My 79 had a gear box between the cable and the xfer case. The ground speed indicator was about 10% slow, so I looked at the gear ratio that was stamped on the side, and took it to place that make them. I simply added 10% to the gear ratio I already had and told them what ratio I wanted. They made me one up and it cost me about $40. Dialed it in perfectly.
So, if you assume you don't have any little gear box, you would need a 1:1.20 ratio. that's one turn into the box = 1.2 turns out.
If you don't have a gearbox as Mudflap spoke of, just pull your speedo gear out and count the teeth. Then just subtract 20% from the number of teeth you have and this will tell you how many teeth your new speedo gear will require.
I got it at a place called northwest truck parts in Woodward, OK. Lemme know what gear ratio you need and I can have my dad get it for ya. You could just send him the $ and he could get it for ya. I just figured that was a pretty standard thing. There's a place here in Wichita that does it too. I'll check into it. Lemme know the gear ratio you need and I can take mine to them and tell them what I need. Then I'll let ya know the price.
Pull your speedo gear. Count the teeth. Each tooth is good for roughly 3 mph @ 60 MPH. If you want your speedo to read faster, use speedo gear with less teeth. Sounds like you may need one with 4 less teeth to get you close.
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