QuestionQUESTION: Hi Barry.
Great info here. The placard on my Ford says original tires were a LT245/75R16D. When I purchased the truck from a Ford Dealership it already had LT275/95R16 BFGoodrich A/T K/O's on it and custom wheels. The truck looks and rides well with the 33", 11.5" tires. As the BFG's are almost worn out, and the rims are looking pitted, I decided to purchase a set of Voxx 20" rims. Now that I have the rims, I'm finding that 20" rubber is very expensive. The other problem I am having is that it is difficult to find a 20" tire size that will keep my tires at 33" and about 11.5" high/wide. I had thought I would get some 275/60 20's which are about the same (slightly narrower but the same height). I was all set to purchase a set of Cooper Zeon XLZ 275/60 20's but realized these are not a true 'LT' tire. The Zeon's have a load capacity of 119S (about 3000 lbs) and are a 4 ply tire.
Apparently my F150 has a trailer towing package which I suspect has resulted in the placard calling for 'LT' tires." I tow an open trailer with two street motorcylces once or twice a year, and maybe carry one of the bikes in the back of the truck once or twice a year.
The load range on the stock tires was D, but this doesn't appear to be much different than the '119S' designation of the Cooper Zeon XLZ's... There are no BFGoodrich A/T K/O's that come in the size I am looking for, and I suspect that most of the 'LT' tires are going to be too rough of a ride and maybe 'overkill' for my application.
Do I need to go to an actual 'LT' tire or can I use a tire such as the Cooper Zeon XLZ that is rated as a XL tire?
Regards, Al.
ANSWER: Al,
First, you should try to match the load carrying capacity of the original tires on the truck - that means the load carrying capacity of the original tire size at the inflation pressure specified. If I have this right the placard should say the fronts were inflated to 40 psi and the back - 60 psi.
If true, the rears are the issue and they aren't quite at the maximum allowable pressure.
Put a different way, they are using 112 of the available 114 Load Index.
So if you use an LT metric tire, you need to find a tire with at least a Load Index of 112.
But there is a small and little know quirk when going back and forth between LT tires and Passenger car tires. That is that Passenger car tires have to be derated 10% if use in an LT application - this includes trucks, vans, and trailers. So if you want to use a tire with a "P" in front of the tire size (or one without) you will have to get one that has at least a 116 Load Index.
All of this is independent of Load Range and XL/SL and other ways of designating load carrying capacity.
So the Coopers have enough load capacity and you need to use 38 psi in the rear, 26 psi in the front. The front tires are a little over inflated for the load, but the minimum I recommend for passenger car tires is 26 psi so the beads stay seated during handling maneuvers.
Hope this helps.
---------- FOLLOW-UP ----------
QUESTION: Thanks very much for your reply.
I just looked at the placard and it actually shows the original equip. tires/rims (LT245/75R16D) at 42 psi front and 42 psi back? The GVWR of the truck is 6750 lbs. Front says 3600 and back 3800. Does any of this make sense?
Regards, Al.
AnswerAl,
Yes, it makes complete sense!
GVWR = Gross vehicle Weight Rating - meaning the most this vehicle is designed to weigh when it is fully loaded.
You mentioned front and rear weights. Those are GAWR = Gross Axle Weight Rating - meaning the maximum weight that axle was designed to carry. Notice that if you add the front and rear GAWR's, you get less than the GVWR. This is the way it is supposed to be. If a vehicle is loaded up to its maximum capacity, it ought to have some reserve in case the load is off center.
So let's go backwards. 3800 rear GAWR is 1900 # per tire. At 42 psi the load carrying capacity of the tires is....... ta da!.... 1931#.
Well! That means the tires have no reserve in case the vehicle is overloaded - poor engineering practice. This was but one of the lessons learned from the Ford / Firestone situation. But the good news is that later models have reserve capacity.
AND..... That is the equivalent of 106 Load Index. So using the tire size you indicated - well, that's the lesson we learned - a bit more is safer. So I would recommend you use 26 psi front and rear.
Hope this helps.
