fast or slow speedometer

[70ford1]

I'm sorry to ask this question but I am confused--take a speedometer that shows you are doing ,say 55mph, when in reality you are doing only 45mph. Is this a fast or slow speedometer? Now, to correct this problem & bring the speed & speedometer together would you choose a driven gear with more or less teeth? I guess I need an explaination a 6 year old will understand. I know this is answered somewhere on this thread but I just can't seem to reason it out.

[fireguywtc]

If it says 55 and you are doing 45 then the speedometer is reading faster then you are actually going, so it is called a fast speedometer. And obviously the opposite would be true for a slow one. To correct a fast speedometer you will need a larger gear (more teeth). More teeth take more speed to turn so it will slow down your speedo.

[70ford1]

Thanks fireguy---You have helped me more than you know--I'm really not that dumb--just more confused than everyone else.

[BobbyFord]

The reason the speedo is fast could be from a rear end gear swap, smaller than stock diameter tires, etc.

[70ford1]

The speedometer is not working at the moment--I am going to install some 16" wheels in the near future & I wanted to clarify the fast/slow speedo & also know which way I may have to go as far as speedo driven gear teeth are concerned. There may not be a big difference but at least I will know which way to go to try to even the speed & speedo. My tires will still be in the 26" to 28" in diameter so all may be O.K. I have the stock 3.50:1 differential--same NP435 4 speed tranny-so all may stay the same. I am looking into converting to a C6 tranny later on if I can marry my stock transfer case to it. Never was a fan of a standard trans but I have had this truck since new, 41 years ago, & now the lega & arms are tired. I want power steering w/auto trans for ease of driving. I won't try to do this myself--to old--but I will find a good shop to do it for me. Lots of love for this truck, & want to drive it as long as I can drive. Named it Blue Herc because it has always been on the go. As long as the tires were all on the ground it went. Got stuck once only & bogged down in mud that was 4" up on the door. Had to let it sit over night & let the ground freeze so I could drive it out the next morning. Would love to send pictures but I can't master that operation--gave up trying. Sorry to rattle on but love to talk about my Bumpside 4x4.

[BobbyFord]

If you venture away from a 28" tire then your speedo will need calibrating or you will need a different speedometer driven gear.

[70ford1]

Thanks for the info BobbyFord--I thought maybe that was the case but I feel better hearing it from someone that knows--