Posted by Clint Dixon on Tuesday, July 01, 2003 at 5:45PM :
In Reply to: Question about Tall Tires? posted by Willy-N on Tuesday, July 01, 2003 at 3:17PM :
Mark, I had a '48 B1PW with modified Ford AA wheels. The tires on these were size 8.25 X 20. If I remember correctly, the loaded rolling radius on these tires was somewhere around 18-1/2 to 19 inches. In any case, this all worked out mathematically to a 10% increase over the 9.00 X 16 tires. This 10% was hardly noticable when traveling down the road. At an RPM at which I normally would have been traveling at 45 MPH with 9.00 x 16 tires, I was now traveling at about 50 MPH with the 20 inch wheels and tires. The gain in speed was not very much. Where the real gain was found was in ground clearance. I knew the long time owner of this truck and know that ground clearance was what he was after by mounting these tires and wheels. Acceleration was slower with these tires because of the increase in weight, but I could not detect any loss in power.
On my current trucks I have kept the 9.00 X 16 tires, but have added 4.89:1 gearing at the differentials. This equates to a 20% increase. At an RPM where I would have been traveling at 45 MPH with the 5.83:1 gearing, I am now traveling almost 55 MPH with the 4.89:1 gearing. The tire and wheel weight has not changed any and acceleration remains unchanged also. I have not gained any ground clearance, but that was not important to me. What I have gained is the additional top speed with no noticable loss of power. Our terrain here in Western Illinois is probably not as hilly as in your area, but we do have hills where I have to down shift to 3rd gear when carrying a load. On these same hills, I still had to down shift even with the 5.83 gears. 50% of my time driving is with my trucks 1/2 to 3/4 fully loaded. Drive train and engine were left unmodified and have not suffered any increased wear or additional problems because of the increased gear ratio.
One think I would think about upgrading with taller tires would maybe be the brakes although these original brakes do work quite well when kept in PROPER ADJUSTMENT. You can lock up all 4 tires at 45 MPH with a Power-Wagon still equipped with original brakes as long as they are kept properly adjusted and clean. Should have seen the look on the guys face who switched lanes right in front of my bumper a couple of months ago.
Clint
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