"And wouldn't it be easier trying to maintain a cadence with a smaller chainring thus raising your average speed over the same route?"
No. It's harder to maintain the same cadence with a smaller chainring. The higher the gear (e.g. a 70 inch gear compared to a 75 inch gear), the harder it is to turn the cranks.
There is no way around the reality that you have to work harder to go faster, and I think you might mistakenly think you can go faster with a different chainring, without putting in the extra work. At the relatively slow speeds you're riding - that is, the relatively small amount of work you're putting out - the difference in average speeds with either chainring will be very small.
However, 15-16mph is, for most humans, a very decent speed to average in a single gear over 24 miles. You can, in fact, go faster by getting stronger, just continuing to ride your 24 mile route and pushing a littler harder. Over time, with either chainring, your average speed will begin to rise.
And you don't have to push that much harder in either chainring. If you're riding about 15-16 miles in both of them, all you need is to average about 10 more rpm over the course of your ride. You don't even have to hit 90rpm to do that, and you'll gain your extra 2 miles per hour.
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Last edited by icyclist; 03-19-13 at 12:58 AM.