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My bike has 53/39T chainrings and a cassette with 12,13,14,15,17,19,21,23,25 sprockets, but I like to ride up steep mountain roads. Is it possible (without great expense) to replace two sprockets to make it a little easier to spin instead of mash?

My rear derailleur is a Shimano RD-6500. I tried checking online, but I couldn't find a clear description of what the maximum sprocket size it supports is. Would it be practical to replace, say, the 23T sprocket with a 24T and the 25T with a 27T?

Or is there something else I should be considering to make the gearing a little more friendly for spinning on steep roads?

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No, you will need to get a whole new cassette, the largest 3 rings are connected to each other, you cannot just purchase individual cogs. You may want to get a new chain as you will probably need to make a chain length change with jumping from a 25T to 27T or 28T largest cog.

Do you know if you have the SS or the GS?

With the SS (short cage) you can go as big as 28T, Shimano sells a 12-28 cassette. With the GS (long cage) you can go as big as 27T, look for a 12-27 cassette.

Another option for easier spinning is a new compact crankset, Shimano comes as a 34/50, but you give up high end speed.

EDIT:

Added screenshot of the relevant tech info for the RD-6500. Hope it's helpful.

Snip of Shimano RD-6500 Tech Docs

Although the derailleur has the capacity for 28t as an SS derailleur, Shimano never produced a 9 speed 28t cog. Therefore, as a practical matter, the 27t is the largest option available for the RD-6500 Ultegra 9 speed derailleur from Shimano. There are cassettes available from Interloc Racing Design which fill this missed niche.

CS-6500 cassette options

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    A very great answer!
    – heltonbiker
    Commented Apr 27, 2012 at 16:36
  • How do I know if I have the SS or the GS? The stamp on the inside of the derailleur that has the model number does not say either SS or GS. If it's relevant, I believe the derailleur is from around 2003.
    – amcnabb
    Commented Apr 27, 2012 at 17:23
  • By the way, thanks for clarifying that I would have to get a whole new cassette. I was hoping that this wasn't the case, but it's good to know. :)
    – amcnabb
    Commented Apr 27, 2012 at 17:25
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    Seems worth pointing out that if you switch to a compact (34/50) crankset and an 11-27 or 11-28 cassette you will keep the high end speed. 50:11 is a higher gear ratio than 53:12, actually a smidge higher than 54:12.
    – freiheit
    Commented Apr 27, 2012 at 18:11
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    @StephenTouset, donations are graciously accepted. :)
    – amcnabb
    Commented Apr 27, 2012 at 18:47
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You can buy individual cassette sprockets! You don't need to buy a whole new cassette.

I don't know where you are, but SJS Cycles in the UK stock individual Shimano sprockets, and Rose Bikes in Germany do too.

Another route is to use a Marchisio sprocket. These can be made up in any combination of ratios you like. Again, you'll need to do a bit of Googling to find a stockist, but you will be able to order individual sprockets or a cassette in any combination of ratios you like. They are Shimano and Campag compatible.

It's also worth looking at SRAM, Miche, BBB and other compatible cassettes if you can't find exactly the ratio you want in Shimano or Campag.

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    You can buy loose cogs ( and even the 3 cog integrated part from ultegra and up ) but if you want to make your own custom cassettes, it's often more cost effective to just buy whole clusters and mix and match.
    – Fred the Magic Wonder Dog
    Commented Aug 17, 2014 at 16:21

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