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Old 02-14-2021, 07:08 PM   #1
tony1960
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Default 47 pickup 100vs60 mph speedometer

hi, all. I finally got my 47 1/2 ton on the road after collecting parts for 19 years!! I have 2 instrument clusters, one with a 60 mph speedo,the other a 100. I want to use the 100 mpg cluster,it's nicer and is made more for a 1/2 ton. problem is that it shows about 60% of my speed,so I know my truck it set up for the 60mph speedo. I have a 20 and a 18 tooth driven gear for the transmission, but neither one changes the speedometer read out by much, maybe 8 mph. the transmission is a 1947 3-speed ,on a 52 flathead V8. don't know about the rear end,but it looks original with the 47 frame. So how do I get my 100mph speedometer to read right? or do I just suck it up and put in the 60mph speedo. I will send pictures of truck I know you guys all want, thanks , Tony
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Old 02-14-2021, 07:41 PM   #2
Bob C
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Default Re: 47 pickup 100vs60 mph speedometer

Do the odometers read accurately?
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Old 02-14-2021, 09:16 PM   #3
JSeery
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Default Re: 47 pickup 100vs60 mph speedometer

No idea on your speedometer issue, but welcome to the Barn!
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Old 02-14-2021, 09:56 PM   #4
Zeke3
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Default Re: 47 pickup 100vs60 mph speedometer

There is a chart in the "Green Bible", Ford Service Parts catalog, that shows the correct speedometer gear for the rear axle ratio and tire size. A part of this chart is shown in this discussion topic on post #10,
https://www.fordbarn.com/forum/showthread.php?t=294334

Ps: welcome to the Ford Barn.

Last edited by Zeke3; 02-14-2021 at 09:57 PM. Reason: added Ps
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Old 02-14-2021, 10:11 PM   #5
alanwoodieman
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Default Re: 47 pickup 100vs60 mph speedometer

go to the rear axle housing and look at the bottom rib on the input side--there will be two number located there, may need to scrape off accumulated crud to read--this is you rear axle ratio. divide bigger number by the smaller and voila!
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Old 02-14-2021, 10:48 PM   #6
rawlir
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Default Re: 47 pickup 100vs60 mph speedometer

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1/2 tons came with the 100MPH speed...Tonners and above with the 60MPH which also shows when to shift.
Don't know why one may work and why one won't.....too technical for this cowboy!!
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Old 02-14-2021, 11:47 PM   #7
Fritz
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Default Re: 47 pickup 100vs60 mph speedometer

Maybe someone put a 100 mph faceplate on a 60 mph speedometer? I think I read somewhere the speedometer can be recalibrated, so you could make it a 100 mph, if they really are both indeed 60 mph.
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