My tires say 45 PSI is maximum pressure on the sides, so I always try to keep them around 38. Optimal pressure is one thing, but you should never fill your tires up to their maximum rated PSI.

I'm thinking some people might need to calibrate for their tires, as Ramman18 said, because I am throwing a B.S. flag out for some of these numbers.

 

my own tests on 2nd Gen Ram
MPG versus speed in mph
is that there is not enough difference in MPG
between 55 mph and 60 mph
to reliably measure in the 'real world',
but above 60 mph the MPG goes down rapidly.

Depending on weight, wind, temperature, ethanol in gasoline, etc
20 to 24 MPG at a steady 60 mph is typical for my 1995 2wd 5.9V8 auto 3.21
and 16 to 17 MPG at a steady 80 mph,
but it does have several mods,
plus high mileage on piston rings that has lowered engine friction

I have done one 300 mile test between Knoxville TN and Burlington NC
at a steady 55 mph where I got a little over 25 MPG

If you are science minded
you know that the HP to push through the wind goes up as the cubic math function,
but that tire rolling resistance is nearly constant at normal speeds:

http://www.us.pirelli.com/web/techno...e/default.page

so that the combination of these two factors along with engine efficiency
is usually something like the square math function
so that twice the speed results in one fourth the MPG

there is a fairly accurate online MPG calculator at:
http://www.bgsoflex.com/mpg.html

use c of drag of 0.47 for a 1994-2001 2nd Gen Ram 4wd
and frontal area of 35 square feet

the question of what psi to run in your pickup's tires
is one people should more thoroughly investigate,
as it is complicated (try different psi in above calculator)

there is an optimum psi for ride comfort
and this is the psi you usually find on the door sticker
and it is chosen to help sell the new vehicle on test drives,
but still (barely) pass the safety maneuvers
like Consumer's Reports magazine does independently
(remember when Jeep had to raise tire psi and lower suspension height on the first year of the Liberty?)

The optimum psi for:
* highest fuel economy
* most resistance to hydro-planing on wet pavemetn
* and all out panic braking
is either at (or even slightly above,)
the maximum psi on the side wall of the tire

on a road course style race track
the highest psi in the tire usually also is where you get
lowest lap time,
but it does depend on the particular layout of the race track

high psi in the tire
will wear the tire out in the center of tread,
so many wrongly say that the 'most economical'
psi is the level that keeps tread wear constant across the width
and you may have heard of the tricks
like putting a strip of duct tape across the tread for 50 miles
or putting a chalk mark across the tread for a few miles
to find the "ideal' psi for lowest tread wear,
but be aware that this will change with load carried in the truck

However, 'most economical' should really include both
cost of tire per mile
and
cost of fuel per mile.

With the cost of fuel above $3 per gallon
in most cases if you include both of the above costs
you will find that it is actually more economical
to wear out the center of tire sooner
but get the MPG gain of a high psi pressure in the tire,
but this does depend on the so called
'rolling resistance' of your tire design
and tires are SIGNIFICANTLY different in both
rolling resistance and rate of tread wear

Bob Bondurant,
a former champion racing driver in multiple divisions
and a owner of racing training schools
says he routinely inflates his tires to the maximum
for both performance & safety
... the safety part comes from
better all out braking (secret here is weight transfer to front wheels),
better resistance to hydro-planing
better emergency cornering ability
cooler temperatures inside the tire

How (and who) decides the maximum psi on the side wall of the tire?

It will be a surprise to most
that engineers at Tire making companies will tell you that
Lawyers set the psi for this
usually at half the psi the tire will take
when inflated and then run at the
maximum speed in mph
at maximum load in pounds.

The tire company's lawyers believe this is what
they can tell a jury hearing a tire explosion or failure case
and have the most chance of winning,
for having set a reasonable 'factor of safety' .

 

and thats why you change gears and some other componets to compensate for the bigger tire's its not really that hard for a dodge to get good gas milage

 

I have a 2000 2500 quad cab ram, basically stock, and figured out staying just shy of 65 MPH gave me the best mileage.(don't have any numbers, just go by fill-ups) As for tire pressure, I go by what is on the sticker on the driver's side door jamb.

 

the trouble with any large diameter tire
is that it lifts the pickup
lets more air flow against the rough and un-streamlined under carriage,
exposes more of the tire tread outside the fender into the wind,
and makes aerodynamic drag worse

the problem is compounded
when 'mudder style tread'
big tires are used
which have much higher rolling resistance
.... generally the deeper the tire tread the greater the rolling resistance,
this is one of the 'break-in' factors that give better MPG as a vehicle ages and runs up miles, and is discussed in the official Cummins white paper on better MPG for big trucks.

but .... larger diameter tires can be low rolling resistance if designed for that purpose, and can even have inherent advantages

ever seen a picture of those 1890s bicycles with the huge diameter front wheel?

it was big to reduce rolling resistance