QuestionI just bought a 2007 ford f350 crew cab fx4 superduty powerstoke diesel. It has 265-70-r17's on it. The truck sais use r20's. What is the difference?
AnswerErik,
Let's start with the basics:
On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.
BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.
One word of caution: Some trucks use tires with the letters "LT" in front of the tire size - and some trucks use tires with the letter "P" in front of the tire size - and some trucks use tires with the letter "C" after the tire size. You should not use "P"'s in place of "LT"'s and vice versa.
You should be sure the pressure on the sidewall equals or exceeds that pressure on the placard. If you do this the issue about the letter "C" in the sidewall will take care of itself (which is not true with "LT"'s vs "P"'s.)
If you are using a tire size that is different than what is listed on the placard, then the pressure has to be recalculated. The calculation is not difficult, but it requires tire load tables which are not allowed to be published in the Internet because of copyright laws. However, I have a copy and would be glad to do the calculation, but the starting point is the vehicle placard: Tire size and inflation pressure. I will also need to know the new tire size.
Normally, after I've pulled this "canned" statement out, I look up the vehicle and try to determine what the vehicle tire placard should say.
But not only are there quite a few different versions of Ford F-350's, but none of the ones listed in my book shows a 20" tire as standard.
So you need to find the vehicle tire placard and tell me what it says: Tire size and pressure.
Plus you need to check your tires to see if there is a letter in front of the tire size. With all that info, I can start to make some sense of what you actually have and whether there are any changes that need to be made.
