Speedometer off...

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Joshua Suehs

Joshua Suehs

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I have been looking through threads and getting really confused... a lot of good information but I am now more lost than at the beginning. I have the diagram for the speedo gear for my application and bought a 33 tooth gear but after install I see that at 35 (speed app) the speedo reads 45 so it’s 10 off. Don’t want to keep buying gears so can anyone help me with the math??? Sorry I know there are a lot of threads on this, just wanted help. Thanks in advance guys.
 
Tell us tire size & rear end ratio for a good start. Then go find the sale post today for 29 tooth speedo gear. The chart is there
 
72Dart6pack said:
Tell us tire size & rear end ratio for a good start. Then go find the sale post today for 29 tooth speedo gear. The chart is there
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I used the chart to find my original speedo gear. I have seen the thread that talks about which gear i need. I bought the gear based off of the chart and it is still 10 off. I have read a few people post how to do the math on the speedo gear where you just use math to close the difference and walk it in to the right speed. I just can't do the math. also, 26" tires 3:23 gears. Thanks!

Does anyone know whether I need to go up or down in teeth and how much?
 
Chart says you have the correct gear, so did you 'clock' the housing correctly when you installed it?
 
Joshua Suehs said:
I have the diagram for the speedo gear for my application and bought a 33 tooth gear but after install I see that at 35 (speed app) the speedo reads 45 so it’s 10 off.
Click to expand...

Joshua Suehs said:
Does anyone know whether I need to go up or down in teeth and how much?
Click to expand...

My charts says 33 tooth as well based on 26" tire and 3:23's.

Before going any further I would double check the accuracy of that app against an actual GPS unit, or you could have someone pace you. If you still get 45, then something is amiss.


speedo 002 (Medium).jpg
 
Bad Sport said:
My charts says 33 tooth as well based on 26" tire and 3:23's.

Before going any further I would double check the accuracy of that app against an actual GPS unit, or you could have someone pace you. If you still get 45, then something is amiss.


View attachment 1715264095
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I have used this app for many other applications with no issues. When you say amiss what else could it be? I really do not want to replace the speedometer or spend a lot of money. I was just hoping to play the speedo gear game and get it close so I have a speedometer and I am not getting 10 more miles on my odometer every hour.
 
Joshua Suehs said:
When you say amiss what else could it be?
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Well, what I mean is verify everything, tire size again (actually measure it), gearing (is this car new to you or did you do a gear swap), double check that the housing is clocked correctly in the trans as mentioned above.

I know when I bought the appropriate gear for my car (which you seem to have done) mine was spot on.
 
Bad Sport said:
Well, what I mean is verify everything, tire size again (actually measure it), gearing (is this car new to you or did you do a gear swap), double check that the housing is clocked correctly in the trans as mentioned above.

I know when I bought the appropriate gear for my car (which you seem to have done) mine was spot on.
Click to expand...


OK, I will go back and check everything. is there anyway to verify the gears without tearing off the diff cover? The rear end was swapped before I bought it. That is why I was hoping someone could help me with the math on the teeth. If we were able to work the math out we would see that maybe it is a 3:51 which in that case I need a 36 tooth gear. Or if someone knows a rule of thumb maybe it is each tooth is 2 mph down/up then I would know I need to do up 5 teeth. But I can dive back in and make sure i did it all correctly. Thanks!
 
One other thought, did you actually count the teeth on the gear to make sure you got the right one?
 
I don't want to sound like i'm defensive, just never seen that before. Does that happen a lot on these? Im new to the A body scene. Did a lot more with 60's mustangs and newer vehicles. But I really appreciate having a group of people who know what they are talking about. So keep the ideas coming!
 
Joshua Suehs said:
is there anyway to verify the gears without tearing off the diff cover?
Click to expand...

Put a jack under the pumpkin and jack it up so the tires are off the ground, put a couple jack stands under the axle tubes. Make a mark (chalk, masking tape etc) on the right side tire and a corresponding mark on the ground, quarter if using say painters tape, make a mark on the driveshaft and a corresponding mark on the center section.

Turn the wheel one complete revolution while counting how many times the driveshaft turns.

If it turns 3 and 1/4 turns it's probably 3:23's, if it turns 3 and 1/2 times it's probably 3:55's.

Joshua Suehs said:
I don't want to sound like i'm defensive, just never seen that before. Does that happen a lot on these?
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Not really, that's why I suggest checking everything.
 
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So here is a plot twist... jacked it up, spun the wheels, and the driveshaft rotates 3 3/4 times. Would that mean that it has 3:91 gears?
 
Sounds like you don't have 3:23's.

That isn't exact science, are you sure it isn't between 3 1/2 and 3 3/4?
 
Bad Sport said:
Sounds like you don't have 3:23's.

That isn't exact science, are you sure it isn't between 3 1/2 and 3 3/4?
Click to expand...

Ha! Very possible. yeah there is a very solid chance. Well I tell ya what, I'll order the 36 tooth and see what happens. Worst case scenario it closes the gap and gets the speed close! Thanks a lot!
 
If your 3.23's are not Sure-Grip, then you have to do two complete revolutions of the tire to get the correct count on the pinion. Put some tape on the drive shaft to get a more exact count when turning.
 
If the speedometer reads 1% high, go to a gear 1% bigger to make it more accurate. Reading 10% high, use a 10% bigger gear. Etc... You're saying 45 mph indicated at 35 mph actual, so that's 45/35=1.286 (or 128.6%). So, 33 tooth x 1.286 = 42 tooth approximately. (I think you have a 4.10 rear gear!)
 
No one has suggested a instrument issue. When the speedometer and cable were new it would show a proper 5,7,10 mph. Does it?
As the cable develops a twisting twitch in its rotation and/or the clearances go away in the instrument,, it will show in the needle movement at very low speeds. Rather than read properly the needle will bounce from 0 to 10. The bounce disappears above 20 mph?
The fault is still present at higher speeds but doesn't reflect drastically, not seen in needle movement. The needle return spring is partial cause. Anyway…
You might pull the inst' panel and clean the crud out of the instruments input port, route the cable into it as straight/neutrally as possible, no forces applied in any direction, and hope to get more accurate readings at all speeds. The instrument will live longer also.
Sure you can get it closer with a drive gear change. You could also change it in the direction you need with a couple heavy coats of paint on the needle.
Continue the path you're on until you have no doubt the speedo drive gear is mathematically correct then consider the other cause of wrong speedometer reading. Good luck with it.
 
I seem to recall reading that the magnet in the speedo can become weak... think that makes it read high too. Could send the speedo to a shop and have them check it out and remagnetize if needed...
 
In the past what I did was:
Take the car out on the highway note the odo setting at any mile marker. then drive 10 miles and note the odo setting again.

if your odo says you went 13 miles then your speedo cable is turning 30% too fast so you need 30% more teeth in your trans gear. so 33 * 1.3 = 43

if your odo says you went 8 miles then your speedo cable is not turning fast enough so you need 20 % less teeth.
or .8 * 33 = 26 roughly.

so you don't need to know your gear ratio or tire size etc. just the size of your current gear.

If your odo is reading correctly then your speedo will also.

The reason you need to go 10 miles is to eliminate the error in reading your odo. +- 5/100
 
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