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Old 09-25-2015, 12:22 PM   #1 (permalink)
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Wheel offset / tire aero vs LRR and weight ...

I thought I was all set with a tire and wheel package for my 1996 F150, but because of availability issues I'm having to revisit things.

I'm looking at a lot of factors for this, including but not limited to ...
  • Price
  • Tread Width
  • Load Capacity
  • Ride / Handling Characteristics
  • Weight of the tire
  • Weight of the wheel
  • LRR
  • Aero considerations
  • Aesthetics

Everything I'm considering will have a tire diameter of about 29" to keep the speedometer / odometer happy.

I'm considering tire and wheel options that vary from 53 pounds at 15" wheel size to about 61 pounds at 18". Both would be LRR tires.

I'm trying to figure out offset trade-offs since some wheels (mostly 15") have a negative offset of -19mm to a positive offset of 20mm for 18" wheels. It's my understanding that negative offsets push the outer side of the tire to the outside of the wheel well and positive offsets pull things back under the wheel well.

Of course the 15" wheels are 15x8" and the 18" wheels are 18x9" so there's a width difference of an inch right there. At the extremes you looking at a range of 39mm, or 1.54 inches.

I think having less of the tire out in the airflow would create less drag, but I wonder how that compares to up to 7-8 pounds per corner as a result of upsizing?

I know that more weight is bad because of that whole moment of inertia thing, but I wonder how much difference it makes in terms of energy required to keep a tire rolling versus how much it costs to start and stop a heavier wheel and tire?

Here I am comparing the difference between different sizes of different LRR tires. If the differences are very small no matter what you do (.1 mpg) then that argues in favor of buying what's cheap or pretty or has the best ride or any one of a number of other factors. If it could be .5 mpg or more, well that's something else.

Thanks in advance for your thoughts.

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Old 09-25-2015, 01:10 PM   #2 (permalink)
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I would bet even if the 18s somehow did fractionally better (which I doubt) they would never recover the added cost that aftermarket 18s cost. If you find a good deal on used but basically new 18s off a newer f150 (because the owner went to 20-22s) then sometimes it is worth it but I think they changed the bolt pattern around 2002. I would just put new 15" tires on cheep factory wheels.
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Old 09-25-2015, 03:44 PM   #3 (permalink)
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Hersbird: Some limitations apply. At 15", there is only one LRR option for me: The General Tire HTS Grabber. The advantages are that it's a cheap tire that I could put on a cheap wheel, but the ride and handling characteristics are unremarkable, as you might expect with, well, cheap.

My bolt pattern is 5x139.7, which you won't find in later model wheels for this vehicle.

At 16" wheel size, the LRR tires won't carry enough load to suit me.

At 17" & 18" sizes, I get lots of options, ranging from great performance but heavy and expensive to much lighter and merely adequate performance wise.

I may still end up going with the cheapest / lightest LRR option at 15".
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Old 09-25-2015, 04:15 PM   #4 (permalink)
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OK: So I decided to keep it cheap & simple. I got wheels that were only 20 pounds each in 15x8, but which will carry up to 2600 pounds, matching the Grabber HTS LRR, @ 109T-XL/2271 pounds per tire and weight of 32lbs per tire.

The tire and wheel are cheap and available and the performance is adequate. They match the speedometer/odometer perfectly and I'm picking them up on Monday. The tread width is 7.2" vs 6.5" for my current Kumho Solus KR21, and the load carrying capacity is slightly greater.
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Old 09-25-2015, 05:03 PM   #5 (permalink)
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What size tire are you running? Is there a problem with the wheels on the truck now? If I am not mistaken the truck would have had 15x7's on it originally. Especially with an aluminum wheel, you would save some weight by running that.
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Old 09-25-2015, 06:13 PM   #6 (permalink)
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Quote:
Originally Posted by iamnotahippee View Post
What size tire are you running? Is there a problem with the wheels on the truck now? If I am not mistaken the truck would have had 15x7's on it originally. Especially with an aluminum wheel, you would save some weight by running that.
It was a 15x8 steel wheel that weighed a ton. Got rid of that almost immediately, except for the spare.
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Old 09-26-2015, 07:38 AM   #7 (permalink)
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Considering PU has the aero of a brick, I don't think you will notice any change due to offset.

That being said - and considering you have a long commute - you will probably be way further ahead economically if you stick with the original P235/75R15's, and chose a good wearing tire in a LRR.

If you want to move up in rim diameter for other reasons, then there are a whole range of options. Directionally going up in rim diameter is going to help dry traction, handling, and RR, do nothing for wear, and hurt wet traction, snow traction, ride, and cost. But most of these are fairly minor, so it just becomes a matter of do you want to spend the extra money.

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