Manual and doorjam call for 38 front, 36 back, Max on tire is 40 lbs. What to do?
listen to the manual and doorjam. The PSI on the tires is the maximum the tires can be pumped up to
“Maximum” and “recommended” are not the same thing. Use the recommended pressure.
However, that’s an unusually low maximum pressure. When the tire heats up during driving, you’ll actually be a bit over the maximum pressure, which concerns me. Exactly what tire are you using here?
Use the doorjamb. I have no concern over the small difference between the max on the tire and the doorjamb, that max is for when the tire is cold, not after being driven.
@CapriRacer - is this correct?
You go by what the placard/label on the doorjamb says, always. The maximum pressure on the tire doesn’t really factor into this. The reason for going with the placard/doorjamb is that the engineers who designed your specific car model determined what the proper tire tire pressure should be. The engineers who made the tire don’t necessarily know what vehicles the tire is going on, so they just concern themselves with what the maximum pressure the tire can hold (at max load) without failing. For example back in the 80’s the tire size 205/70/R14 was used in everything from light trucks to compact sedans. If you had a Ford Ranger with that size tire you needed to make sure it could handle a 1000 pound load in the bed and not fail. In this case the vehicle manufacturer would likely spec a higher tire pressure. . Or you might have a Chrysler K car with that sized tire as well and while you weren’t carrying 1000 pounds of cargo, you might want a softer ride, so the manufacturer would call for a bit less air in the tire. In both cases the same tire is used, but the vehicle manufacturer would specify different tire pressures since the vehicles are made to do different things.
Perhaps you’re right. I’ve never personally seen such a small difference, so I’ve never had to think about it before.
I also noticed the small difference ( 40 psi ) . I doubt if the person will return just like so many lately . I wonder if they read the tire size ( 235 18 40 ) and thought that the 40 was the max tire pressure .
Here’s what Tire Rack says:
“A tire’s maximum inflation pressure is the highest “cold” inflation pressure that the tire is designed to contain.”
It’s a little bit complicated, but the max pressure listed on the sidewall of a tire is the maximum cold usage pressure - with some exceptions. The tire is NOT designed around the max pressure. It is designed around the max load and its corresponding pressure.
[quote=“lion9car, post:3, topic:153127”] ……However, that’s an unusually low maximum pressure. When the tire heats up during driving, you’ll actually be a bit over the maximum pressure, which concerns me. Exactly what tire are you using here?
First, good catch.
For passenger car tires the max pressure will be listed as one of the following: 35, 36, 41, 44, 50, or 51 psi - Not 40 psi. Long story why it is only those numbers.
It is likely that the OP is reading the safety warning not to inflate over 40 psi when seating the beads when mounting.
Also, tires can handle much more pressure than the max listed on the sidewall - that is, the burst pressure for a tire is on the order of 3 or 4 times the max pressure, and the reason for that is in the concept of fatigue strength. (https://en.wikipedia.org/wiki/Fatigue_(material))
- BUT - a tire should NOT be inflated higher than what is listed on the sidewall (with a few exceptions!)
OK, now I’m confused. How is that different than what I said?
The door is the corrrect pressure . All tire max pressures will be considerably above the door placards normally .
Well, it seemed to imply that the max pressure is used when the tire is cold, rather than the placard value being the one to be used - also measured when cold.
This comes up here on a regular basis.
It is simple, you should use the door placard values.
That tire can be used on many different vehicles. Each has it’s own demands.
The vehicle manufacturer has specified the recommended inflation pressure for their application.
The tire manufacturer doesn’t know which vehicle the tire will be installed on so they simply provide the “not to exceed” value on the sidewall.
If a head bolt can take 400 ft lbs before breaking are you going to use that value or are you going to use the vehicle manufacturer specified torque value for their application?
I didn’t mean that, see my earlier post. I was only addressing the question about whether there was a problem if the tire, when inflated to the placard value when cold, exceeded the maximum value on the tire when hot.