Speedo Gearing question

[Cornwallrc]

I've got a Speedo Gearing question, Looking for some help.. I've swapped the gears in the rear Diff to 3:7:3, running a 350CI with TH350 Tranny, running 295/50/15 tires which = a tire height of 26.61" tall, My Speed seems to be roughly 40MPH off??? I think the truck originally had 3:4:2 rear end.. When I'm doing approx 55Mph its reading 100Mph Here's a picture I took along time ago "When building the truck" of the speedo gears.. Can someone help me out with possibly the propper gear combo I need to get or what my options are?? Thanks

[MARKDTN]

From what I see using
http://www.tciauto.com/tc/speedometer-gear-calculator/

(assuming you change to a 7 tooth red drive gear) you need a blue 20 tooth driven gear.

[geezer#99]

If you can't find the right combo of gears and don't want to pull the tail housing off then use a ratio adaptor to correct it.
http://www.transmissioncenter.net/sp...n_______va.htm

[Yooper truck]

Isn't that the correct conversion to Kph? Sorry..... I couldn't resist when I saw you were from ON.

I understand the frustration as I have my own speedo issues right now.

[Cornwallrc]

LOL..... Your right.... it is really close to the Kmph.... which would be fine with me but..... Its adding so many miles to the Odometer and when on the freeway the speedo only goes to 100MPH... so umm ya.... running out of space to run the 120Kph the truck is doing... LOL

[davepl]

It's not adding miles to the odometer. It's adding Kilometers to the odometer!

I think its covered, but if you can find one of those speed signs add about 1-2mph to what it indicates (unless you're a good half block back, then it should be ok). Then use any of the numerous websites to calculate the gear colors you need to adjust for the error.

Have to do the same thing myself so was just asking about it at my favorite "Old guys that fix old cars shop" yesterday.

[Cornwallrc]

Correct me if I'm wrong.. But my drive gear is the 7 tooth??? and I believe what I have is the 18 tooth driven gear.. So assuming that is right (by counting teeth in the pics). Do I only have to swap out the drivin gear to a 20 from the 18?? Someone told me rule of thumb that each tooth is worth approx 5Mph?? At what point do I need to swap the drive gear so that the mesh of the 2 gears is ok?? Meaning I don't think I can fit a 24 tooth drivin gear in and still mesh up well with the 7 tooth.. the Drivin gear will be getting larger in diameter with more teeth, at some point I would need a way of adjusting the mesh of the gears?? I'd like to order some gears, but I don't want to order and by something that won't work.. Thanks for the help.. Jamie

[geezer#99]

You would be correct. Drive gear 7 tooth and driven gear 20 tooth. The calculation is in the link I gave you.
http://www.bgsoflex.com/speedo1.html

[Cornwallrc]

I just can't wrap my head around how only 2 teeth on the driven gear is going to change my speedo from being off by almost 40Mph. When my speedo says I'm doing 100mph I'm actually only doing like 60. ?