Speedo Gear Question

[dgold_881]

I searched the forum and I’m more confused then I started.. so I'm posting a new thread. I have no clue what the rear diff gearing is I'm assuming stock. I'm currently running 33" 12.50x15 tires and my speedo is off 10mph over what I'm actually doing. So at 55 my speedo says I'm doing 65. Any suggestions on what tooth count I need to fix it? I believe the gear in there now is white. Thanks for the help. ?:?

 

[73azbronco]

All things equal, if you only change tire size from stock and the speedo read right at that time, then larger tires would turn slower to cover the same distance, in other words the tires turn slower to do the same speed, therefore, larger tires would make the speedo read less than what you are really doing, say 50 instead of what the speed radar says is 60. Or, if you went 55 on your speedo with bigger tires, you would actually be going 60ish.

You problem is just opposite, which leads me to believe you do not have stock gearing.

In any event, your reading about 16% off to the fast side, so you need a speed gear which turns 16% slower, or has more teeth, 16% more. In your case that would be two more teeth depending on how many teeth you now have. I have usually seen 16-19 teeth on our speedo gears.

 

[dgold_881]

Is there a way to figure out my gearing with out cracking into the case?
My EB runs like a raped Ape and is fast off the line and shifts great, no slugish hesitation at all. but it seems to be a little high on the RPMs at higher speeds I just figured it was like that since its a 3 speed and no OD.. so I'm guessing it has a higher gear ratio. When I bought it it had
35's on it and it didn't run.

All things equal, if you only change tire size from stock and the speedo read right at that time, then larger tires would turn slower to cover the same distance, in other words the tires turn slower to do the same speed, therefore, larger tires would make the speedo read less than what you are really doing, say 50 instead of what the speed radar says is 60. Or, if you went 55 on your speedo with bigger tires, you would actually be going 60ish.

You problem is just opposite, which leads me to believe you do not have stock gearing.

In any event, your reading about 16% off to the fast side, so you need a speed gear which turns 16% slower, or has more teeth, 16% more. In your case that would be two more teeth depending on how many teeth you now have. I have usually seen 16-19 teeth on our speedo gears.

 

[Broncobowsher]

55/65 = 0.846
Your speedometer is only reading 85% of your actual speed
What you need to know is the number of teeth on the driven gear on the end of the speedometer cable. Take the number of teeth and multiply by .846 to get your corrcted number of teeth you need. If you currently have 19 teeth, multiply that by .086 and you get the number 16.0769. Round to the nearest tooth so you need a 16 tooth driven gear providing the inaccuarcy was done with a 19.

Now the fun part, how many teeth do you currently have? The ONLY way to know is to pull what you have and count. There were different gear ratios in the axles, different tire sizes offered, different number of driving teeth in the transfer case, and then someone could have changed something in the past 40 years as well. There is no accurate way to know the number of teeth you are starting with unless you pull it and count. Good news is there is only 1 small bolt to pull and the cable comes out above the fluid level so at most there will only be a drop or two of oil on the end of the gear so it won't even make a mess.

Little note, the fewest number of teeth you can get on a driven gear is 16.

 

[dgold_881]

Thanks for all the help guys I will check it when I get home from work.

55/65 = 0.846
Your speedometer is only reading 85% of your actual speed
What you need to know is the number of teeth on the driven gear on the end of the speedometer cable. Take the number of teeth and multiply by .846 to get your corrcted number of teeth you need. If you currently have 19 teeth, multiply that by .086 and you get the number 16.0769. Round to the nearest tooth so you need a 16 tooth driven gear providing the inaccuarcy was done with a 19.

Now the fun part, how many teeth do you currently have? The ONLY way to know is to pull what you have and count. There were different gear ratios in the axles, different tire sizes offered, different number of driving teeth in the transfer case, and then someone could have changed something in the past 40 years as well. There is no accurate way to know the number of teeth you are starting with unless you pull it and count. Good news is there is only 1 small bolt to pull and the cable comes out above the fluid level so at most there will only be a drop or two of oil on the end of the gear so it won't even make a mess.

Little note, the fewest number of teeth you can get on a driven gear is 16.

 

[dgold_881]

I took the speedo gear off and counted the teeth and there is 17. Adding a tooth reduces the reading by 5% right? So would I have to replace my 17 tooth for a 20 tooth? Since my speedo is reading 10 miles faster than what I'm really doing?

55/65 = 0.846
Your speedometer is only reading 85% of your actual speed
What you need to know is the number of teeth on the driven gear on the end of the speedometer cable. Take the number of teeth and multiply by .846 to get your corrcted number of teeth you need. If you currently have 19 teeth, multiply that by .086 and you get the number 16.0769. Round to the nearest tooth so you need a 16 tooth driven gear providing the inaccuarcy was done with a 19.

Now the fun part, how many teeth do you currently have? The ONLY way to know is to pull what you have and count. There were different gear ratios in the axles, different tire sizes offered, different number of driving teeth in the transfer case, and then someone could have changed something in the past 40 years as well. There is no accurate way to know the number of teeth you are starting with unless you pull it and count. Good news is there is only 1 small bolt to pull and the cable comes out above the fluid level so at most there will only be a drop or two of oil on the end of the gear so it won't even make a mess.

Little note, the fewest number of teeth you can get on a driven gear is 16.

 

[Broncobowsher]

So the speedometer is reading too fast, I got it backwards. It is usually reading slower then you are actually going due to larger tires. You must have put in lower gears.

65/55=1.181818 (determine speedometer inaccuracy)
1.181818x17=20.0909 (apply the inaccuracy factor to the reference number of teeth on the driven gear)
So a 20 tooth gear should work perfect for you.

 

[dgold_881]

yea my speedo is reading 70mph when I'm actually doing 60mph.

So the speedometer is reading too fast, I got it backwards. It is usually reading slower then you are actually going due to larger tires. You must have put in lower gears.

65/55=1.181818 (determine speedometer inaccuracy)
1.181818x17=20.0909 (apply the inaccuracy factor to the reference number of teeth on the driven gear)
So a 20 tooth gear should work perfect for you.

 

[Broncobowsher]

yea my speedo is reading 70mph when I'm actually doing 60mph.

70/60*17=19.8333, still close enough to 20