My 2001 Silverado C1500 has never gotten over 19 MPG, and 19 was only
once on a long highway trip when I wasn't in a hurry. Usually it gets
about 15 or 16 in the summer and 11 in the winter. (I think part of the
problem is the oxygenated gas they sell up here doesn't have as much
energy as straight gasoline)

The original equipment tires were General P255/70R16's and the door
placard says to keep 35 pounds of air in them. These tire always had
good traction on dry pavement, mediocre in rain, and were terrible in
the winter.

I put four LT215/85R16 all-season radials on a couple of weeks ago. Been
experimenting with different tire pressures to get it to handle right --
ended up with 55 pounds in the fronts and 50 in the rears. And the ride
is not noticeably rougher than it was with the old tires. With 55 in
the rear tires, it gets scary going around a curve on a rough road if
there's no weight in the back. I can increase them to 60 or 65 when I'm
hauling.

I just checked the gas mileage for the first time yesterday and I got
18.4 mpg! That doesn't sound like much, but it's at least a 15%
improvement and it didn't cost me anything because I needed new tires
anyway. This was about half highway driving and half stop-n-go city
driving. It might actually break 20 now on the highway (and they should
give me better winter traction too because they are narrower.)

So why are all the auto makers putting bigger and bigger P-metrics on
their trucks now (P265/70R17 seems to be typical) instead of narrow load
range D tires?

Bob

On Thu, 06 Aug 2009 11:43:35 -0500, zxcvbob <zxcvbob (AT) charter (DOT) net>
wrote:

Don't know if this is naive or not, but I would imagine that the
wider tire would give you a bigger contact patch and especially at
a lower pressure, more traction - all other things being equal.

scrape wrote:

I'm pretty sure the contact patch ends up the same size; it's just short
and wide with fat tires and long and skinny with thin tires. The wide
contact patch resists sideways forces better. The long skinny patch
gives you more straight-line traction, especially in ice and snow. And
in loose sand, you want wide tires with very low air pressure.

I could be wrong about parts of this.

Bob

zxcvbob wrote:
Did you lookup the rolling radius of the two tires in question? If your
new tires have a larger rolling radius you simply changed your overall
drivetrain gearing to a lower effective ratio which will improve MPG at
the expense of some towing / hauling capacity. It will also affect your
speedometer and odometer accuracy so it should be recalibrated.

You could actually do this deliberately if you wanted and had a
programmer handy to correct the calibration, have a truck with a
relatively high axle ratio for towing capacity, and have a second set of
tires/wheels with a notably larger radius to swap on when not towing for
better MPG. Similar effect could be achieved with an add-on overdrive
gearbox, but that's a lot more complex a task.

Pete C. wrote:

Yes I did. (I thought about putting 7.50R16 or 235/85R16 tires on for
just that reason) The LT215/85's have exactly the same rolling radius
as the original P255/70's.

I checked my speedometer yesterday with a GPS and it's dead-on. :-)

Bob

"zxcvbob" <zxcvbob (AT) charter (DOT) net> wrote


You put on roughly a 50 mm narrower tires and pumped up the air pressure
from 32 to 55 and got better milage. And less traction around corners .
Thats what should happen. The milage increase sounds reasonable too. The
other posters statement about snow traction was correct too. narrow , is
better. Its probable you are going to get better winter traction if the
tread design allows it . Manufacturers put low profiles and wider tires
on to sell trucks. And help out the tire manufacturers .

"zxcvbob" <zxcvbob (AT) charter (DOT) net> wrote

The wide tires just look better, and suggest high performance. Not only have
you gained by increasing your tire pressure, but narrower tires have less
aerodynamic drag. If you can live with the appearance of your new tires,
they will probably serve you better than the old ones.

Another area where narrower tire are a plus, is driving through deep snow at
highway speeds. You get better steering and a lot less drag.