Speedometer is off after putting in new rearend

[cdawglean]

My 1970 Dart's speedometer doesn't show the correct speed (120 when i'm about 50mph) This I imagine is because of the new rear end. How do I fix it?
3.91 Gears
8 1/4 rearend
904 Tranny

Any help in this would be awesome.

 

[dingo77]

different speedo gear

 

[1968FormulaS340]

We will need your tire height (or at least the size) if we are going to give a "Best Guess" on what gear to get.

You posted the other info we would need.

 

[1970duster360]

one tooth on the speedo gear is roughly 5 mph
see whtts in you trans and go from there
one more tooth will slow it down one les will speed it up
mike

 

[cdawglean]

The tire size is P205/70R14/93s, The tires are Radial G/T Road Huggers. Thanks for any info you guys can give me. Is the speedo gear in the tranny and how difficult is it to change out?

 

[doktor_x]

This should get you pretty close.

http://www.themoparshop.com/shop/userDocs/speedo.htm

I'd suggest measuring the tire diameter for this. They often differ from the calculated value.

Then again, the tire sizes on this chart are pretty coarse. Calculation should do.

 

[2 Darts]

The tire size is P205/70R14/93s, The tires are Radial G/T Road Huggers. Thanks for any info you guys can give me. Is the speedo gear in the tranny and how difficult is it to change out?

Try a 40 tooth speedometer gear for the tire/gear combination you listed. My experience has been 1 tooth per 3 mph of speedo reading change. It is not a difficult swap. Before you start, however, make sure that everything around the speedometer cable going into the transmission is spotless. It would wreck your weekend if a piece of debris were to find its way into the transmission. Recommend raising the driver side higher than the pax side. There may be a little tranny fluid spill, so be prepared.

IMO The absurd reading seems to be more of a speedometer problem than a gear problem. Seen something like this before that was corrected by making sure the cable was properly seated in the speedometer input and no kinks in the cable.

 

[dingo77]

the speedo gear is in the tail shaft of the tranny. as demon said you will want to clean the area up and raise the drivers side up. loosen up the retaining bolt and pull the retaining fork off. then the assembly will pop out. keep in mind where the fork was placed on the housing.