I have a compact double (50-34) with a 12-32 rear cassette. I put that in when I could not get up most hills with the 12-28 that the bike came with. So, with your 39-25 combo, riding up hills would be very difficult.
I live in an area of mountains. Even the 34-32 combo is not really enough low gearing for me. I'm 59. I ride with a club and most are faster than me. I'm starting to be able to keep up on the flats and downhill, but still am slower on the hills (but I get up them now without walking - YEAH!). But all the women and most of the men here ride with triples and mountain bike gearing in the back - even the experienced riders. You don't need it all the time, but when you need it, you do.
Yesterday I was riding with a woman friend and she wanted to do the hilly route back (she has a triple with MB gears). I said I'd go with her if she would wait for me at the top. Her response was that I must be really strong to get up that hill without a granny gear, and that I would have no trouble if I had the right gearing.
So, I am looking to increase to an 11-34 or 11-36. I was so jazzed that Shimano came out with 10-speed MB gearing (Dyna-Sys), only to find out that they are incompatible with road bike shifters and derailleurs. Why would they finally make what I need and make it not work? I have been looking on forums to see what other people have done. So far the recommendation has been to use the new 10-speed Shimano cassette or a SRAM 10-speed cassette, with a MB derailleur (and not the new Dyna-Sys one which is incompatible, but the older one). I think I will do that.
I don't think anyone should tell you that you should just suck it up and work harder. I've been doing that and I have improved, but it's still a drag. If you have grades over 10% and they last for a long time, I think you just need lower gears. Again, it depends on the geography of where you live, your age, your weight and your fitness. And I think long legged men and some bike mechanics just don't understand.



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