Correcting Speedo after Wheel change.

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Wrencher

Wrencher

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Is there an easy answer to this question I have always overlooked somehow?

I went from the factory 13" (original tire size unknown) wheels to a 15x7 wheel with a 225/60.

The car is a 225/auto 66' Dart. I know a speedometer gear is the fix, but how do I go about locating the correct retrofit? Please advise. TYVM
 
need rear axle ratio There are charts that give proper gear tooth count based on gear ratio and tire size.

225/60x15 is 25.6" tall.
6.50x13 approx factory tire size 175/70x13 22.6" tall

factory std rear axle ratio is -
auto trans 2.93 gear needed part number 2538930, Yellow, 30 tooth
manual trans 3.23 gear needed part number 2538933, Yellow, 33 tooth

http://www.mymopar.com/speedometer.htm

30 tooth for sale here.
http://www.forabodiesonly.com/mopar/showthread.php?t=262176&highlight=speedometer+gears
 
Yes, rear ratio and actual tire diameter.
But sometime in the mid-60's, they changed the speedo pinion. So you need to find out if your trans takes the common "long stem" speedo gear from the mid '60's through the '80's somewhere, or the earlier type. When they went to more electronics, they stated using a short stem speedo gear. (I'm not familiar with the early speedo gears. At work and don't have time to search for more details.)
 
Go down the highway at a steady 60 mph. When you pass a mile marker start your stopwatch. How many seconds does it take to get to the next mile marker?

Repeat a couple of times for accuracy.

You can use that to determine if you have to increase your teeth and how many from what is already in the transmission.
 
krazykuda said:
Go down the highway at a steady 60 mph. When you pass a mile marker start your stopwatch. How many seconds does it take to get to the next mile marker?

Repeat a couple of times for accuracy.

You can use that to determine if you have to increase your teeth and how many from what is already in the transmission.
Click to expand...

***********
That's pretty much what I did originally over 10 mile stretches.

But last tire upgrade, I used a GPS and compared that to the indicated speed. Did the math to find a multiplier. 68 indicated and 60 actual is 68/60 = 1.133 for example . Then checked what speedo gear was in the trans and applied the constant. The speedo gear is right there when you didconnect the speedo cable from the trans.

If your speedo is reading high, you need more teeth on the gear. So if you have a 30 tooth gear now, multiply by 1.133 to get 34.0 teeth required. Went on line and bought a new gear with the correct number of teeth (or as close as you can get)
Now the speedo matches the GPS.
 
This chart is applicable to early a-bodies.

Tire Size Tire Diameter 2.93 3.23 3.55 (1) 3.91(2) 4.56 (3)
6.50 x 13 23 19 21 19 21 --
7.00 x 13 24.3 19 21 18 -- --
7.00 x 14 25.4 18 20 18 -- --
7.50 x 14 25.9 18 19 21 19 21
8:00 x 14 26.4 17 19 21 18
8:50 x 14 26.6 17 19 -- -- --
9:00 x 14 26.8 17 19 -- 18 --
6.70 X 15 26.3 17 19 21 -- --
7.10 x 15 27.4 17 19 -- -- --
7.60 x 15 28.3 17 18 -- -- --

(1) use .800to 1 ratio adapter
(2) use .800 to 1 ratio adapter
(3)use .800 ratio adapter

Oops...lost formatting in post. guess you'll have to count rows.
 
KosmicKuda said:
***********
That's pretty much what I did originally over 10 mile stretches.

But last tire upgrade, I used a GPS and compared that to the indicated speed. Did the math to find a multiplier. 68 indicated and 60 actual is 68/60 = 1.133 for example . Then checked what speedo gear was in the trans and applied the constant. The speedo gear is right there when you didconnect the speedo cable from the trans.

If your speedo is reading high, you need more teeth on the gear. So if you have a 30 tooth gear now, multiply by 1.133 to get 34.0 teeth required. Went on line and bought a new gear with the correct number of teeth (or as close as you can get)
Now the speedo matches the GPS.
Click to expand...


x2


but I would make sure you don’t have a trunion type trans before continuing
 
KosmicKuda said:
***********
That's pretty much what I did originally over 10 mile stretches.

But last tire upgrade, I used a GPS and compared that to the indicated speed. Did the math to find a multiplier. 68 indicated and 60 actual is 68/60 = 1.133 for example . Then checked what speedo gear was in the trans and applied the constant. The speedo gear is right there when you didconnect the speedo cable from the trans.

If your speedo is reading high, you need more teeth on the gear. So if you have a 30 tooth gear now, multiply by 1.133 to get 34.0 teeth required. Went on line and bought a new gear with the correct number of teeth (or as close as you can get)
Now the speedo matches the GPS.
Click to expand...


Exactly, that's how to match what you have. :cheers:

Sometimes your combination does not fit into the charts...
 
excellent answers guys. Thank you so much for the help. I have no idea what the rear ratio is, but the method suggested by KosmicKuda seems like a great way to do it. I will try to get that figured out as soon as I figure out what is causing the sppedo clock to jump incessantly. I posted in another section, and have gotten a few good ideas. Once I get to the bottom of that, I will figure this little problem out. Thank you very much
 
sounds like you need to clean and lube your speedo cable.
If that does not take out the jump then it may be time to replace your speedo
 
trapster said:
x2


but I would make sure you don’t have a trunion type trans before continuing
Click to expand...

Please educate me? Im sorry but Im not sure what I have. I'm 6'6" 370lbs+ so I dont exactly fit under the car for long periods of time to do these types of inspections =P~
I've not even fixed adjusted the T.V. linkage yet after my 2BBL retrofit because its such a P.I.T.A. to get under it.

Im guessing that you are referring to the way the driveshaft hooks up to the output shaft?
 
trapster said:
sounds like you need to clean and lube your speedo cable.
If that does not take out the jump then it may be time to replace your speedo
Click to expand...

It jumps around 10-20mph. The noise (grinding dry speedo cable type) came in right after I re installed the cluster. I had pulled it to unwire the ammeter and install a solid state cluster voltage regulator and capacitor.

I may be stuck on the wrong track here, but as I stated in my other thread, the fact that the noise came in right after removing the cluster suggests to me that it is not the cable. It bolted back in with no problem, no binding or anything. I even pulled the cable and lubbed with wd-40. However, that didnt even make a dent in the noise. no change or anything.

The jumping is much worse now, however, even when I was a little kid riding in this car with my grandfather, I always remember seeing the speedometer jump around. Although if I had to venture a guess I'd say it moved maybe 5-7mph.
 
Well I took it out and drove it today, and it looks like Im indicating 43mph @60mph on the GPS. Im going to get it up on a rack monday after work and remove the speedo gear and apply the .72 multiplier to get the correct gear. Thank you very much for the help, that was a great idea.

As for the speedo noise, Im pretty sure its a speedometer clock problem. Hopefully someone has a good one laying around.
 
Wrencher said:
Please educate me? Im sorry but Im not sure what I have. I'm 6'6" 370lbs+ so I dont exactly fit under the car for long periods of time to do these types of inspections =P~
I've not even fixed adjusted the T.V. linkage yet after my 2BBL retrofit because its such a P.I.T.A. to get under it.

Im guessing that you are referring to the way the driveshaft hooks up to the output shaft?
Click to expand...


If your speedo gear looks like this and your drive shaft is bolted to your transmission. you have a pre 1966 transmission and you cannot use the charts provided above. and good luck finding a gear.

if you drive shaft slips into you trans and the gear has a metal shaft coming out of it your ok and you should not have too much trouble finding a gear.
 

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trapster said:
If your speedo gear looks like this and your drive shaft is bolted to your transmission. you have a pre 1966 transmission and you cannot use the charts provided above. and good luck finding a gear.

if you drive shaft slips into you trans and the gear has a metal shaft coming out of it your ok and you should not have too much trouble finding a gear.
Click to expand...

oh goodness..... so with mine being a '66 im guessing it can go either way.... Thank you very much for the description. Heres hoping to having the newer style that Russ H pictured!
 
More pictures of pre 66 trans.

the socket that goes in is about 1" in diameter

for the later models it is about 2 inches an offset for larger gears.
 

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X 2 on the recommendation to use powered graphite on the speedometer cable. Get it at Home Depot or Lowe's.

A general rule of thumb to correct a speedometer (assuming it was accurate before tire/gear change) is one tooth per 3 miles per hour. Using this rule, the transmission would need six fewer teeth on it. I'd prefer to use Tony's chart, however.
 
Easy way to match speedo gear is to take a odometer tenths reading at first mile marker, drive to next mile marker, read tenths again. note tenths offset + or - one mile.
pull speedo gear in trans and count teeth and devide count by 10.
multiply by offset , add or subtact to original count to get new tooth count needed.
Lon;
 
Wrencher said:
oh goodness..... so with mine being a '66 im guessing it can go either way.... Thank you very much for the description. Heres hoping to having the newer style that Russ H pictured!
Click to expand...

Brewers performance has used early type speedo gears for sale at their on line store. They are showing them available in 16,17,18,19, 20 and 21 toothed gears.
 
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