Cooper S/T Maxx - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 9 Old 10-14-2014, 09:20 AM Thread Starter
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Cooper S/T Maxx

I've got some Rugged Ridge XHD wheels coming in today and after doing a lot of researching, I got the local Discount Tire to order me some Cooper S/T Maxx tires in 285/70r17. It is a heavy load range E tire and I know I'll probably need to do the chalk test, but does anyone else run something similar and can give me a good starting point on what pressure to run in these tires?

I ran E range Terra Grapplers on my Nissan Titan Pro-4x, but I know this is going to be a different deal.
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post #2 of 9 Old 10-14-2014, 10:05 AM
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I would start at 28psi and go from there. that is what I did with my Toyo E OC's in that size.
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post #3 of 9 Old 10-14-2014, 10:06 AM
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I'm running a 255 width ST Maxx, so it's not quite apples to apples, but I'll let you know my experience so far. I haven't done the chalk test yet.

The door says 37 PSI with the stock 255's and I decided to start at 35 with the ST Maxx tires. The tires have a bit more of a "domed" or bulging tread surface than some other tires. After a few hundred miles I noticed that the little nubs left from where the mold hairs used to be were completely gone from the center treads. However, the inner and outer tread blocks still have slight nubs (the hairs are gone, but there are still little bumps where they used to be). That makes me think I should go a bit lower, though I don't think it's too bad.

Your tires are wider, so I would think that 32-ish would be a good starting point. That's where I would start. A chalk test would be a good idea, but I don't think you are going to have any issues running 32 or so for a bit until you can get a chalk test done.
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post #4 of 9 Old 10-14-2014, 02:57 PM Thread Starter
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Thanks for the feedback. It'll be interesting to see what the tire shop thinks they should be set at. I'm betting it will be way too high.
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post #5 of 9 Old 10-14-2014, 09:02 PM Thread Starter
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One other note...the tires are supposed to weigh right at 60 lbs each. I know the MPG is going to suffer a bit as will acceleration and braking. I also am stuck with 3.21 gears for now. Anyone know what I should expect? I'm thinking a loss of around 2 MPG. My only concern about braking is just the weight of the tires being harder on the brake pads and wearing them out faster.

I had thought about getting Duratracs, but they're only 7 lbs lighter than the S/T Maxx. I didn't think that was much and I trust the quality/durability of Cooper more than Goodyear.
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post #6 of 9 Old 10-14-2014, 09:16 PM
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Quote:
Originally Posted by DuneVol View Post
One other note...the tires are supposed to weigh right at 60 lbs each. I know the MPG is going to suffer a bit as will acceleration and braking. I also am stuck with 3.21 gears for now. Anyone know what I should expect? I'm thinking a loss of around 2 MPG. My only concern about braking is just the weight of the tires being harder on the brake pads and wearing them out faster.

I had thought about getting Duratracs, but they're only 7 lbs lighter than the S/T Maxx. I didn't think that was much and I trust the quality/durability of Cooper more than Goodyear.
I've heard the ST Maxx run heavy (bit heavier than the specs), but I don't know if that's true because I didn't weigh mine. May not be true, but that's what I've heard. I don't believe Cooper lists the weight on their site, so I take weights listed on other sites with a tiny grain of salt. Not saying they are wrong, just that I wouldn't bet my life on them being correct.

Not only the weight alone, but the increased diameter vs. stock (I don't know what you are coming from, size wise) will also play a role in fuel economy and performance changes, as well as being harder on the brakes (larger diameter tires present more leverage against the brakes). The tire width also is a factor in fuel economy. Increased width presents a larger surface for wind resistance, as well as potentially increasing rolling resistance.

How much difference it makes is hard to say for sure. What size are you coming from?
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post #7 of 9 Old 10-14-2014, 11:06 PM Thread Starter
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I'm coming from the stock 255/75r17 Goodyear SR-A tires. I'll be looking at around 28-29 lbs more per tire and wheel over stock if the Coopers weigh 60 lbs. I'm extremely easy on my brakes anyways, so I doubt I'll deal with any excessive brake wear. Any loss in acceleration can be remedied with a gear change down the road. I've been reading up on everything I can this evening to see what I should expect with the added weight and found a great dyno test from an XJ where they tested it with a 68 lbs combo (wheel/tire) vs a 96 lbs combo (same size tire diameter and width). I think it took 2 more seconds to 60, very little difference in acceleration from 40-60, 60-0 took 24 more feet, and gas mileage dropped by about .63 MPG. I think it's a pretty good baseline from what I would be doing by going to the S/T Maxx...although the mileage should definitely suffer more from going with a wider tire (I'm guessing closer to a 2 MPG loss).

I can live with the loss in mileage if it's only around 2 MPG (my Titan averaged around 14 MPG). I think my main concern now is that it tracks well on the highway and doesn't feel unpredictable. I want it to feel safe on long trips on the interstate to the beach where hitting 80 mph isn't uncommon.
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post #8 of 9 Old 10-15-2014, 02:22 PM
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Dunevol, thanks for your support. We look forward to your report on the tires.

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post #9 of 9 Old 10-17-2014, 08:37 PM Thread Starter
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Sad to say that the Coopers were just too heavy. DT didn't have a scale, but I found a few instances online where the 285/70r17 was coming in at 66 lbs instead of the listed 60 lbs. The Duratrac was much lighter. I've been concerned about the sidewall on the Duratrac, but they gave me a great deal that I couldn't pass up...so I got them. REALLY wanted the Coopers, but it wasn't worth another 13 lbs per tire. Acceleration and braking seem to be mostly unaffected. Mileage dropped by about 1 MPG so far after an 80 mile trip...according to the dashboard readout, and I know that's off a little. I think the ride and handling have improved. Happy so far.
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