I just finished putting an '86 Astro van T5 in my '48 AD truck. The truck is all orig. the rest of the way and since it's a 60K survivor of which I'm just the third owner I want to keep it that way. She's a 3/4 tonner which means the rear is a 4.56:1 full floater and I'm still running the orig. 15" split rims. This explains the main reason for the swap, but it also explains why my speedo shows 78 when a GPS says 50mph. I've been searching all over the web to get info on speedo gear options and haven't really found all the answers. It had a 19 tooth driven gear originally and I swapped to for a 21 'cause I had one. That improved it from completely rediculous to just mostly rediculous. My Questions are: 1) How much more driven gear can I get to mesh with the drive gear? 2) How do determine what drive is in it now (it's red)? 3) Where do I find a chart or formula to determine the best combination? 4) has anyone used a ratio adjuster to rectify this problem and which one? Any help from you guys will be greatly appreciated!! P.S. Other than the speedo deal this is absolutely the best thing going for this old girl. What a difference in drivability!
Try this site. I had a site saved that sells a correction unit that goes on the transmission but I lost it. It was a place where you drive a measured distance and do a calculation to see how for off the odemeter is and then select the corrector. http://www.bgsoflex.com/speedo1.html If you Google speedometer/odemeter ther are several sites.
They aren't exactly cheap but if you figure the percentage that the speedometer is off most shops that cater to semi trucks have speedometer calibration boxes that go on the side of the trans and the cable fastens to them. They use little quickchange style gear sets to recalibrate the speedometer. Changing the drive and driven gears is probably a lot less expensive if you do it yourself though. Here is another site with good info. http://www.transmissioncenter.net/speedometer_calibration_______va.htm
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