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Old 09-21-2008, 01:44 PM   #1 (permalink)
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change in tire size

I have a 2002 4x4 chev silverado 1/2 ton pickup that I need for towing. I have done a few mods to it like upper grill block, partial bellypan, lowered ride height 2" front and rear that has helped.
Next I was going to try different size tires. It has the stock size tires on it now (245/75-16 on 16"x6.5" rims) max sidewall air pressure of 44 psi. Was thinking of trying a 215/85-16 E rated tire with a max sidewall pressure of 80 psi. Even though they are thinner tires (1") they are higher load rated tires (E-rated) and the diameter are roughly the same.
Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/75-16 7.2in 15.2in 30.5in 95.7in 662 0.0%
215/85-16 7.2in 15.2in 30.4in 95.5in 664 -0.3%

My question is will this help my mileage much or will I just get a rougher ride and less traction? Guess I'm just asking for pro's and con's on this type tire change.
thanks

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Old 09-21-2008, 06:16 PM   #2 (permalink)
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tougher sidewall is the efficent part. I would assume better from my own experience.
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Old 09-25-2008, 05:18 PM   #3 (permalink)
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Skinnier "Truck" summer Tires are always better on the fuel. If you can find the goofball types that are real skinny with thick tread (similar to some trailers) you get even better.

My old diesel suburban lost about 3 mpg going to big winter tires, in my oppinion not worth it. After that I stuck to skinny tires.

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Old 09-25-2008, 06:08 PM   #4 (permalink)
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I would think that the ride will actually be better with a less wide tire and FE should also be better...I would bet that those tires would also weigh less.
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Old 09-25-2008, 08:46 PM   #5 (permalink)
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Sorry for taking this off topic, but you can use the vbulletin code tags to pretty up that chart.
Code:
Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/75-16     7.2in    15.2in 30.5in   95.7in        662        0.0%
215/85-16     7.2in    15.2in 30.4in   95.5in        664       -0.3%
Click the QUOTE button to see how it works.
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Old 09-25-2008, 10:38 PM   #6 (permalink)
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Mully,

You have to be careful here. You have 2 different types of tires that you are comparing, and that gets complex. So let's go over the details.

Your vehicle placard (It's on the driver's doorframe) ought to say the original tire were P245/75R16 (Notice the "P") inflated to 35 psi and mounted on 6.5" rims. This means the load carrying capacity of the tires is 2065 #.

The other tires are LT215/85R16 (Notice the "LT") Load Range E, and in order to carry the same load they need to be inflated to 55 psi. LT tires are built with the idea that they are going to used where the unit loading is high. (Meaning the load compared to how much material is actually used in the tire.)

Normally this might mean better fuel economy, but because these tires are small compared to the load they are trying to carry, the rubber compounds are stiffer - and if everything else were the same, that means more rolling resistance.

Without a doubt, this is going to hurt your ride quality - and my best guess is that you will not improve the RR and perhaps even hurt it.
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Old 09-25-2008, 10:59 PM   #7 (permalink)
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A tire rated as an "LT" takes higher pressure and is much heavier than a "P" series tire. If you tow heavy loads, then I suggest LT245/75R16 BFG Long Trail tires for that truck.
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Old 09-28-2008, 01:03 AM   #8 (permalink)
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Thanks for the replies! I found the specs for my oem tires that are still on the truck...Goodyear Wrangler ST

245/75R16
Load Index 109 = 2271lbs (1030kg) per tire
Speed Rating S = 112mph (180kph) 109S SL Treadwear: 340
Traction: B
Temperature: B340 B B
2271 lbs. max load
44 psi tire pressure
12/32" tread depth
33 lbs. tire weight

9.8" section width

30.5" dia
674 revolution per mile
Country of Origin "CA" = CANADA


And the specs for the narrower but same diameter E-rated Goodyear Wranger HT for comparison.

215/85R16
E
2680 lbs. max load
80 psi tire pressure
13/32" tread depth
40 lbs. tire weight

8.5" section width

30.4" diameter
677 revolutions per mile
Country of Origin "US" = UNITED STATES


I'm still confused on the couple of replies that don't reccomend this change in tire size as they are roughly the same diameter and have a heavier load rating. They are narrower, putting about 4.5" less rubber on the road. Wouldn't that mean more mpg's? They are heavier though weighing 7lbs more per tire. Still learning here and love this web site!
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Old 09-28-2008, 08:13 AM   #9 (permalink)
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Mully,

There are primarily three physical properties that affect RR - deflection, volume of the material being deflected, and the hysteretic properties of the material being deflected.

Hysteresis is just a fancy word for the internal friction that generates heat. Anytime something is stretched, bent, compressed, etc., the internal friction of that material generates heat.

In the case of tires, it's the amount of deflection - the difference between the tire being a circular shape, and the shape it takes when in contact with the road surface (basically flat!) Most of this deflection is in the tread area, so the tread rubber contributes the most to RR.

In your example: While the volume of tread rubber would be smaller (narrower tire), and the amount of deflection would be less (due to the higher pressure being used), the tread rubber itself more than offsets any apparant gain.

I just came back from a week long seminar on tires, and one of the papers detailed a coorelation between the various RR tests in use (and under development). Their conclusion was that it didn't matter which test you ran, you could you grade tires by a single point test. What they didn't address was the apparent differences within a tire line - some of which seem to be size related. This means that the goal of having a simple, easy to follow guide for selection of tires by RR is still eluding everyone.

But buried deep in this paper was some RR data of both P metric tires and LT metric tires. I was struck by the fact that the LT metric tires, while having lower RRC's, when tested at the same load as a P metric, had much larger RR values.

I found a formula used to calculate RR in a P metric tire once a test has been performed. However, accompaning the verbage on this formula was a comment that LT metric tires behaved differently and the formula would be different and more complex. This is obviously the difference between what you might think would happen and what actually happens.

Also, I think there is a clue in the data between the 2 tires: In spite of the LT metric tire being physically smaller, it weighs more.

Bottomline: If you are interested in more mpg's, then going from a P metric to an LT metric doesn't appear to be the way to go. (which is one of the reasons I always identify tires with the letters that come as part of the tire size - in this case P245/75R16 and LT215/85R16.)

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