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 !!
