Bdox, yes, a smaller gear. I just sent jpjahn a PM telling him to read the newest post.

That's how I figured out my speedo set up, but I wrote it down differently and missed read it. That was 19 years ago when I did the tire/rear end change.

Man I feel stupid.

Time for another beer.

 

OK, I had the same problem, not stock tires, new gears(4.10s) and speedo off by who-knows-what. I went to a real speedo shop and heres what THEY told me.
All speedometers are set for 1000 revolutions per mile, regardless of speed thats it for all american cars.
So start with the odometer, go find a measured mile on a highway somewhere, and watch the ODOMETER, if one mile on the ground is one mile on the clock the speedo is working pretty right. If your are off by a tenth low (.9) your speedometer SHOULD be slow by the same, one tenth over (1.1miles) your speedo SHOULD be fast.
Now go back and TIME one mile and what the speedo says is sixty. SHOULD be one minute. Same rules above apply. Once you figure the percentage you can get a new gear.
Heres the catch, they only make replacement gears so big and they are color coded. Mine was black and I went to red, the biggest that fits in the hole. I was off by 15% the gear took care of 5% so I am still off by 10 %. Gave me a perfect reason to buy 33/12.50s . Speedo spot on now.
efstop22

 

I was wondering why the speedo driven gear size that we came up using math on here was opposite of what a speedometer gear calculator came up with on the internet. When I use the calculator on the internet, its completely wrong though, because when I enter in my old info, it comes up with me needing a 17 tooth gear(I have 20 originaly) and when I enter the info with new tires, i need a 14 tooth gear. So I don't believe what the calculator tells me.
Anyways, I just got back from a trip to Idaho. I took my truck so I could see how good of gas mileage I get, and to see whether it would make it all the way. When I was going down the highway, I used a mile marker and timed how long it took to do a mile. I figured it out so that when I am going 50 according to the speedo, i'm actually doing 60. I lose 2/10's of a mile per mile. I went 146 miles and used 9.3 gallons, thats 15.6 mpg, which i'm pretty happy with for the old 351.

 

I am not familiar with that internet calcualtor you mentioned, but it sounds to me like it was for a vehicle other than yours.

I know all these guys are figuring out their tire circumference etc., but all you need is the diameter of your stock tires, and that of your new tires, that you can measure with any tape measure or even a yard stick. (OK a yard stick doesn't work on monster trucks!) The percentage is the same either way you figure it out. I prefer easy.

For example: Orignial tire=29 inches. New tire=33 inches. 29 deveded by 33 gives you 87.87 percent. Therefore your new speedo gear must be 87.87 percent the number of teeth of the origianl. So if your original is 20 teeth, .8787 X 20 teeth = 17.57 teeth and you get as close to that as you can.

You can do the same type of thing for ring and pinion ratios. Such as: Old gear ratio is 3.5 and the new ratio is 4.1 Same deal, divide 4.1 by 3.5 you get 1.17, or 117% Next, take the old speedo drive gear's number of teeth times 1.17 and you get your new number. (Remeber, in this case we are speeding up the speedo.)

If you are changing to larger tires and changing your differential ratio at the same time, do the first calculation for the tires and save the number. (17.57 teeth in my example.) Then use that number (don't round it off yet,) and use that in the final drive ratio formula. So using my (hypothetical) numbers, 17.57 teeth times 1.17 gets you 20.55 teeth. If your numbers are close to those that I'm using here, your original gear should work. Generally speaking that is a good goal. The power to the pavement remains the same as does your gas milage.

 

I am not familiar with that internet calcualtor you mentioned, but it sounds to me like it was for a vehicle other than yours.

I know all these guys are figuring out their tire circumference etc., but all you need is the diameter of your stock tires, and that of your new tires, that you can measure with any tape measure or even a yard stick. (OK a yard stick doesn't work on monster trucks!) The percentage is the same either way you figure it out. I prefer easy.

For example: Orignial tire=29 inches. New tire=33 inches. 29 divided by 33 gives you 87.87 percent. Therefore your new speedo gear must be 87.87 percent the number of teeth of the original. So if your original is 20 teeth, .8787 X 20 teeth = 17.57 teeth and you get as close to that as you can.

You can do the same type of thing for ring and pinion ratios. Such as: Old gear ratio is 3.5 and the new ratio is 4.1 Same deal, divide 4.1 by 3.5 you get 1.17, or 117% Next, take the old speedo drive gear's number of teeth times 1.17 and you get your new number. (Remember in this case we are speeding up the speedo.)

If you are changing to larger tires and changing your differential ratio at the same time, do the first calculation for the tires and save the number. (17.57 teeth in my example.) Then use that number (don't round it off yet,) and use that in the final drive ratio formula. So using my (hypothetical) numbers, 17.57 teeth times 1.17 gets you 20.55 teeth. If your numbers are close to those that I'm using here, your original gear should work. Generally speaking that is a good goal. The power to the pavement remains the same as does your gas milage.

 

http://www.tciauto.com/tech_info/speedo_gears.htm

This is one that I found. But there are several different ones, they all come up with the same info though.
You guys should check the numbers using your own vehicles, to see if the calculator is wrong, or if something has been changed in my truck that I don't know about yet.
Let me know what you come up with
Thanks

 

jpjahn, read my last post. I took a quick look at that calculator you just posted, but using it is more work than using my method. Bdox

 

I know. I need a 17 tooth driven gear. But at this point, what I want to do now is find out why the numbers are so far off when I use the calculator. With my rear end ratio, and tires I should not have this combination of drive and driven gear. Its nowhere near correct.

 

jpjahn, what did you put as the drive gear teeth number?

I punched in all for numbers listed, and of course got 4 different driven gears.

 

7 drive gear teeth, with 3.55 gears, and 29 inch tire (original stock)

 

I too came up with 14 teeth on that calculator.

Are you sure you have a 7 tooth drive gear?

I'm not doubting you. Did you actually count the number of teeth on?

You have 35 tires, right?

 

yeah i used a mirror and shined a light inside of the whole. I also used my finger to count the ridges.

 

And you double and tripple check it.

Original ratios on the drive train?

 

well, yeah, I guess i could check it tomorrow again for a third time, but i think the two first times did the trick. I know that when I used the calculator, i could use 8 drive gear teeth to get the right numbers, so i went out there expecting to find 8 teeth, but i only found 7, nomatter how hard i tried. I'm sure i have 7.

I am beginning to wonder if I have the original ratio's in back. That is the only thing that i can think of that can explain the wierd numbers.

 

Since you have a 4x4, I hope both front and rear differentials are damn near the same.

Worse comes to worse, you can always buy the 17 tooth driven gear.

Price out the gear. It shouldn't cost that much.

Ford has two types of driven gears, or at least for my truck.

Here's a pic of both of them.

http://www.428cobrajet.com/speedo-calc.html

Look at the pic. There is a difference in both driven gears.

Where gonna get this speedo gear resolved, even if it kills.......you.

Edit: I found another site with a calc. and same number (14).