Rear tire pressure?

[ViewRVs]

I am currently running 40 psi in my rear tires per the manual. I recently saw that the chassis came from the factory with 55 psi in the rear. I am concerned that the 40 psi is hurting my gas mileage. I have been running between 7 and 9.5 MPG in my Argosy 20 after the carb rebuild. Before that, it was around 8 MPG.

What psi are you guys putting in the rear tires?

(I am running with the factory 70 psi in the front tires.)

[mprokop87]

Im running 55-ish psi and I wish my mileage was that good! I just did some work on my carb so I guess we will see on the next trip.

[HowieE]

Tire pressure is a function of the load on the tires not what is printed on the tire as max nor what is in the manual.

After weighing your rig look on the tire inflation chart and see the recommended tire pressure.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

Note that dual tires run at a lower pressure than singles.

The chart is good for any manufacture as the government require that tires of a given size all meet the same specs.

If your tires are not on this chart look for the Michelin RV tire inflation chart.

[jadatis]

You better use my made pressure/loadcapacity lists, they take better care of giving the same deflection of tire over the whole range of Pressure/loadcapacity.

https://onedrive.live.com/?cid=A526E...E092E6DC%21904

First open the USA map with given in PSI and LBS.
then you can choose the submap for if you want it given per tire, axle single or axle dual, I would take the axle single , so you dont need to devide the axle loads by 2 anymore.

then first determine the reference-pressure(Pr)/Atpressure/maxloadpressure/pressure needed for the maximum load up to 160km/99m/h, wich is not the maximum pressure of tire.
If we are talking about your 70 psi in front tires, I think you have E-load tires with Pr/// of 80 psi or mayby D-load with exeption 70 psi Pr/// instead of the usual 65 psi.
Then download or leftclick the PDF beginning with the Pr.
In that list first search the Loadindex/maximum load in the first rows then in that line the load you want to know it for, and look above or below it for the needed pressure.
Always first add a 10% extra to the load before looking it up in the list.
The car-makers use nowadays the GAWR's to fill in for normal use , American pressure stickers only give that advice. But sometimes for heavy duty maximum load of tire is used or even maximum pressure of tire , wich gives terrible bumping when normally loaded.
The difference in gas- milage between this determined pressure and high pressure is not that big, but enaugh and not to high pressure gives better comfort and gripp, and less nervous stearing when on front.

[CapriRacer]

Originally Posted by ViewRVs

I am currently running 40 psi in my rear tires per the manual. I recently saw that the chassis came from the factory with 55 psi in the rear. I am concerned that the 40 psi is hurting my gas mileage. I have been running between 7 and 9.5 MPG in my Argosy 20 after the carb rebuild. Before that, it was around 8 MPG.

What psi are you guys putting in the rear tires?

(I am running with the factory 70 psi in the front tires.)

This doesn't sound right. Are you sure your manual says 70 psi front/40 psi rear?

What about the vehicle tire placard? What does it say? It ought to be the same as the manual. The placard be on the driver's doorframe (if you have a 2008 and newer model, but may be elsewhere if older.)

[ViewRVs]

Originally Posted by CapriRacer

The placard be on the driver's doorframe (if you have a 2008 and newer model, but may be elsewhere if older.)

..... except that this is the Argosy motorhome forum and my vehicle is a 1977 Argosy 20 MH on a Chevy P-30 truck chassis.

[ViewRVs]

Here is a picture of the page from my manual showing 70 psi up front and 40 psi in the rear.

[Smartstream]

Without knowing your actual weight that agrees with the P-30 manual from GM. I believe it is somewhere here on the forum. I have a pdf of it but can't copy the page.

[CapriRacer]

Originally Posted by ViewRVs

Here is a picture of the page from my manual showing 70 psi up front and 40 psi in the rear.

Good. That seems very clear.

And just to dot the i's and cross the t's, you are using 8.75-16.5's right? That would be bias ply tires. If you are using radials, you'll want 5 psi more.

And considering how long ago the manual was written, it would be advisable to use even more. say 10 psi at both ends.

But if you aren't using the 8.75-16.5's, then we have a whole bunch of recalculation to do.

[jadatis]

Is the back by any chanche dual so 4 tires on the ( imaginary) axle?
That would explain the low advice pressure , because rear GAWR is mostly more then front.

Then if the advice is for bias ply , those are calculated in their loadcapacity with a formula that gives almost 70% of maximum load at 50% of AT-pressure.

To get a right advice for radial ( if you have those) I need GAWR's and maximum load and AT-pressure of tires at least.
GVWR and speedcode of tires would be nice too.
And ofcource even better axle or better 4 point weighing in loading you drive with.