Speedo gear size and replacement on Dodge 1970 Charger

[TomsPinkPanther]

I don't know if this subject has been covered or not, but I guess I have the excuse of being a new guy..ha.. anyway, I have had the speedo in my 1970 Charger 500 with automatic trans, reading off about 20mph, speedo shows 70mph actually only going real speed of 50mph.? I don't know if the owner before me had different sized rims / tires or not? At this time I have 225/70R X 14 tires on it 26" height of tire and it has a 32 tooth black colored speedo gear that goes into the trans. what would be the new tooth gear that I would need to bring it to right speed? I tried a formula that I had seen on You Tube that jacked up rear and turned drive shaft and rotation of rear tire but was still kind of confusing to my simple mind?? any help appreciated. thank you.

 

[66Satellite47]

Well if you jacked the car up and counted how many driveshaft rotations for one tire rotation, you have the gear ratio. Then you go to the factory service manual for the correct speedo pinion for the diameter of tire you are using. Search on this site for a copy of the speedo pinion chart.

 

[R/T Boy 67]

You should check your odometer reading against the mile markers on a highway, and correct if necessary with a gear swap. If the Speedo is wrong after that, then it needs to be adjusted, or replaced.

 

[R413]

Seems that is the correct gear for a 26” tire and 3.23 gears. So you need to know your gear ratio.

with 26” tires the speedo gear is the same as the ratio.
3.55=35 tooth, etc.

 

[66Satellite47]

The speedo gear USUALLY has the number of teeth marked on the underside. You can't go by the color as had been said over the years. The color varied over the years. If you can't see the molded in tooth number, you have to count the teeth. The gear ratio to teeth number is not a one to one correlation. Maybe with a 26' tire, I don't know. My bet is a 3.23 w/26" tire would use a 31 tooth speedo gear.
To OP another issue you may have is the "clocking" of the housing. The housing has various markings that must be oriented to match the number of teeth on the speedo gear.

 

[Dragon Slayer]

You don't have to guess you can use the chart and 32 is what mopar recommended for the combo mentioned. But 20 mph is a big difference. So you must have a high gear ratio and would need to go to a bigger gear. If you clock the housing wrong, your not engaging the speedo gear correctly and would either jam it, or stripe not engage it. The offset is for properly engaging different gear diameters.

 

[66Satellite47]

If clocked wrong, it won't necessarily strip. I do agree the Mopar chart will be real close. Since the FSM gives tire sizes from "70 not diameter, I'm guessing at what size is a 26" diameter. But 30 or 31 should be correct.

 

[hunt2elk]

The charts will get you close. I seem to always vary a bit from them to get the speedo as close as possible.

 

[toolmanmike]

Here's a good thread: Speedometer Gear Charts - Clarification/Differences

 

[R413]

Clocking it wrong won’t make it read 20 mph higher,

you have the wrong gear.

you need to know the rear gear ratio.

A30 won’t do it, dont believe me, read it for yourself.

Our tires are slightly shorter than the measurement because of the flat spot on the bottom of the tire.

 

[66Satellite47]

413, the circled combo is for a 23.7" tire and a 3.73 gear. Where does that come from? OP says he has a 26" tire and an unknown gear. In post #2 I suggested counting the pinion rotations for one tire rotation, that will tell him what the gear ratio is. I presumed the most likely gear, 3.23. I could be wrong.

 

[moparmarks]

Did you check for a ratio tag on the diff bolts?
Like Hunt2elk I too have a box of gears. They are marked and the color code never changed but there 2 different green, orange, yellow, black and red ones.
The chart above is a good starting point and seems to be dead on most of the time.
If clocked in too far it will strip out the gear. Out too far gear doesn't turn.
Over the years I've calibrated a butload of speedometers. Used a stop watch and miles marker for years but now they have GPS apps that are more accurate and easy to use. Using mile markers and the odometer doesn't always work because the odometer and the speedometer may not calibrate the same.

 

[R413]

413, the circled combo is for a 23.7" tire and a 3.73 gear. Where does that come from? OP says he has a 26" tire and an unknown gear

That red circle it doesn’t mean anything, It came with the chart for free. Just read the chart for whatever combo suits the situation.

who even runs a 23” tall drive tire with these speedo gears?

 

[R/T Boy 67]

If the odometer is right and the speedometer is wrong, then the speedometer needs to be calibrated. If it is close then you can adjust the gearing to fix the Speedo if you want, but it might not be accurate at all speeds.

 

[66Satellite47]

That red circle it doesn’t mean anything, It came with the chart for free. Just read the chart for whatever combo suits the situation.

who even runs a 23” tall drive tire with these speedo gears?

So I don't understand the point of your post. A chart with a red circle that has nothing to do with the issue at hand?

 

[R413]

exactly! Ignore the red circle, it has nothing to do with the issue at hand, Done deal.

Now that you’ve done that its a chart with all the tires sizes and all the gear ratios. Find the combo that applies to you and move on.

 

[R413]

Seems that is the correct gear for a 26” tire and 3.23 gears. So you need to know your gear ratio.

with 26” tires the speedo gear is the same as the ratio.
3.55=35 tooth, etc.

And read the above post and look at the yellow underlined group, all matching the number of teeth to the gear ratio for a 26.3” tire.

 

[69Bee]

Clocking it wrong won’t make it read 20 mph higher,

you have the wrong gear.

you need to know the rear gear ratio.

A30 won’t do it, dont believe me, read it for yourself.

Our tires are slightly shorter than the measurement because of the flat spot on the bottom of the tire.

View attachment 1301272

Exactly, the clocking of the adapter only accounts for the different sizes of speedo pinion so that they will actually mesh with the teeth on the output shaft, that's all...

 

[66Satellite47]

And if the teeth don't mesh properly, what happens?

 

[moparmarks]

And if the teeth don't mesh properly, what happens?

Can strip all of the teeth off of the gear. I learned that 40 years ago and have never done it since.