Need help from a speedometer gear size expert!

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cruiser

cruiser

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Hi guys. I have a 225 powered 74 Duster with the TF A-904 auto tranny. The tranny was rebuilt last year and I don't know the tooth size of the speedometer drive gear that they put in. My completely stock speedometer reads four mph slow. If I change this gear, will my speedo read correctly? I think I need fewer teeth. How many teeth reduction would it take to make my speedometer speed up by 4 mph? Any ideas? Thanks.
 
Thanks, Mike. I've never removed one before. How hard is this to do? Will tranny oil go all over the place? Are the drive gears readily available?
 
cruiser said:
Thanks, Mike. I've never removed one before. How hard is this to do? Will tranny oil go all over the place? Are the drive gears readily available?
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They are easy to get. Brewer Performance has them. Megaparts USA does as well to name a couple. Easy to pull and it will make a little mess. Put a small drain pan or a old cookie sheet underneath. There should be a bunch of threads here to show you how.
 
You need your tire diameter, and rear end ratio and the chart below will tell you what gear you need

Screenshot_20200510-113240_Samsung Internet.jpg
 
OP first thing you need to do is read a service manual. Mopar speedo gears are the easiest in the industry to deal with. Go over to MyMopar and download a factory shop manual.
 
4 mph at what speed ? The way I do it is to find a mile marker and note your odometer. drive 10 miles and note your odometer. Divide the difference by 10 and multiply that number by the number of teeth. so if your odo says you drove 9 miles and your tooth count is 30 then you will need a 30 x 9/10 = 27 tooth gear. or just use the charts.
 
Convert the error to percent, then change the existing gear by that percent.

For instance 4 mph at 30 mph is 13%, but at 60 it is only 6.66%
If you have a 27 tooth the former is 3.5 teeth and the latter is 1.8 teeth

If your speed-O reads high, and you need to slow it down, this will require more teeth, and vice-versa.
 
trapster said:
4 mph at what speed ? The way I do it is to find a mile marker and note your odometer. drive 10 miles and note your odometer. Divide the difference by 10 and multiply that number by the number of teeth. so if your odo says you drove 9 miles and your tooth count is 30 then you will need a 30 x 9/10 = 27 tooth gear. or just use the charts.
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My very accurate speed app on my cell phone tells me that I'm actually traveling four mph faster than what my speedometer is showing. At 74 mph on my phone, my speedometer reads 70 mph. I've checked the speed app against the very accurate speedometers on my Lexus and my Mercedes, and the app is right on. So I'm pretty sure I'm 4 mph high at 70 mph. I'm thinking about installing a pinion gear with one fewer teeth, which should speed up my speedometer.
 
4 at 74 is 5.4%
on 29 teeth that is 1.57 teeth. You are borderline as to making it worth-while. Changing the gear, you will still be off. Since it is off in the right direction, personally, I would make a mental note of the 5.4 %, and just leave it. It's pretty hard to get a speeding ticket, when traveling 5.4% slower than the limit.
5.4% at 37mph is just 2mph, at 55.5 it is 3 mph. And no matter what, that speed-O is still gonna be out .43 tooth or ~1.1 mph@74mph, you can't get closer than that.
The thing of it is; you gotta take the current one out to determine what you have, put it back in. Then find what you need, get it shipped to you, then swap it in;
meh, too much work for me, when 5% is so easy to calculate mentally.
jus saying.....

Oh crap I got it backwards
You said;
cruiser said:
I'm actually traveling four mph faster than what my speedometer is showing.
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In that case ok, ya got me, I would speed it up
 
With an auto tranny just jack the butt up in the air as high as you can, use jack stands of course, and you shouldn't loose any fluid.
Here is what the tooth setting looks like.
I would go one tooth smaller.

20200816_133419.jpg
 
You should check the accuracy with your odometer, because it is gear driven and not prone to becoming inaccurate over time like your speedometer is. If your Odometer is not off by the same percent then you probably need to replace it.
 
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