Bicycle Mechanics - Changing Sprockets?

Hi Guys & Gals....
Even though I ride quite a bit, I'm not too knowledgable about certain mechanical aspects of my bike. A few years ago I got a used road bike, 21-speed Cannondale. Love the bike an awful lot except if I really had my way, there's one thing I'd really like to change about it but I am unsure if it can be done without costly upgrades. What I'd like to to is change the gear ratio on the rear hub. I hate to say it but I have become way too accustomed to my hybrid-touring bike gear scheme and have difficulty conquering some of Atlanta's hills on that road bike with its faster gear scheme.
Logic tells me that the sprockets can be changed out on the rear hub (?). If so, should this require a new derailleur system? I would hope not but you folks are the experts. What would parts and labor on a job like this cost (if it can be done at all, of course)?
Thanks for your help, the folks here are the best!

Hi Guys & Gals....
Even though I ride quite a bit, I'm not too knowledgable about certain mechanical aspects of my bike. A few years ago I got a used road bike, 21-speed Cannondale. Love the bike an awful lot except if I really had my way, there's one thing I'd really like to change about it but I am unsure if it can be done without costly upgrades. What I'd like to to is change the gear ratio on the rear hub. I hate to say it but I have become way too accustomed to my hybrid-touring bike gear scheme and have difficulty conquering some of Atlanta's hills on that road bike with its faster gear scheme.
Logic tells me that the sprockets can be changed out on the rear hub (?). If so, should this require a new derailleur system? I would hope not but you folks are the experts. What would parts and labor on a job like this cost (if it can be done at all, of course)?
Thanks for your help, the folks here are the best!

How much of a change are you looking for and what's your definition of a costly upgrade?

A road bike rear derailleur will handle rear cogs up to 27 teeth. Actually, you can "cheat" on that a little, but that's Shimano's spec. If you want rear cogs much bigger than 27 teeth, you probably need to replace your rear derailleur with a mountain bike derailleur. It'll work fine with your road bike shifter.

At the very least you'll need a new cassette or freewheel and chain figure $40.00 or so if you do the work yourself. You'll need some special tools for replacing the cogs and for sizeing the chain. If you don't know anybody who has them, figure another $25.00 or $30.00 for labor. On a bike that old, there's a good chance that stuff is worn out and should be replaced anyway. If you decide to go with a 32 tooth cog and a mountain rear derailleur, figure another $20.00 and up. Sometimes the cheapest way to get it done is to find a garage sale of thrift shop donor bike that has the parts you need on it and throw the rest of it away.

Sometimes the cheapest way to get it done is to find a garage sale of thrift shop donor bike that has the parts you need on it and throw the rest of it away. Or, keep the rest out of the landfill and give it to a local organization or collective that refurbishes bikes.

Hi Guys & Gals....
Even though I ride quite a bit, I'm not too knowledgable about certain mechanical aspects of my bike. A few years ago I got a used road bike, 21-speed Cannondale. Love the bike an awful lot except if I really had my way, there's one thing I'd really like to change about it but I am unsure if it can be done without costly upgrades. What I'd like to to is change the gear ratio on the rear hub. I hate to say it but I have become way too accustomed to my hybrid-touring bike gear scheme and have difficulty conquering some of Atlanta's hills on that road bike with its faster gear scheme.
Logic tells me that the sprockets can be changed out on the rear hub (?). If so, should this require a new derailleur system? I would hope not but you folks are the experts. What would parts and labor on a job like this cost (if it can be done at all, of course)?
Thanks for your help, the folks here are the best!

You can switch the rear cassette and derailleur for about $75 plus labor. If you are a casual rider, you could see if your shop has an old 7 speed mountain bike crank. I'd offer $30 plus labor. That would significantly lower the gearing.

There are 2 ways to lower gears. Make the rear cogs bigger or the front rings smaller.
You can get the cogs changed at any bike shop. most derailleurs can handle slightly bigger than their specs. You may be OK with a 28t rear cog.
The crank bolts may limit your min chainring size. With Shimano, that limit is 38t.
If you need to get much lower, then fit a different chainset. A road triple chainset can take an extra small ring, but the mid/large stay the same. A low-ratio double has a smaller bolt circle, so can take eg a 34/48 combination.
Switching to lower gear ratios makes a lot of sense for a non-athletic rider. I'm using a Stronglight chainset which lets me fit a 28/38/48 combo. Its just right for scooting around a hilly town.