Posted by Clint Dixon on Saturday, December 08, 2001 at 9:33PM :
In Reply to: 5:83w/9:00x16 tires vs 4:89w/7:50x16 tires posted by RDavis on Saturday, December 08, 2001 at 4:52PM :
I had several formulas for calculating tire size, engine RPM's, and road speed when two of the above variables are known and one wants to find the third. I was looking for them just today so we could purchase new tires for our family Jeep Cherokee and hopefully correct an innacurate speedometer in the process. I seem to have missplaced the formulas, but I did find a chart I had made of all of the different size tires on the Power-Wagons I have owned and the axle ratios that they had.
Keep in mind that different tire manufactures, styles, tread patterns, and degrees of tread wear can result in varying rolling radii and overall tire diameter. The information was taken from tires I had at hand, all of which were used and with various amounts of tread left.
Assuming that one is driving in fourth gear (direct drive), my calculations tell me that a combination of 7.50x16 tires having a loaded rolling radius of 14-3/4" and an axle ratio of 4.89:1 will produce engine RPM of 2784.81 at 50 MPH. A combination of 9.00x16 tires having a loaded rolling radius of 16-5/8" and an axle ratio of 5.83:1 will produce engine RPM of 2945.68 at 50 MPH. Not a big differance. This makes sense because that was the whole reason Dodge offered the 4.89:1 gearing in the Power-Wagons, in order to match and compensate for the optional smaller tires.
Now if you use 9.00x16 tires in combination with 4.89:1 gearing, this will give you engine RPM of 2470.74 at 50 MPH. A whole 16% decrease in engine RPM's.
I am running 4.89:1 gearing and 9.00x16 tires on both of my trucks. I have my governors set at 3000 RPM and regularly carry loads and cruise at 55 to 60 MPH up and down our rolling midwest landscape. I have been doing this for over 14 years on one 230 engine and have noticed no lack of power nor other ill effects. I believe if a Power-Wagon can't pull at least 45 MPH with stock tires and gearing, then the engine must be in need of attention.
Hope this gets your gears aturnin. Clint