I have no speedo...no...not the swimsuit, but the indicator. I was off about 10 mph when it did work and want to change the gear reducer anyhow, even if that doesn't fix it (might be a gear in the t-case). My ordeal now is that the local chevy dealer has no idea how to find out which unit to use. I was showing to go about 50 when running 60mph or so. I am running a 350 th trans, 3.73 gears and 31/10.5/15 tires. Isn't there a method of calculating this? The gear reducer on my truck is 1: .866 which is discontinued anyway. I need to make the speedo see me going about 10mph faster at highway speed. Which number needs to change? The (1) or the (.866) is my question I guess. This is on my 69 gmc with 75 axles. Hope someone has an idea, I may get a ticket guessing my speed all the time!