replaced speedometer driven gear with recommended 40 tooth and still not accurate...

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nitronut

nitronut

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I recently installed a new speedometer cable and the mph readings were off by 20-30 plus mph. Called Mancini Racing, gave them my tire size (P225R1595S) along with the rear end gear size (3.91) and was told I needed to install a 40 tooth driven gear, which I purchased and installed. (The old gear I replaced was a 23 tooth). It still seemed off so I used the speed app on my phone and now reads accurate to 10 mph and then jumps the speedometer reading 10-13 mph OVER my actual speed when driving faster than 15 mph. Is there a rule of thumb for speedometer speed readings per gear tooth? Do I need a 42-44 driven gear instead of the 40 tooth? Any thoughts? Thanks in advance... UPDATE. Issue Fixed! Working perfectly now. Thank you to all who helped with advice...
 
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Heres a gear chart from brewers performance In regards to tire diameter, and final drive ratio to determine speedometer gear. If you have a 225 60 R15 which is a 15" tall tire,

((( however i cannot decipher what you have tirewise in your post. Is it a 50, 60, 70 series?)))

and a 3.91 rear gear it says 41T

Screenshot_20200510-113240_Samsung Internet.jpg
 
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Your speedometer most likely needs rebuilt. I am dealing with the same issue. Using the correct gear, speed shows low according to gps speed. If I change the gear to show accurate on the needle it throws the odometer way off. There are two tabs that spin around a wheel which moves the needle.

With the correct gear in your car, google map the distance driven with the odometer and see if it is close.
 
threewood said:
Your speedometer most likely needs rebuilt. I am dealing with the same issue. Using the correct gear, speed shows low according to gps speed. If I change the gear to show accurate on the needle it throws the odometer way off. There are two tabs that spin around a wheel which moves the needle.

With the correct gear in your car, google map the distance driven with the odometer and see if it is close.
Click to expand...
If you want to do the math you can double check using this formula. If the math is correct, the speedo is out of calibration like others have mentioned.

62-5 SPEEDOMETER DRIVE GEAR CHART-PIC.PNG
 
Been there done that. That's why I suggested just sticking any gear in there you could find that's close; then figure from there to refine it. Not trying to start another tire size/diameter free for all, but there's more to tire height than just tire size. The width of your wheel changes diameter, along with tire pressure. Also, that's why I suggested that you first find out EXACTLY how far off your speedometer actually is. At least now you're a lot closer, and you should be able to dial it in near perfect. Here's the real test tho; for instance, if your speedometer is off by 5mph at 25 mph, then it should be off by 10mph at 50mph...otherwise speedometer is bad. These gears can get expensive and it's time consuming to find them, order them, wait for them, install them...repeat process....not fun.
 
is there more than one mopar speedometer gear housing? most are 26-45. but you said you had a 23 tooth gear originally.

what size is your tire 295r1595s does not compute
 
Sometimes you can only get them close. My dart sport is 5-8 mph off. If I do up one tooth or down its off the opposite way. So I ended up using the one that showed me going a little faster than I really am.
 
If you have a working Speed-O;
The correct speed-O gear is simple to determine.
The error in speed when converted to a percentage is always the same percentage in tooth count change required.
Example;
If your speed-O is off by 20%, then you will need a toothcount change in the same 20%

Which way to go,bigger or smaller, is indicated by the direction of speed change required; more teeth will slow it down. Less will speed it up.

You say that you had a 23t, I think that might be a typo, and you mighta meant 33t
If that's right then;
and now have a 40t.
That is a change of 33/40=82.5% which points to a speed-O of the same percent. So for it to now read correctly at 60 mph, it must have been reading 60/.825= ~73mph before, with the 33t.

BTW
You can use this same logic on the rear gear ratio. If the 40T turns out to be correct, and you did not change tire sizes,then
the previous ratio must have been a; 3.91 x.825= 3.23s
Logically then, knowing the gear change and the tooth count, you can determine the new gear required from them.
Example
going from 3.23s to 3.91s is 3.91/3.23=1.21 or plus 21%, and from a 33t that would be 33 X1.21=39.93=40t
presto!
 
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A big Thank you to @moparmat2000 for this chart. I want to point something out; On the rear tire height axis, find the dividing line between 26 and 27, and follow it down. Notice that the tooth count is always ~10 times the gear ratio. So if you have a 26.5 tire, and you know the rear gear, then you just have to multiply the gear ratio by 10.
But if you don't have a 26.5, then you just convert the difference to a percent and add/subtract it. Knowing this, you can now commit every number on that chart to memory with one simple rule.
Using a 26.5 tire, just multiply the gear ratio by 10.
So in your case that is 3.91 x 10=39.1 teeth. Your tire must be slightly smaller than 26.5, for Mancini to recommend 40. the math says 39.1/40 x26.5= 25.9 tall.
screenshot_20200510-113240_samsung-internet-jpg.jpg
 
el5dart said:
is there more than one mopar speedometer gear housing? most are 26-45. but you said you had a 23 tooth gear originally.

what size is your tire 295r1595s does not compute
Click to expand...
My mistake. The driven gear I pulled was a 27T ... The rear tire size is P225/60R15
 
moparmat2000 said:
Heres a gear chart from brewers performance In regards to tire diameter, and final drive ratio to determine speedometer gear. If you have a 225 60 R15 which is a 15" tall tire,

((( however i cannot decipher what you have tirewise in your post. Is it a 50, 60, 70 series?)))

and a 3.91 rear gear it says 41T

View attachment 1715600342
Click to expand...
Thanks. That's what I was thinking (41T) before buying the the 40T but Mancini insisted it was a 40T
 
AJ/FormS said:
A big Thank you to @moparmat2000 for this chart. I want to point something out; On the rear tire height axis, find the dividing line between 26 and 27, and follow it down. Notice that the tooth count is always ~10 times the gear ratio. So if you have a 26.5 tire, and you know the rear gear, then you just have to multiply the gear ratio by 10.
But if you don't have a 26.5, then you just convert the difference to a percent and add/subtract it. Knowing this, you can now commit every number on that chart to memory with one simple rule.
Using a 26.5 tire, just multiply the gear ratio by 10.
So in your case that is 3.91 x 10=39.1 teeth. Your tire must be slightly smaller than 26.5, for Mancini to recommend 40. the math says 39.1/40 x26.5= 25.9 tall.
View attachment 1715600470
Click to expand...
Thank You...
 
AJ/FormS said:
If you have a working Speed-O;
The correct speed-O gear is simple to determine.
The error in speed when converted to a percentage is always the same percentage in tooth count change required.
Example;
If your speed-O is off by 20%, then you will need a toothcount change in the same 20%

Which way to go,bigger or smaller, is indicated by the direction of speed change required; more teeth will slow it down. Less will speed it up.

You say that you had a 23t, I think that might be a typo, and you mighta meant 33t
If that's right then;
and now have a 40t.
That is a change of 33/40=82.5% which points to a speed-O of the same percent. So for it to now read correctly at 60 mph, it must have been reading 60/.825= ~73mph before, with the 33t.

BTW
You can use this same logic on the rear gear ratio. If the 40T turns out to be correct, and you did not change tire sizes,then
the previous ratio must have been a; 3.91 x.825= 3.23s
Logically then, knowing the gear change and the tooth count, you can determine the new gear required from them.
Example
going from 3.23s to 3.91s is 3.91/3.23=1.21 or plus 21%, and from a 33t that would be 33 X1.21=39.93=40t
presto!
Click to expand...
Thank you so much! Numbers are not my strong point...
 
AJ/FormS said:
A big Thank you to @moparmat2000 for this chart. I want to point something out; On the rear tire height axis, find the dividing line between 26 and 27, and follow it down. Notice that the tooth count is always ~10 times the gear ratio. So if you have a 26.5 tire, and you know the rear gear, then you just have to multiply the gear ratio by 10.
But if you don't have a 26.5, then you just convert the difference to a percent and add/subtract it. Knowing this, you can now commit every number on that chart to memory with one simple rule.
Using a 26.5 tire, just multiply the gear ratio by 10.
So in your case that is 3.91 x 10=39.1 teeth. Your tire must be slightly smaller than 26.5, for Mancini to recommend 40. the math says 39.1/40 x26.5= 25.9 tall.
View attachment 1715600470
Click to expand...
Great info. Thanks...
 
CudaFactHackJob said:
Been there done that. That's why I suggested just sticking any gear in there you could find that's close; then figure from there to refine it. Not trying to start another tire size/diameter free for all, but there's more to tire height than just tire size. The width of your wheel changes diameter, along with tire pressure. Also, that's why I suggested that you first find out EXACTLY how far off your speedometer actually is. At least now you're a lot closer, and you should be able to dial it in near perfect. Here's the real test tho; for instance, if your speedometer is off by 5mph at 25 mph, then it should be off by 10mph at 50mph...otherwise speedometer is bad. These gears can get expensive and it's time consuming to find them, order them, wait for them, install them...repeat process....not fun.
Click to expand...
Thank you. The speed was spot on until above 10 mph. Then it increased. You're right. when the speedo was reading 62mph the gps was reading 50mph. Do you know what the mph difference reading on the speedo would be by going up 1 tooth, generally speaking?
 
It depends on the toot count. at 40t, a 1 tooth change is 2.5%. It sounds like you need more like 20%. According to these calculations, you would need at least a 45t. Doesn't make sense. Are you sure you don't have a 4:56? Does your speedometer read 31mph at 25gpsmph?
 
CudaFactHackJob said:
It depends on the toot count. at 40t, a 1 tooth change is 2.5%. It sounds like you need more like 20%. According to these calculations, you would need at least a 45t. Doesn't make sense. Are you sure you don't have a 4:56? Does your speedometer read 31mph at 25gpsmph?
Click to expand...
Close to it. I don't remember the exact numbers. Speedometer is accurate to about 12mph, then the faster the speed the higher it is off. Doing 50mph (per gps) the speedometer was reading 62mph. Had the rear end rebuilt a couple of years ago and was told it's a 3.91 gear... I was thinking about putting a 41 T in it but sounds like I need even more?
 
I don't think it's the gear. 40 tooth is as close as you'll ever get with 25.6" tire and 3.91 gears. Sounds like the speedometer itself isn't working right. You could also check your rear end gear ratio. Guessing at that just makes things even more difficult to diagnose.
 
nitronut said:
Close to it. I don't remember the exact numbers. Speedometer is accurate to about 12mph, then the faster the speed the higher it is off. Doing 50mph (per gps) the speedometer was reading 62mph. Had the rear end rebuilt a couple of years ago and was told it's a 3.91 gear... I was thinking about putting a 41 T in it but sounds like I need even more?
Click to expand...
Thanks... I'll get there. I did find out today however that one tooth equates to 3mph on the speedometer. Talked to a tech guy at A and A Transmission in Indiana. Mancini didn't know...
 
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