s10 speedo recalibration help

i have a 1999 s10 ext cab 4cyl 2wd 5 sp with a 410 rearend, i am going to switch it with a 373 rearend. however i spoke with a chevy dealer, they tried to get a new code from the corporation to enter through the scanner (they used my vin # and said the rearend was switched to a 373) but gm said that they would only go back 2 years to give a proper code. does anyone have any ideas what i could do to recalibrate my speedmeter i would really like to have it accurate. thank you

99 might need re-programmed, it may also have a chip in the back of the Insterment pannel

You can buy one of the hypertech power programmers for about $300 and reprogram the entire computer on that truck. Change everything from timing to tire size speedo calib. everything. It costs alot in the beginning but the uses of it are well worth it.