can't get speedo to read right

[tootall]

I got a 04 f250 6.0, bought it with a lift and 355/60/20 and other than that thats really all i know, got the sct tuner and have been trying to set the tire revs and the axle ratio, but no matter what the speedo is always off, the best i had it was 5 mph off, but the truck was shifting all kinds of funny, and just seemed gutless....like it was stock again on the extreme tune, 4.10 axle ratio and 560 rpm on tire setting, the axle tags are missing so im not sure what to set and what to set it to? anyone have any ideas or solution, im going off my navigon gps, holds true in my other car so i don't think its off, also if i get the tire/axle ratio setting correct i'll be able to see how many real mpg im getting right? cuz if the speedo is off than the odometer is off as well so i can't do the math =) the lie o meter up top says 12-12.5 and i know i should be getting more than that..any help would be appreciated. sory for the long winded story

[dirtbikeguy2003]

You need to know what gears are in it for sure. You can try and different ratio and the rest to see if it gets closer. 3.73, 4.10, 4.56

[tootall]

i don't know how to tell what gears it has, axle tags are missing so i can't get the code from it, just lil annoying...its been on 3.73 as i thought it was stock and the previous owner just replaced the diff covers, but its 8-10 mph off at freeway speeds, going 55 tach reads 1550-1600rpm with 3.73 setting and 560 on tire rpm