speedometer off by 9 mph

[69roadr]

i confirmed this by using my Garmin GPS. when reading 60 on the gauge, the gps is reading 69.
26" diameter tires and orange 40 tooth speedo pinion. do i need to go up or down on tooth count.

 

[kiwigtx]

What's your rear end ratio?

My initial thoughts are 38 tooth pinion - Blue
Playing with my pocket calculator would suggest possibly 36 tooth - Red

Your speedometer needs to be running faster - so a smaller pinion (fewer teeth) is required to spin faster - sometimes it's trial and error even after the best intended mathematics calculations.

 

[ckessel]

One tooth is approximately 2.5 mph.

 

[69roadr]

What's your rear end ratio?

My initial thoughts are 38 tooth pinion - Blue
Playing with my pocket calculator would suggest possibly 36 tooth - Red

Your speedometer needs to be running faster - so a smaller pinion (fewer teeth) is required to spin faster - sometimes it's trial and error even after the best intended mathematics calculations.

thanks kiwi. the funny thing is, it had a red 36 tooth years ago, and i thought that was wrong. replaced it with the orange 40 tooth and sold the red one. from what my fender tag says, it supposed to be 3:91 ratio.

 

[kiwigtx]

One tooth is approximately 2.5 mph.

Number of teeth required depends upon percentage ratios, and therefore, tyre height and diff ratio play a major part in the equation.
The only constant is the transmission tail shaft output.

- - - Updated - - -

thanks kiwi. the funny thing is, it had a red 36 tooth years ago, and i thought that was wrong. replaced it with the orange 40 tooth and sold the red one. from what my fender tag says, it supposed to be 3:91 ratio.

No problem, if it were me, I'd start at 36 tooth pinion and measure from that - calculate the percentage difference from the current 40 tooth pinion, and it will be obvious how many teeth are required.
I used the same method to calculate accurately the speedo out from a Corvette trans running a Hall effect operated speedometer. That was dipswitches, but the principle is the same.

 

[djais1801]

fender tag will show you what rear gear ratio you have?

thanks kiwi. the funny thing is, it had a red 36 tooth years ago, and i thought that was wrong. replaced it with the orange 40 tooth and sold the red one. from what my fender tag says, it supposed to be 3:91 ratio.

 

[69roadr]

fender tag will show you what rear gear ratio you have?

the fender tag has option "A31" which according to "Charger, Roadrunner & Super Bee Restoration Guide" translates to 3:91 axle ratio. possible previous owner may have swapped out the 3rd. member for 3:55's.

 

[Cranky]

If you have a good limited slip (Sure Grip) in it, you should be able to jack up the rear end and spin the tires over and count the number of revs the drive shaft turns in one tire rev and confirm the rear end ratio.....but going from a 3.91 to a 3.55 is around a 10% change....

 

[69roadr]

If you have a good limited slip (Sure Grip) in it, you should be able to jack up the rear end and spin the tires over and count the number of revs the drive shaft turns in one tire rev and confirm the rear end ratio.....but going from a 3.91 to a 3.55 is around a 10% change....

i did that, and it seems to point to 3:55. so maybe po changed the 3rd member to be a little more freeway friendly. also think i should go with the red 36 tooth as kiwi suggested.