road bikes, so a triple built around a 50t or 48t outer chainring might give you combinations you'll actually ride.

If you decide to open up all possibilities, start by using a gear chart to calculate what gear ratio you do most of your level ground riding, what you want as a low, and what high gear you actually would use. Then find a cassette which combined with a granny will give you the low end needed. With that cassette in mind pick a chainring that will give you your level gear using the outer middle of the cassette (sweet spot), and the necessary high with the outermost. Lastly slot in a middle (if you have a choice) which gives you a good working low range off the middle and inner cassette sprocket for the rolling terrain (or when you begin to tire). I try to have the middle chainring gear values be a half interval off from the outer so rather than duplicate values, I can use either middle or outer rings with the middle of the cassette to create even finer spacing.

The first thing to do is measure your rear dropout spacing... Older 7-speed bikes generally used 126mm dropouts. Newer 9- and 10-speed hubs require 130 mm spacing. So tou will likely need to "cold-set" (i.e. bend) the rear triangle to fit the larger hubs. That is probably best left to somebody that has some significant shop smarts.

You may find it easier/better to change the front chain wheels instead. What are you running in front?

Use one of the many online gear calculators to get a feel for how changing front and rear tooth counts affects overall gear ratios and rideability. Google is your friend!

The SRAM PG730 cassette is 7-speed and 12-32T (and Shimano-compatible). I've thought about trying one.

I agree with FB's observations, but assuming you indeed don't want to bother with compact/triple I would recommend you get a 7-8 speed with as large a high gear cog as you can get to minimize the spread in the middle. Absolutely not a 12-34 or 11-34. Otherwise you get a larger spread AND a useless high gear (or two). I think the best you can do is a 13-34 8 speed or a 14-34 7 speed, which are not too bad in the center of the range. With a 7 or 8 other equipment issues are minor as well.

Another option is to replace the 39t with a "triplizer" ring. If the chainstay clearance permits it without moving the crank out, this is an easy cost effective option. Depending on what you add as a granny, it may or may not require a new front derailleur. (most, but not all left shifters allow for a triple, even if sold as doubles) .

It's something to consider.

Also note that going to a larger cassette will almost guaranty needing a long cage RD, but if you're willing to lve with limitations on the use of the granny, you can often keep the RD with a well configured triple. (it'll depend on the ratios, and what limitations you can accept)

For maybe $60, you can get a Deore-level 9s 12-36 cassette, which I have on my mountain bike. 9s chain is a bit more expensive than 8s, but very much cheaper than 10s. The 36t cog should work fine with any vaguely recent Shimano MTB derailleur, and the new ones are designed for it.

Since a 53x12 top gear is silly, you might swap the 12 for a 13 bottom to give you something like 13-14-16-18-21-24-28-32-36 cogs. This isn't much different than your current cassette -- there's just a moderately big cog and huge dinner plate added.

39x36 should get you up pretty much anything.

IF your frame spacing will handle 8 speeds, it'll handle 9 or 10.

You'll spend about the same for the cassette if you shop around and a few more $ for a 9 speed chain than 8.
The variety of cog combinations in 9 vs 8 is MUCH better.
Plus you have that extra gear to help "fill in" the middle.