Why 3.91/4.10 Gears make the best short term expressway gears!

Like others have said overdrive is just an extra gear. You can keep short rear gears and do all the same things you mentioned in your OP but when you aren't running balls-out you can shift one more time and have the RPMs drop to a nice mellow level. It's not like you HAVE to drive in top gear all the time, or the overdrive somehow affects your other gear ratios... Try driving a good high-performance car with overdrive (better yet a Mopar musclecar) and you'll quickly figure out why everybody wants one. My friend's 360-powered '73 B200 has 4.10s and it is fun around town but it gets old kinda soon on highway trips longer than 5 minutes...