Tires: tire inflation, tire placard, ford f 250

Question
QUESTION: Hi Barry,

I just purchased an '85 Ford F-250 with 35/12.5 x 16.5 tires. No owners manual. I estimate the truck to be around 4500 lbs. Could you tell me what the tire inflation should be for running with no cargo?

Thank you so much,
Thomas

ANSWER: Thomas,

On every vehicle sold in the US - and I understand this is a worldwide practice as well - there is a sticker – commonly called the vehicle 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.

For this situation, 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.




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

QUESTION: Hi Barry,

Thank you for your quick and thorough response. Yes, my problem is that the tires I am running are not original equipment. They are: 35/12.50x16.5 load range D. I mentioned that the truck I thought to be 4500 lbs. I thinking about that now and not so sure. Anyway it's an '85 Ford F-250, stock except for the extra tire weight and a lightweight steel canopy.

Thanks so much again,
Thomas

ANSWER: Thomas,

The starting point for a calculation for a different tire size than what is listed on the vehicle tire placard is the information that is on the vehicle tire placard - the original tire size and inflation pressure.  That will tell me what load carrying capacity the vehicle manufacturer says is needed for the vehicle.  

According to Tire Guides - a publication that summarizes vehicle tire placards -  a 1985 Ford F-250 came with a variety of tire sizes and different inflation pressures  - both depending on the particular model of F-250.  For example, there is a difference between 4X2's and 4X4's.

Plus, Tire Guides has been known to have incorrect information.  If I can match what is listed in Tire Guides with what the vehicle tire placard says, then we can be assured that the information is reliable.  If the vehicle tire placard differs with what Tire Guides says, then I will use the placard information as it is the most reliable bit of information available.

So it would be much more precise if we start with what the vehicle tire placard says.  If I remember correctly, the vehicle tire placard will be on the driver's door frame - lower right hand corner.

If the placard is not readable, even partial information would be helpful.  Tire Guides lists F-250 by GVW (Gross Vehicle Weight#.  It also has listings for 4X2's and 4X4's.

Let me list what possibilities exist:  

4X2 GVW 6300# - LT215/85R16 at 51 psi front and rear - and LT235/85R16 at 44 psi front and rear

4X2 GVW 6500# - LT215/85R16 at 51 psi front / 58 psi rear - and LT235/85R16 at 44 psi front and rear #this seems wrong!#

4X2 GVW 7300 and 7800# - LT235/85R16 at 34 psi front 65 psi rear #The front seems wrong!)

4X2 GVW 8600 and 9100# - LT235/85R16 51 psi front 80 psi rear

4X4 GVW 6600# - LT215/85R16 at 51 psi front and rear - and LT235/85R16 LT235/85R16 44 psi front and rear

4X4 8600# - LT235/85R16 44 psi front / 80 psi rear

4X4 8600# with snow plow option: - LT235/85R16 58 psi front / 80 psi rear.

As you can see, there are quite of few variations.

So go look at your truck and tell me what it says.




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

QUESTION: Hi Barry,

Thank you, this has been very helpful so far.
My numbers: 4x4 8600# - LT235/85R16 44 psi front/ 80 psi rear.
Sorry for leaving out some of the info.

Thanks again,
Thomas

Answer
Thmas,

Thanks for the information.

First, I have my doubts about the front inflation pressure - 44 psi.  That seems too low for me.  It would be well to check that out to be sure.

But assuming that is correct, then a 35x12.50R16.5LT would carry the same loads at 25 psi front /44 psi rear.

But one of the fundamentals is to match the pressure split of the original in order to keep the front to rear spring rate balance.  The original difference was 36 psi, but the new one is only 19!  This concerns me as the new split might make the vehicle a little more over-steer prone - very bad for a pickup.

No matter what I try to do to these numbers, I can't come up with a balance that works for me - so, for what it's worth, those are the numbers.