Higher bike gear vs lower gear

Would it be better for workout to ride a high gear, go slower, but push harder to travel. Or to be in a low gear, and sustain a higher speed with less effort? When I do my daily 3 mile ride, I try to do it all in 15th gear, 1.5 mile light down grade, and the other is slight up. The up hill is hard to keep a high speed on, and I did't know if I would be doing better if I lowered the gears but peddled faster... Thoughts?

depends what you want, i guess: lower gears for more cardio, higher for more strength training.

Most cyclists try to maintain a steady pedaling cadence between 85 - 95 ppm.  That pedal speed would seem very fast to someone not accustom to it.  They shift gears inorder to maintain the cadence.  As you become fit and accustom to the high cadence, you can maintain a good speed for hours. 

However, cyclists will also do various kinds of intervals to improve strength and speed.  One of those intervals is to intentionally push too much gear at a lower cadence as a way of building strength.

So you can do both.  But be careful, cycling is knee freindly when spinning faster gears but you can injure a knee by pushing too hard a gear.

trh is right on!

Maybe think of increasing your distance to increase your fitness, or vary each day do one sprint in low gear and one high gear and hard, then one easy to recover. Mixing it up will help your fitness improve as well.