Different MY, different PSI

[Johnny248]

My '11 Ram calls for 35 psi. Most everyone I've seen in recent years does. A friend just purchased a new Ram, same set us my 11. Same tire size etc. His placcard calls for 39psi.

The tires on both trucks have a max inflation of 44 psi, however the load rating of the tire is max at 35 psi. They don't over any more load carrying when the pressure is increased above 35 psi, so the switch to 39 psi for the new MY doesn't have anything to do with load carrying.

I'm guessing Dodge increase the PSI to lower rolling resistance and squeeze out every last bit of fuel economy available since right now in time, fuel economy ratings on truck seems to be a pretty hot comparrison between the makers.

Do you think I am correct in my theory? Or is there any other reason why the pressure has been increased?

 

[rslifkin]

Do they have the same model stock tires? Different tires could respond differently in terms of their pressure needs.

Also, are you sure about the max load at 35psi thing? Most tires I've seen list max load at max inflation pressure.

 

[Rand]

does his dodge ram have bigger wheels?

some larger wheel sizes spec higher psi.

For example on an accord 18" performance oem wheels were spec'ed at 38psi

The dealer even put an additional tire pressure sticker on the doorjam for the summer wheel set.

 

[eljefino]

Are these XL (extra load) tires?

 

[Johnny248]

Both trucks are configured the same. Both trucks have the same size wheel and tires, in fact they are the same model tire.

I have been the in tire industry for years and completely understand load ratings, pressure changes, plus sizing, etc.

Not all tires have max load carrying at max psi. These P-Metric tires max load carrying is at 35 psi. Load carrying does not increase above 35 psi even though max pressure is 44 psi. Any increase above the 35 would simply be for vehicle manfuacture prefference in terms of ride quality, handling characteristics, tire wear etc.

Feel free to take a look at load tables for a P275/60R20 114 and you will see how to load carrying changes. You will see that you max load is 2601 lbs at 35 psi as well as 44 psi.

 

[Johnny248]

Originally Posted By: Rand
does his dodge ram have bigger wheels?

some larger wheel sizes spec higher psi.

For example on an accord 18" performance oem wheels were spec'ed at 38psi

The dealer even put an additional tire pressure sticker on the doorjam for the summer wheel set.


Not applicable here.

P-Metric and Euro Metric tires have different load tables. Then throw in different sizing, XL ,SL, LL, etc and the the required pressure for different load changes.

In this case they are the same size, same load rating, and same model.

 

[Quattro Pete]

Any differences in towing/cargo load ratings between the two trucks?

 

[NMBurb02]

Originally Posted By: Quattro Pete
Any differences in towing/cargo load ratings between the two trucks?

Or curb weight?

 

[leeharvey418]

Originally Posted By: NMBurb02
Originally Posted By: Quattro Pete
Any differences in towing/cargo load ratings between the two trucks?

Or curb weight?

Or new Chrysler internal spec that calls for a higher load factor for the tires?

 

[Johnny248]

Originally Posted By: Quattro Pete
Any differences in towing/cargo load ratings between the two trucks?


Id have to look. Both trucks are the cab/bed/4x4 configuration. The newer one has the newer trans and stuff, but there shouldn't be that much difference in gvwr. Even so, there is no load benefit from the 39 psi

 

[CapriRacer]

Originally Posted By: Johnny248
My '11 Ram calls for 35 psi. Most everyone I've seen in recent years does. A friend just purchased a new Ram, same set us my 11. Same tire size etc. His placcard calls for 39psi.

The tires on both trucks have a max inflation of 44 psi, however the load rating of the tire is max at 35 psi. They don't over any more load carrying when the pressure is increased above 35 psi, so the switch to 39 psi for the new MY doesn't have anything to do with load carrying.

I'm guessing Dodge increase the PSI to lower rolling resistance and squeeze out every last bit of fuel economy available since right now in time, fuel economy ratings on truck seems to be a pretty hot comparrison between the makers.

Do you think I am correct in my theory? Or is there any other reason why the pressure has been increased?



Chrysler seems to be the only one who does this: Specify more than 35 psi on P metric (or hard metric) Standard Load tires for normal usage.

The Germans (Mercedes and BMW in particular) will specify a normal usage spec, then specify a high load/high speed spec that is over 35 psi - and the Germans seem to be the only one to do that.

Why did Dodge do that? Good question, but your fuel economy thing seems reasonable.