Tires: 2003 F150 tire size question, question question, question thanks

Question
QUESTION: Hello,

I currently have P235/70/R16 tires on my truck, but its time for new ones. The tag on my truck also says P235/70/R16 is the way to go, but I'm on a budget and I've found a good deal on some Pathfinder 245/75/R16 tires. I don't know yet (Won't until Friday) whether or not these are P245 or LT245 so I'm wondering if they turn out being LT245, is it important that I don't buy them? I've read here that LT shouldn't switch to P, but I wasn't sure if it goes the other way as well. Another thing I wanted to ask is will switching from 235/70 to 245/75 cause any problems? The person I asked said they would be fine, but I wanted to make sure.. they could have just been wanting to make a sale. I would assume these would be slightly bigger tires, correct?

I've also noticed that the tires I currently have on my truck bulge a bit on the sidewalls, even though they have the correct ammount of pressure. They have been like that for as long as I can remember, they always look like they're a bit flat compared to 99% of trucks I see around, which has nice firm looking tires. With my very small knowledge of tires, I just left them that way, but I really don't like it.. Honestly it seems like my truck is somehow too heavy for the tires it came with. Any info on this?

Thanks a ton for the help.

Pete

ANSWER: Pete,

I do not recommend replacing LT metric tires with P metric tires - and vice versa.

In this particular case, you are thinking of going larger by 2 sizes - and that brings its own set of problems.

So I would recommend you stick with the current size.  It is a very popular size and you should be able to find deals on them as well.

The bulge?  Nothing to worry about.  The physics involved in tires says there is going to be a bulge and some tires will do it more than others.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply! If possible, can you please elaborate a little more on what problems switching to bigger tires would cause? In this case, switching from 235/70 to 245/75. I have more than enough room in the wheel well if that is a possible issue, there will be no rubbing. I really do like the idea of going bigger, but this is the first I've heard of problems coming from it. Everyone I've asked said as long as it fits the rim and wheel well I would be okay.

Thanks,

Pete

Answer
Pete,

Because I don't know how big the fenderwells are and what possible limitations there might be - such as hitting the suspension components, the fenders or the frame, I usually don't recommend anyone do that.  Put another way, if I can't be sure a larger tire will fit, then I don't feel comfortable commenting on larger tires.

But I will make an exception in your case, simply because, you've shown a willingness to accept the consequences if they do rub.

First, there is the issue of the speedometer and odometer being in error.  Going from a P235/70R16 to a P245/75R16 means the speedometer will always read low by 5%.  In other words, at 60 mph, your speedometer should read 57 mph.

The second thing is that this raises the vehicle by 3/4".  This not only makes the vehicle more prone to rollovers, but changes how the front suspension contacts the pavement.  This might not be a problem, but since I don't know how the suspension is set up, I can't be sure.

Third, some ABS systems seem to know when the tire diameter changes and they don't always react the way they should.  Again, I don't know that this is going to be a problem, but I have heard of some problems associated with this type of diameter change.

That, in a nutshell, is why I don't normally recommend changing tire size - too many unknowns.