Changing Tire Pressure When Changing Load Rating

[smithcreek]

Can anyone confirm what I am doing below is correct?

I recently put a set of Michelin Defender LTX, E-rated (120) tires on my Toyota Tacoma. The tire shop installed them and set the pressure at the same 34psi as the old tires and placard says.

As soon as I started driving I noticed they had a softer ride and significantly more squirm than the D-rated tires they replaced. The opposite of what I was expecting. Yesterday I hooked up the trailer and took it out for a quick Spring shakedown ride. The squirm was even worse with the extra weight and when I jumped on the highway for a couple miles I felt a bit of sway for the first time ever.

After some research I think I should be running significantly higher pressure in these new E-rated tires than the old D-rated ones. I found a chart online that looks to be the instructions employees of Discount Tire use when selling a customer tires that are a different load rating than the OEM tires.

From the chart, the old D-rated tires (109) with 34psi had a load carrying capacity of 2143. To get the same load carrying capacity in my new E-rated (120) tires I should be at about 50psi. I took the truck out for a spin with 50psi and it was a major improvement.

It would be nice if the tire companies would make inflation charts available. I can't find one for the new tires.

[AirMiles]

Originally Posted by smithcreek

Can anyone confirm what I am doing below is correct?

I recently put a set of Michelin Defender LTX, E-rated (120) tires on my Toyota Tacoma. The tire shop installed them and set the pressure at the same 34psi as the old tires and placard says.

As soon as I started driving I noticed they had a softer ride and significantly more squirm than the D-rated tires they replaced. The opposite of what I was expecting. Yesterday I hooked up the trailer and took it out for a quick Spring shakedown ride. The squirm was even worse with the extra weight and when I jumped on the highway for a couple miles I felt a bit of sway for the first time ever.

After some research I think I should be running significantly higher pressure in these new E-rated tires than the old D-rated ones. I found a chart online that looks to be the instructions employees of Discount Tire use when selling a customer tires that are a different load rating than the OEM tires.

From the chart, the old D-rated tires (109) with 34psi had a load carrying capacity of 2143. To get the same load carrying capacity in my new E-rated (120) tires I should be at about 50psi. I took the truck out for a spin with 50psi and it was a major improvement.

It would be nice if the tire companies would make inflation charts available. I can't find one for the new tires.

I have Michelin LTX AT2 E rated tires on my 8,000# F250. My tire placard says to use 60psi front and 65psi rear. I typically run them at 55psi when not loaded to get a nice smooth ride and 65psi when towing to take the squirm out of them. You are using the correct weight table and are fine at 50psi with your lighter truck. Weight tables are not tire brand specific.

[Bill M.]

Are they the same size tire as the ones they replaced? What were the old tires? It does sound strange. But I expect using the load table is a good idea. Sometimes I think new tread squirms a lot and needs to wear in a little.

[smithcreek]

Thanks for the responses. Yes, exact same size. The old tires were Firestone Destination le2, light truck tires. It didn't occur to me that a more heavy duty E-rated tire with heavier sidewalls would need more pressure, but I guess that's the case. I'll probably hook the trailer up again tomorrow and see how it feels with the higher pressure.


Bill, I wondered if part of what I was feeling was not the fact that I now have 13/32" of tread compared to the old tire that was down to 4/32" or so.

[smithcreek]

Just took the rig out for a spin down the highway and all seems fine with the higher pressure. I'm going to get some chalk and do the chalk test to see if I can dial the pressure in even more, but I'm much happier now at 50psi than 34psi. Squirm is gone as is the hint of sway I felt yesterday. Felt steady at 65mph, yesterday felt shaky at 60mph.

If any of the tire experts on here read this post your comments are more than welcome, I don't want to be giving out bad advice. With my lack of expertise in mind I'll say if anyone changes to a higher rated tire you may want to consult the attached pdf to see what pressure you should run in the new tires.

[rustyoldman]

It is not the tire manufacturer who is at fault. It is the distributor from whom you purchased the tires and yourself. Typically, E rated tires have a much higher pressure rating. It is printed on the sidewall. Before you purchased the E rated tires you should have done your homework. The distributor that you got your tires from certainly isn't doing his job. He should have explained everything to you before you drove off. I have 10 ply E rated Michelins on my Silverado. I run them at 60 psi around town and 90 psi when towing. Get yourself a good pressure gauge a check your tires often. Good luck.

[smithcreek]

Originally Posted by rustyoldman

It is not the tire manufacturer who is at fault. It is the distributor from whom you purchased the tires and yourself. Typically, E rated tires have a much higher pressure rating. It is printed on the sidewall. Before you purchased the E rated tires you should have done your homework. The distributor that you got your tires from certainly isn't doing his job. He should have explained everything to you before you drove off. I have 10 ply E rated Michelins on my Silverado. I run them at 60 psi around town and 90 psi when towing. Get yourself a good pressure gauge a check your tires often. Good luck.

I didn't start this thread to blame anyone for anything, just looking for confirmation that running higher psi is normal in an E-rated tire when switching from a D-rated. I know the sidewalls are stiffer, that's the reason I made the switch. I know it is possible to run higher pressure but did not know it was necessary if the load did not change.

Try doing an internet search on switching from D to E rated and tire pressure, there is surprisingly little out there. As far as Michelin goes, they should publish a load/inflation chart, easy to find, right on their website.

BTW, what E-rated tires do you have that you can inflate to 90psi? I thought the max on E-rated was 80psi?

[Bill M.]

E rated tires must have stronger sidewalls to hold the air pressure. I doubt if radial tire sidewalls are really "stiffer" in terms of supporting the load or resisting sideways movement. That "stiffness" would seem to be more of a factor of the tire shape/aspect ratio. But if they are the exact same size then that would be the same also. The way a 75 ratio radial blooms out I just do not see the sidewalls holding anything on their own.

[Gleverett]

I had the same issue when purchasing Michelin tires from the Ford dealer. They put 38lbs of air in the tires when the side wall says 80psi at max load. I was told by them that everyone runs those truck tires at the lower pressure for better ride. Never have been able to find any type of pressure chart for loads. I now run 70 psi in them when towing the 25'FC and about 50psi when not towing. I carry 70psi in the AS tires all the time as we are never heavily loaded.

[CapriRacer]

Lots of misinformation in this thread.

First, a Toyota Tacoma (Year unknown) came with P metric tires. That will be stated on the vehicle tire placard pasted on the driver's doorframe by the letter "P" in front of the size numbers. Technically, that letter is part of the "size" - though technically it's the "size designation" .

Second, Tire Guides says the spec inflation pressure for a Toyota Tacoma is not 34 psi. Depending on the original tire size (what's on the placard) it could be 29,30, 32, or 33.

Next, Firestone Destination LE2's don't come in Load Range D (LT type). They only come in P metric (That pesky "P" again.)

LT type tires only come in Load Ranges and Michelin Defender LTX's do come in LT sizes (as well as P type sizes.)

As a general rule, in order to carry the same load a P type tire does, an LT type tire with the same dimensions needs about 15 psi more.


The spec inflation pressure (What's on the placard) has NOTHING to do with the load range - except to say that the tire MUST have enough to carry the pressure and the load. Put another way, Load Range D and Load Range E don't have different load carrying abilities at the same pressure (except to say that a LR E can be inflated and loaded higher.)

[smithcreek]

Originally Posted by CapriRacer

Lots of misinformation in this thread.

First, a Toyota Tacoma (Year unknown) came with P metric tires. That will be stated on the vehicle tire placard pasted on the driver's doorframe by the letter "P" in front of the size numbers. Technically, that letter is part of the "size" - though technically it's the "size designation" .

Second, Tire Guides says the spec inflation pressure for a Toyota Tacoma is not 34 psi. Depending on the original tire size (what's on the placard) it could be 29,30, 32, or 33.

Next, Firestone Destination LE2's don't come in Load Range D (LT type). They only come in P metric (That pesky "P" again.)

LT type tires only come in Load Ranges and Michelin Defender LTX's do come in LT sizes (as well as P type sizes.)

As a general rule, in order to carry the same load a P type tire does, an LT type tire with the same dimensions needs about 15 psi more.


The spec inflation pressure (What's on the placard) has NOTHING to do with the load range - except to say that the tire MUST have enough to carry the pressure and the load. Put another way, Load Range D and Load Range E don't have different load carrying abilities at the same pressure (except to say that a LR E can be inflated and loaded higher.)

Thanks for the response. Yes, 32psi is correct according to the placard. I got 34psi stuck in my head because that's what the Toyota dealership puts in every time I've had the vehicle serviced, and have even used that number any time I've added air myself.

With that said though, I noticed when replacing the original tires that they were worn more on the outside than center. Outside was down the 3/32" while center was closer to 5/32" My thinking is towing the AS with 34psi they may have been underinflated a bit. Does that make sense?

Good to know that an additional 15psi is about what I should use. So, the big question is, will I see any benefit in less sidewall flex from the E rated tire vs. the D when run at the appropriate pressure? Clearly the E tires have more capacity than the Tacoma will ever need, so if there's no benefit I'll go back to the OEM spec tires next round.

[Mike G]

Originally Posted by rustyoldman

It is not the tire manufacturer who is at fault. It is the distributor from whom you purchased the tires and yourself. Typically, E rated tires have a much higher pressure rating. It is printed on the sidewall. Before you purchased the E rated tires you should have done your homework. The distributor that you got your tires from certainly isn't doing his job. He should have explained everything to you before you drove off. I have 10 ply E rated Michelins on my Silverado. I run them at 60 psi around town and 90 psi when towing. Get yourself a good pressure gauge a check your tires often. Good luck.

Just out of curiosity, what is the maximum pressure allowed for those 10 ply Michelins?

[CapriRacer]

Originally Posted by Mike G

Just out of curiosity, what is the maximum pressure allowed for those 10 ply Michelins?

All LR E tires will have a max pressure of 80 psi. (Yes, that's not quite correct and it's not quite as simple as that, but for the purposes here, it's close enough.)

Originally Posted by smithcreek

...… With that said though, I noticed when replacing the original tires that they were worn more on the outside than center. Outside was down the 3/32" while center was closer to 5/32" My thinking is towing the AS with 34psi they may have been underinflated a bit. Does that make sense? ……

That wear pattern is much more likely because the steer tires are scrubbing in the turns. Drive tires tend to wear in the center. So with a RWD, you need to rotate regularly to even out the wear pattern.

Originally Posted by smithcreek

…….Good to know that an additional 15psi is about what I should use. So, the big question is, will I see any benefit in less sidewall flex from the E rated tire vs. the D when run at the appropriate pressure? Clearly the E tires have more capacity than the Tacoma will ever need, so if there's no benefit I'll go back to the OEM spec tires next round.

I am not a fan of replacing P type tires with LT type tires.

Having said that, I think in your case, the real benefit is with the increased inflation pressure

[WhereStream]

Originally Posted by smithcreek

I'm going to get some chalk and do the chalk test to see if I can dial the pressure in even more...

Would you mind explaining the chalk test? That's something new to me.

Thanks.

[smithcreek]

Originally Posted by CapriRacer

I am not a fan of replacing P type tires with LT type tires.

Having said that, I think in your case, the real benefit is with the increased inflation pressure

Thanks again CapriRacer. As long as I didn't really screw up buying these tires I'm good and can always switch back in a couple years.

Originally Posted by WhereStream

Would you mind explaining the chalk test? That's something new to me.

Thanks.

First thing is if CapriRacer is still watching this thread to get his take on the chalk test for setting pressure. Is it a valid way to dial in psi?

Chalk test is pretty simple, just draw a chalk line across the tread of your tire, then with whatever load you plan on carrying, drive 100 ft. or so and check the line to see if it is wearing off evenly across the tread or unevenly, more in the center, more on the edges. Adjust your psi up or down until you get it wearing evenly. If you start with high psi you can take your rig somewhere like a parking lot and slowly let air out until you get even wear.

[WhereStream]

Originally Posted by smithcreek

Chalk test is pretty simple, just draw a chalk line across the tread of your tire, then with whatever load you plan on carrying, drive 100 ft. or so and check the line to see if it is wearing off evenly across the tread or unevenly, more in the center, more on the edges. Adjust your psi up or down until you get it wearing evenly..

Good to know. Thanks.

[CapriRacer]

Originally Posted by smithcreek

……. First thing is if CapriRacer is still watching this thread to get his take on the chalk test for setting pressure. Is it a valid way to dial in psi? …...

I am not a fan of the chalk test.

If steer tires tend to wear in the shoulders and drive tires tend to wear in the center, how can you use wear to decide if the pressure is correct.

And my experience says that this wheel position effect is stronger than the affect inflation pressure has. Plus there are other things that also have a stronger affect.

When I was a tire designer, I could alter the footprint pressure distribution which has an affect on how even the wear is. I noted that some of my competitor's tires didn't have very good footprints. so their wear patterns (and therefore the chalk test) would lead one to a wrong conclusion.

[JCWDCW]

I believe tire wear on the outside of a rear drive tire is definitely under inflation. Turning wheels may be subject to scrub butI don't think rear wheels are, except in very specific circumstances that are not the normal use and wear & tear.
I run full inflation on my P type rear truck tires just to be ready for trailering. They definitely run harsh at full pressure but I don't want to be resetting pressure every time I pick up the trailer.
JCW

[Tireman9]

As the other actual tire engineer on this thread Capri and I almost always agree.
Some additional comments.


1. It is the inflation that carries the load, not the tire sidewall construction. You will only get the load capacity of LR-E as marked on the tire sidewall if you inflate to LR-E inflation as marked on the tire sidewall (80 psi in this case)


2. Anyone that changes from OE tires to something else MUST become smarter than the person inflating the tires, and must specify to that person the inflation you want.


3. Chalk test is sometimes OK but the best way to learn the MINIMUM inflation for your tires is with actual scale weights when truck & trailer are fully loaded and then use Load Inflation tables (available on my blog and on tire company web sites) to learn the MINIMUM inflation to run.


4. Capri and I both recommend you should have a minimum of 15% more load capacity than needed on trailer application tires.


5. I suggest you add 10% to the inflation number found on the Load tables for motor vehicle application.


6 On Towables with tandem axles, I strongly suggest that people run the tire sidewall pressure as their minimum. The only exception would be if you have increased the LR of your trailer tires more than one level. (LR-C > LR-E could use 65 as their cold) This is because of the Interply Shear that can result in belt separation and tire failure.

[smithcreek]

Excellent information, thanks Roger. I'm good for now, just driving the truck around by itself. We have a trip planned for Memorial Day weekend and first stop is going to be the local CAT scales to do any final changes to the hitch and tire pressures. Our FC20 is a single axle, I'm hoping to get individual weights for each tire this time around. From the layout of the FC20 I've wondered if it was heavy on the passenger side, so it interesting to see.

I printed a Load/Inflation chart to bring along. Knowing your recommendations for increasing load capacity is very helpful.