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

[cal30_sniper]

3.73s work great with a 700r4 and the factory tire size. That's what I had in my '90 R1500 Suburban (350), and it was capable of knocking down 16-18mpg. With a 26 inch tires, you might look at stepping down to a 3.42 ish gear. The ratio is easy to count, just turn the tire one revolution and see how many times the driveshaft rotates. That's your ratio.

[1ndatrunk]

Would the 3.73 and the 3.42 give me the ability to make a little wheel spin off I wanted to?
Thanks for the quick reply

[cal30_sniper]

Depends on tire size and engine mods. I had Vortec heads, a performer intake, quadrajet, HEI, full tube headers, and dual dynomax exhaust on my 350. The peg leg rear in it could do a decent job of tire spin when you wanted to. The warmed up 454 I have in my current burb can spin all you want, even with 32" tires and a locking 14 bolt rear. I really like the 3.73 ratio as long as you've got a truck with an overdrive trans. Its about as high as I'd want to go with the steep 1st gear in a 700r4 though.