differential gear change

F

Ford coupe

Senior Member
79 Granada changed differential from original325 to 272 as that was all i could get..works fine but speedo out about 10 mph..Now is there a different gear for end of speedo cable that can fix that or??? If so where could i maybe get that part? The ****** is a jatco 6000 i beleive..Thanks in advance
 
Yes, you can get a different speedo gear (the driven gear at the end of the cable) with less or more teeth to either slow or speed up the speedometer to read correctly. First you must determine what you have now, how much the speedo is off and whether it reads high or low. These factors will determine the gear that may fix your problem.


There are math formulas to determine this, I have not used them.
 
Since the speedo reads high, you want a gear with more teeth than your current one. Either one or two more is my guess. Actually, ebay is a good place to get them. Note - speedos are not necessarily accurate from the factory so if you get yours within 2 or 3 MPH you are lucky.


Ford coupe said:
tire HOPE THIS HELPS sizehr78-14 cant get no from diff but when going 50 mph speedo reads 58
Click to expand...
 
Tell us what transmission is in the car. Assuming it’s original, and the 3.25 was original, and the speedo was accurate before. Someone with the manual will be able to tell the factory gear and tooth count. That will let you calculate which gear (on tooth count) you need. Your wanting to slow slow the speedo down, so you’ll be looking for (guessing here) about 2 more teeth.

Edit. Paul beat me to it. What’s a jatco 6000?
 
The math in this change from 3.25 to 2.72 does not seem to agree with the change in speedometer reading. The transmission/driveshaft RPM should be running slower with the change (about 0.837 : 1) so the speedometer should be reading slower than before. Are you sure of the 2.72 ratio? Is it possible that the actual change was to a higher numerical ratio? You indicated that at 50 MPH, the speedometer registered 58 MPH. This would indicate that the new ratio would be more like 3.77 rather than 2.72. Of course, none of this is accurate unless the speedometer was acurate with the original 3.25 ratio.

You will need to count the teeth on the current driven gear at the transmission (driven gear is attached to speedometer cable). Then
 
Sorry I got interrupted and sent the last reply prematurely. Anyway, once you have the current tooth count on the driven gear, you will need to increase the tooth count in the same ratio as the error of your speedometer. Currently your speedometer error is 58/50 or 1.16 ratio. You will need to multiply your current driven gear tooth count by 1.16 to get the cable and speedometer reading accurate.

Good Luck,
Dave
 
paul2748 said:
Since the speedo reads high, you want a gear with more teeth than your current one. Either one or two more is my guess. Actually, ebay is a good place to get them. Note - speedos are not necessarily accurate from the factory so if you get yours within 2 or 3 MPH you are lucky.
Click to expand...
So car has a 18 tooth gear and going to try a 19 as it looks like that should have been in it new but possibly since it was a special edition it maybe had different size tires or whatever...Part not to dear so hope for best...
 
The change from 2.72 to 3.25 makes comes very close to agreeing with the speedometer error. The increase in transmission/driveshaft RPM is approx. 19%, so the speedometer would increase the same. Based on that ratio, a change from 18 to 19 teeth on the driven gear would get you part way back to correct speed, but not all the way. It sure won't hurt to try it and see what the difference is, but you will likely need to jump 3 teeth to 21 tooth driven gear (if it is even available).

Good Luck,
Dave
 
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