Speedometer question
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Speedometer question
Hello fellow bump side enthusiasts. First a little about my project. I am fairly new to this forum since I have recently bought a 1969 F100 with a 360 and a 4 speed. It also happens to be a short bed. I am in the middle of rebuilding the entire running gear from the engine to the differential. I am using a very trusted engine builder to take care of ask the stuff I can't do. I got some bad news over the weekend. Apparently my block had some cracks in it so we had to junk it. We found another block abd took the opportunity to turn it into a 390. We are putting in done mild cams, a Holley Dominator intake and a 650 cfm carburetor, so it's going to be a strong motor. We are also going to swap out the differential gears so that it's more highway friendly, which brings me to my question. Will going with taller gears mess up the speedometer abd how can I correct it?
Logistics Pro
Joined: Nov 2003
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From: PA
Hello and welcome to FTE. Congrats on your build and there's nothing wrong with going from a 360 to 390 when you got more cubes to play with right? To answer your question your speedometer will be off. Anytime you change your tire size and or rear end gear ratio your speedometer will need corrected. You need to change the gear on the end of your cable going into the transmission with the right toothed gear that matches your setup. There is ratio calculators online you can use to plug in all your info and will tell you what gear you need to use to correct it.
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Thanks, I'll have to look that up. I only drove the truck a few feet get when I bought it. I knew I was going to do some engine work on something that old anyway so I just drive it into the trailer to bring it home. I did notice that the speedometer was pretty jumpy. It bounced from 0 to about 10 mph in those few feet. Will that fix this problem also?
Hotshot
Joined: Mar 2013
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From: Phoenix, Az.
Thanks, I'll have to look that up. I only drove the truck a few feet get when I bought it. I knew I was going to do some engine work on something that old anyway so I just drive it into the trailer to bring it home. I did notice that the speedometer was pretty jumpy. It bounced from 0 to about 10 mph in those few feet. Will that fix this problem also?
Ford Parts Specialist
Joined: Nov 2006
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From: Simi Valley, CA
Since you are changing the ratio, the speedometer will not read correctly.
The DRIVEN gear you have is probably nylon. 16 thru 21 tooth nylon DRIVEN gears have been used since 1960, all are available from Ford & auto parts stores.
What you'll have to do...is fiqure out how far off the speedo is, then install a new DRIVEN gear with the apropos number of teeth to either slow the speedo down or speed it up.
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I appreciate all the speedy advice but I actually went to see my engine builder yesterday. I gave him more money and asked him the same question. Apparently it has all been taken care of. According to him, all I have to worry about when he's done is the cosmetic stuff. The entire drivetrain from nose to tail will be looked at and made better.
Logistics Pro
Joined: Nov 2003
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From: PA
Sounds like a good deal to me getting it back ready to run all mechanicals looked over and refreshed. Take some pics of this nice clean refreshed engine bay when you get a chance.
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Right now all I have is the picture of when I was bringing it home but I'm not sure how to post it here.
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Already built the engine?
If not swap to a 428 crank.
The 410 engine, used in 1966 and 1967 Mercurys used the same 4.05 inches bore as the 390 engine, but with the 428's 3.98 inches stroke, giving a 410.1 cu in real displacement.
If not swap to a 428 crank.
The 410 engine, used in 1966 and 1967 Mercurys used the same 4.05 inches bore as the 390 engine, but with the 428's 3.98 inches stroke, giving a 410.1 cu in real displacement.
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I spoke with my mechanic about that and we both decided that for what I want, the 390 with the add-ons I'm doing to it will be enough for now. I just want a comfortable highway cruiser with a good exhaust note but still give me great power just in case I need it. Right now, all I want to do is be able to outrun my brother's Dakota R/T.
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