Gearing For a Non-racer's Road Bike
If you will be riding where it is hilly you will want a triple crank. (3 gears in front) This provides for a wider gear range. In particular it allows the addition of lower gears.
A double crank might provide you with 52 and a 42 tooth rings in front and some number of gears (as many as 9) in back ranging from 11 to 24. This gives you:
52/11=4.73 for a highest gear
42/24=1.75 for a lowest gear
Adding an additional chain ring with 32 teeth gives you:
32/24=1.33 for a lowest gear
All gearing arrangements are compromises. The differences are in which compromises you want to make. More gears and greater range add a little weight. More range also means wider spacing of the gears. If it's important to you to be able to pedal your bike up past 50 mph while going down a hill that you might coast down at 40 mph then you might want a gear a bit higher than even the highest gear above. If you want to climb hills, especially long ones, without putting high loads on your knees you might want lower gears than the lowest above. Very few recreational riders have any use for the highest gears above. Racers like large gears on the crank because they want high gears and they want close spacing of the ratios. A recreational rider looking for a wider range of gearing and not needing the highest gears can have a smaller large front gear. A few percent change makes a large difference. It's worth noting that 52/13, 48/12 and 44/11 are all the same gear. My suggestion is something like this:
50/12=4.16 for a highest gear
32/26=1.23 for a lowest gear
for an experienced, fast rider in good shape. Riders also vary in their preferred cadence. Some riders would rather spin the pedals a bit faster while others would rather push a bit harder. A strong, low cadence rider who likes speed might like a 52 tooth gear and a rider who likes high cadence could be happy with a 48 tooth gear. Changing from a 12 tooth to an 11 tooth smallest gear in back makes a huge difference.
52/12=4.33 50/12=4.17 48/12=4.00
52/11=4.73 50/11=4.55 48/11=4.36
This is an easy way to provide a significantly higher top gear, but may require a slightly smaller largest gear in back or in front because all derailleurs have a maximum limit to range of gears they can accommodate.
A touring bike might have a 44 tooth ring as it's largest front gear. It could have a range like:
44/11=4.00 for a highest gear
22/32=0.69 for a lowest gear
A simple guideline when looking for a touring bike is to demand that the largest gear in back have at least 6 more teeth than the smallest one in front. The most serious "loaded tourers" will have a difference of 8 or 10 as in the above example.
Gearing similar to this, but with a low gear more like 24/28 is appropriate on a light road bike for the recreational rider who likes to look at hilly scenery and has little interest in riding in groups at high speed.
A mountain bike might have gearing similar to a touring bike. Its different wheel size makes its effective gearing slightly lower though. For mountain biking it's more critical to have a number of very low gears and less critical to have the wide range that is desirable on a touring bike.
A standard way of expressing gearing is called "gear inches". This is a simple function relating a rotation of the pedal crank to the distance traveled. GI=(gear ratio) X (tire diameter) Its virtue is that it takes wheel and tire size into account. If you have a road bike with wheels other than 27" or 700c you must take this into account when comparing gearing. Changing tires sizes also changes the effective gearing, but not enough for most people to notice or worry about. Two nice gear calculation sites are at: