[fleetsidelarry]

shortbed, the only problem I see with wrenchbender's suggestion is that it is trial and error, requiring you to have different gears to swap BEFORE you know what to get.

I don't know how you determined that you only need to change to a 2.75 ratio from your current 2.71 but, for instance, if you are off 10 mph at 50 (just for example) you are off 20%. changing from 2.71 to 2.75 is only about a 1.4% change if my math is right.

I like to offer another suggestion.
1.as wrenchbender suggested, time your truck through a measured mile or two at a steady speed.
2. note your speedo reading.
3. divide 3600 by the number of seconds it took to go a mile (if you go 2 miles, divide your total time be 2 to get an average time per mile)

this gives your actual speed versus your indicated speed. now you're not guessing

example, say you put your truck on 60 by the speedo and time the mile at 55 seconds: 3600÷55=65mph. if it takes 70 seconds then 3600÷70=51.4mph.

since you know what gears you have you can determine a driven gear to achieve the change you need. if you are reading too high as you said, you're going to need to slow the speedo down, which means MORE teeth on the driven gear.