speedometer drive gearCan anyone tell me what drive

[crytough]

Can anyone tell me what drive gear I need. All i know is it's a sure-grip 3.2 slap stick automatic in a 318 engine out of a 1985. don't know if the differential is a 2.76 or 3.23. took in in to a shop they could not help me. My problem is with the speedometer. Once I get up to about 45 mph it'll show that i'm doing about 80. I replaced the cable and new speedometer. That's why I think it's the drive gear. It had over size tires on it and replaced with P225/70/ R15 The cable and speedometer were bad and had to be replaced anyway. took gear out it had 28 teeth. Any advice will be appreciated.

 

[rogue]

you need to know the axle ratio and the tire height... you should be able to get the gear ration from your build sheet
Mopar Speedometer Gear Selection Guide - MyMopar.com

 

[aussiemark]

If you can't get it close enough with a speedo drive gear you can use something like this to fine tune it they come with UN and metric threads so you have to be sure to get the right one for the cable.
Gear Ratio Adapter to speed your Speedometer up +44% . | eBay

 

[chargerdon]

Ok, so you know its 28 tooth gear. First step is to get an accurate speedometer reading at a known true speed. I would simply use a gps signal that gives speed. Bring car up to 60 mph and then get the speedometer reading. Lets say a true 60 is reading 80. Divide the true by the speedometer. In this case it is .75 60/80= .75. Now divide your current gear tooth by .75. So 28/.75= 37 So you would need a 37 tooth gear... Now, in your case you say you have 225/70/15 inch tires. That would equate to 27.5 inch tires... IF you had a 3.23 axle then the chart says 32 to 33 tooth. A 3.55 gear would require 34 to 35 teeth.

Another thing you can do is jack up the car with both wheels off the ground, and IF it is a sure grip when you rotate one wheel the other side will turn in same direction. If that is the case then rotate the wheel 1 full revolution, while COUNTING the number of revolutions the axle makes. That will be your ratio... Then read the chart at mopar speedometer gear chart - Google Search for a 27" tire and there is your answer. If on the other hand when you rotate one wheel, the other spins in opposite direction, then you have an open differential. On it rotate one wheel 1 full turn, while counting the number of revolutions on driveshaft and divide by two. then take that number into the chart.

Personally, i like the gps vs speedometer reading method.

Good luck !!