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That is the mathematical formula and it is right. But not every 315/75-16 is 34.6 inches in diameter. Some are as little as 34.1 and some as much as 35.4. There may be some outside that too. Going from one brands size to another brands size does not guarantee you a change in size. I learned that the hard way going from Bridgestone to Nittos, thinking I was increasing about half inch when actually I lost a very small fraction. This was on a car, but decease or even staying the same was counterproductive. Can't trust metric sizes, you need to check manufactures specifications on actual size and revolutions per mile to get accurate picture.
I agree. The "formula" is barely a guideline. And I don't know what they're thinking with metric dimensions on "LT" (light truck) or medium truck tires but the standing height and the revolutions per mile are all over the place from one manufacturer to the next. Even when the spec is 35 or 33, the revs per mile can be different and actual dimensions are different. I was playing with this problem in a big off-road store that had recently set up a display of numerous wheels and tires with a lot of wheels with tires fitted so you could put the wheel next to your vehicle and decide if you like the look. I went a step further and jacked up the frame five inches to see how it would look with the lift. Obviously the tire will stand differently once it's laden. Sure, an eight-ply might sit a bit fatter and flatter than a ten-ply etc. etc. And the practical differences between 640rpm and 650rpm might be negligible. Overall, I found that the 35's all seemed about the same but no two were actually the same. And that's not even accounting for tread depth where some sizes had specs showing deeper tread. I think there's so many other variables in the final drive and the various stack height and sag of lift kits that I'm just not even going to worry about the decimal place.
Yep, I've seen the variances in tire specs as well and just use the math as a rough guide. A decimal up or down doesn't make much difference in my book either.
Theodor and your owners manual are right. The stock converter will allow some slippage in OD. That's bad. It will me very minor at first. Your driving conditions and load will also have a great impact.
If you want to tow in OD get a GOOD AFTERMARKET trans temp guage, at least you will know when you are roasting your friction surfaces and oil.
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