Diameter discrepancy with Cooper Discoverer AT3 XLT?

Hey all,

Just got a set of Cooper discoverer AT3 XLT 295/70r18’s installed. Cooper lists the tire as having ~34.2” diameter. Parked in the driveway, my tires measure right around 33” with my tape measure. Does that sound right? Do tires “flatten” that much to make them sit ~1.2” smaller than actual size or is Cooper full of shit? That seems like a huge discrepancy to me.

 

Most tires are like that. Very few tire companies make true to size tires, and even then that's at full pressure with NO weight on them.

 

What psi are your running?

 

Not sure of pressure, I can check in the morning when I go out. Just got them on today at discount tire.

I’m Generally happy with the tires so far, I just thought they’d be a lot closer to true size than that. Now I’m thinking I could have gotten away with the 285/75r18 or just a straight up 35”. Truck has a 3/1 lift and 1.25” spidertraxx spacers with stock sr5 steel wheels.

 

Also curious here. How do you measure, clearly you said with a tape measure, but did you eye ball it or stick a level on top of the tire and measure to it?

 

Im running these tires in a 37.....I think they say they measure 36.5 or 36 and some change.... will have to report back the difference I see

 

Take them back and replace them with 285/75R18s.

 

Tire pressure is at 33 psi all the way around.

I measured by putting the metal tip of my tape against the concrete at the “bottom” of the tire. Straight edge across the “top” of the tire that intersects the tape right at 33”. Tape is centered on the tire and wheel hub measuring it at its widest diameter.

You think discount tire would exchange these even after installing and driving a few miles?

 

I would think you may need a little more air, but maybe not. I think when the tire company measures them, they have them mounted on the rims and inflated, but they don't account for the weight of the truck.

 

More PSI for one thing. I run 50 in my BFGs. My door sticker states 46. Did you measure the original tires and check clearances.

 

You can't measure from the ground, because the tire is squished at the bottom because of the weight of the truck. Especially if you only have 33psi. Measure horizontal, 9 o'clock to 3 o'clock for more accurate measurement.

 

My door sticker says 30 psi front and 33 psi rear. Stock size was 255/70r18. Door stickers don’t mean much for these new tires though, do they?

I might air them up a little and see what they do. They certainly don’t look flat or anything, ride is good, steering is responsive.

ETA: good point about measuring vertically. Just remeasured horizontally and they’re more like 33.5” like that.

 

Door sticker only applies to the stock tires with the exception of BFGs as OEMs. I am surprised that Discount did not put more air in them. Not familiar with the AT3s but I assume they are 10 ply and therefore should have more air.

 

Normal in my experience. I've had luck with recognizing which brands tend to run large and which run small.

I've had Nittos, Coopers, BFGs, Falkens.

BFG KO2s are small enough that I can visibly see theyre smaller than other manufacturers.

Coopers for me seemed to run larger than BFGs and Goodyears

Nittos and Toyos seem to be true to size and run large. Falkens also seemed on the larger side.

Another thing to consider is tread depth. Tires with deeper depth also seem to be the ones that run larger in overall size. The difference seems like it would be marginal but thats not always the case.

 

I know I'm coming in late on this one....but measure horizontally, not vertically. Better to measure off the truck in general, but this will work. Also, tire pressure on door is stock tires.
MFG's usually will have the sizes on their website or some tire websites do this as well...but they will show you their true size. Aside from metric, almost all the other sizes will be off by 1/4 - 1/2". When I bought my BFG KO2's recently, I looked on their website to see what they offered in terms of sizes, but also the true size. Like how a 35/12.5 usually came out to around 34.5/12.25. My 325/60R20 claimed 35.5/12.8 via website, and when they arrived I measured them to discover they're exactly as advertised on their site. These were measured off the truck

 

Tire manufacturers have a standard for measuring a tire. I don't recall how they refer to it but this is how it's done... the tire is mounted on a prescribed rim and left to sit for 24 hours. After 24 hours the tire is inflated to the max cold pressure and then measured. This measurement is taken off of the vehicle, NO weight on the tire. So, the measurements that the manufacturers list won't be the actual measurement once the air pressure is decreased and the weight of the vehicle is on it.
As a side note, after chalk testing 41 pounds seems to be the sweet spot on my 275/70/18 XLT's.

 

Awesome, thanks for all the replies. I’ll probably play with 40-45 psi and see what I like.

maybe next time I pull a wheel off I’ll try to measure the tire without weight on it.

 

Yes. Cooper has a clause where if you are
not happy with your tires you can return them. So just go exchange them out. If that local shop gives you issues then call Cooper and let them handle it.

 

Just had my Cooper AT3 XLT 285/75x17 installed last week or so, and they measure 33.25" with a level at the top of the tire. Pressure was 33 PSI, which seems a little low, but seems to ride just fine. Can't find the tire recommended pressure on the actual tire itself. Cooper spec for this tire is 33.8" so I'm pretty close to spec.(0.5" or so). Very happy with them.

 

Tires only have max PSI on them

 

Tires no longer form a circle when they are supporting a load. Diameter is the distance across a circle, not a 3/4 circle to flat.

 

I aired them up to about 42 psi all around. I might pull one of the wheels tomorrow and see if I can more accurately measure true diameter.

I had to trim the inner fender/bumper liners a bit today, too. Had rubbing issues with left turn in reverse. Only seemed to be rubbing on the passenger side but I trimmed both sides for good measure. Liking the tires so far. A little more noise than the stock tires but totally expected that. Ride is still completely acceptable for a pickup in my opinion, especially for an E rated tire.