Speedometer off

[MileHighDart]

so I know my Speedo if off a bit.

Downloaded a gps speedo app for my phone.
When the app is reading 60mph
My speedometer is reading 67.

So my question is, do I need to go up a tooth or down a tooth on the speedometer gear?
I need to pull the current one out this weekend to see what it is. Changed it out a couple years ago but apparently didn’t get it right.

 

[brian6pac]

You need to slow down the speedo so the gear on the cable needs to get bigger

 

[Alaskan_TA]

There is a chart you can use online at Mopar Speedometer Pinion Numbers - the last two numbers are the number of gear teeth on the transmission pinion gear.

You need to know your rear tire diameter & rear axle ratio.

The pinion gears have the number of teeth embossed on them in a circle.

 

[AJ/FormS]

you're out by 67/60 = plus 11.7%
You need plus 11.7% more teeth

 

[TrailBeast]

so I know my Speedo if off a bit.

Downloaded a gps speedo app for my phone.
When the app is reading 60mph
My speedometer is reading 67.

So my question is, do I need to go up a tooth or down a tooth on the speedometer gear?
I need to pull the current one out this weekend to see what it is. Changed it out a couple years ago but apparently didn’t get it right.

Mine is exactly 5 slow and below is the list of things I have done about it.
1. Nothing, I know what it means when it says 70.

 

[MileHighDart]

Mine is exactly 5 slow and below is the list of things I have done about it.
1. Nothing, I know what it means when it says 70.

I know I could just leave it, or fire up the app when I drive it, but it’s just little **** like that that bugs me.

 

[TrailBeast]

I know I could just leave it, or fire up the app when I drive it, but it’s just little **** like that that bugs me.

LOL, I know exactly what you mean.
Hopefully I'll be changing tire and wheel sizes before long, so I'll get to it after that.

 

[MileHighDart]

There is a chart you can use online at Mopar Speedometer Pinion Numbers - the last two numbers are the number of gear teeth on the transmission pinion gear.

You need to know your rear tire diameter & rear axle ratio.

The pinion gears have the number of teeth embossed on them in a circle.

That looks like the chart I used last time. I had to guess a bit, cause my tires are like 25.5”, and 3:23 gears not 3:21 like the chart says.
Looking at that I probably went with the 33 tooth. Looks like the next one up is a 35. But I’ll pull it and check before I buy one

 

[ir3333]

Is it accurate at slow speeds?
It may be the speedometer needs calibration if it gets worse as speed increases.Mine is dead on at slow speeds but off by 10 % or higher as speed gets above 55 or 60

 

[Dana67Dart]

Current Speedometer Reading = CSR

Actual speed = AS

Ratio = R

CSR / AS = R (67 MPH / 60 MPH = 1.117)


Current Speedometer Gear Number of Teeth = CNT

New Speedometer Gear Number of Teeth = NNT

(Assume you have 33 teeth now)

CNT x R = NNT (33 teeth x 1.117 = 36.85 teeth)


Speedometer Reading with New Gear =SNG

CNT / NNT x CSR = SNG (33 teeth / 36.85 teeth x 67 MPH = 60MPH)

They don't make gears with 36.85 teeth so let's try 36 and 37 teeth

Next Lower Number of Teeth = LNT

SNG = CNT / LNT x CSR (33 teeth / 36 teeth x 67 MPH = 61.42 MPH)

Next Higher Number of Teeth = HNT

SNG = CNT / HNT x CSR (33 teeth / 37 teeth x 67 MPH = 59.76 MPH)

(AJ/FormS perhaps you could check my math)

I created an excel spreadsheet if anyone is interested PM me if you want a copy
Screen shot

(I changed "Delta = D" to "Ratio = R" and replaced all "D"s with "R"s)

 

[oldkimmer]

Don’t forget with ur speedo reading fast it is also adding up miles faster on ur car. Kim

 

[Dana67Dart]

reading fast it is also adding up miles faster on ur car.

But once it rolls over its a low mile car again ;^)

I've got a 1967 Dodge Dart Convertible with only 15,000 miles on it (for the 4th time!)

 

[MileHighDart]

Dana67Dart,

Thanks for the math, I couldn’t figure that out for myself.
So when I get the old one out if it is in fact a 33, then I’ll look for a 37, that would put me closest to 60 by your calculations

 

[Dana67Dart]

I just used 33 as a sample. You would need to redo the math. Just use your tooth count in place of the 33s in the equations . If you have excel on a pc i have a spread sheet you could use. Pm me your email address

 

[ir3333]

Old rule of thumb for 26 inch tire is gear. X tooth
3.23....33 tooth gear
3.55....35 tooth gear

 

[MileHighDart]

I just used 33 as a sample. You would need to redo the math. Just use your tooth count in place of the 33s in the equations . If you have excel on a pc i have a spread sheet you could use. Pm me your email address

Tried to pm you, I think my inbox is full, let me fix that and I’ll try again

 

[harrisonm]

Mine is exactly 5 slow and below is the list of things I have done about it.
1. Nothing, I know what it means when it says 70.

Leave it to Trail Beast to so eloquently describe the best way to handle a speedometer error. LOL Mine has been 5 MPH off for years, and the thought of buying a new gear and installing it makes me think, "Why bother."

 

[Dana67Dart]

So when I get the old one out if it is in fact a 33, then I’ll look for a 37, that would put me closest to 60 by your calculations

I chose 33 based on this chart. (I guessed you had 26" tires and a 3.21 rear ratio) but yes based on the math if you have 33 teeth now you would need 37 to get the closest.

 

[TrailBeast]

Dana67Dart,

Thanks for the math, I couldn’t figure that out for myself.
So when I get the old one out if it is in fact a 33, then I’ll look for a 37, that would put me closest to 60 by your calculations

I found it a ton simpler to just figure 1 tooth for every 2 miles an hour.

 

[Dana67Dart]

I found it a ton simpler to just figure 1 tooth for every 2 miles an hour.

Fine make it simple! 4 years of school for nothing. ;^)

 

[Oldmanmopar]

Mine seems to be 50 off . At 20 miles per hour my speedo reads around 70. what gear do I need so I don't get pulled over.

 

[Dana67Dart]

what gear do I need so I don't get pulled over.

By my calculations you would need a mature-alot gear. (I do get the humor of your post, but really in a residential neighborhood feet from people, cars and machinery!)

Just my 2 cents (flame me if you need to prove your manliness)

 

[TrailBeast]

Fine make it simple! 4 years of school for nothing. ;^)

Sometimes experience trumps schooling I guess.

 

[Dana67Dart]

Sometimes experience trumps schooling I guess.

My boss accuses me of taking a simple thing and making it complicated. It's how my brain works, start at the beginning and work the problem to the end. I envy people who see the end from the beginning, it cuts out a lot of calculations!

 

[jos51700]

you're out by 67/60 = plus 11.7%
You need plus 11.7% more teeth

Exactly, Percentage is a much more accurate way to count this.

I know I could just leave it, or fire up the app when I drive it, but it’s just little **** like that that bugs me.

It'll be off at high speeds or low ones, which is up to you. I prefer accurate in town. There's really only one highway speed I need to worry about, and my speedo errs on the safe side at that speed. There are a lot of smaller speed limits (25mph, 30 mph, 35 mph etc), so I figure accuracy down low is paramount.

Take your tooth count off what you got, multiply by 1.1, and that's your new tooth count. Round per your preference. 33 teeth*1.1=36.3. I'd stab a 36 in there and reclock the housing if needed...