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First datapoint, new tires experiment

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

Reply to
zxcvbob
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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.

Reply to
scrape

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

Reply to
zxcvbob

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.

Reply to
Pete C.

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

Reply to
zxcvbob

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 .

Reply to
None4U

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.

Reply to
Tim

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