Gear ratio for cruising and mpg

[1ndatrunk]

Hi guys. I recently swapped in a rear from a 74 2wd c10 with a 305, into my stock 2wd 88 tbi/700r4 Suburban because the axle took a crap on me. I have no idea what the c10 ratio was, nor do I know what was originally in my suburban. What I do know is that my speedo is reading 10-15mph lower than actual speed, and that the truck seems sluggish.
I know I can get a new plastic gear to help with the discrepancy in my mph reading, but what about the sluggishness? Is there a gear ratio that would give me some decent pep, while still having good mpg and a low cruise speed? My friend mentioned 3.73 gears, but wouldn't that cause me to cruise at too high of an rpm? I plan on going bone yard pillaging in the next week or so, so can you guys help me out? I'll also be adding 26" rims/ with low profile tires in the next couple of months also, if that makes a difference.
Thanks in advance