Tire Talk - 17s or 18s

OK Bros,

Impart some wisdom on me.

Looking at new fleet vehicles.  

Dodge Ram 2500, gas, 4x4, CCLB, 3.73 gears, no rear locker (this part is locked in)

Two wheel and tire options from Dodge.  17x7.5s with 245/70R17 and 18x8s with 275/70R18

Which tire/rim size is the wisest choice (safety, comfort, longevity, cost) considering:

• These trucks get 40-60k miles a year on them, 90% of it slabbing down the freeways
• They are always loaded at or close to GVWR
• No trailer towing
• No off-road use - worst case would be gravel or snow

For your use the 17s all the way.  Narrower is going to be more fuel efficient in all the highway miles, easier to get in and out from being shorter, and cheaper to replace in what is a very common and available size.  The only reason for the 18s would be off road clearance/traction and looks.  

I agree.  More tire sidewall will help ride.  It will also keep more of the moment of mass near the inside of the tire, meaning a wee bump in acceleration and possibly mpg.

Edit to add... tires get their weight capacity in part by the volume of air inside the tire.  I think you'll find that for the same diameter tire, the 18" will likely have a little less capacity than the 17".  The other part of weight capacity is sidewall integrity, ie, pressure capacity.

If those sizes are correct, the 18" package is 2.5 inches taller overall.  That'll give taller effective gearing and more drag, so likely worse highway MPG and potentially worse performance when loaded. 

Considering how large the overall tire size difference is, I'd make sure there aren't any GVWR or payload rating differences between the wheel/tire packages.  It's possible that one is rated for more weight than the other. 

I think the aspect ratio is off on one of those tire sizes.   Also I agree for the use listed 17's are the way to go.

Go to the Ram configurator. Those are the right two sizes, with a 2.66" difference in diameter.

I agree that narrower will help mpg, and smaller will be cheaper. However...

Larger circumference and wider should wear slower when loaded up. The same well-reviewed replacements for each look to be ~$235 for the smaller and ~$275 for the larger, a 17% increase. What are the chances of seeing a 17% loaded life increase to match? No idea.

If the suspension is effectively 'lowered' 1.33" to maintain the same ride height between the two options, as the configurator implies might be the case, the larger tire reducing the fender gap might not be a bad thing for aero. But if the larger tire sits the truck 1.33" taller, that's definitely a negative from this standpoint. This would be easiest verified from trucks on the lot, but I'm not sure how measurable of a difference it would actually make either way for your use.

Sitting taller might make loading and unloading either harder or easier though, depending on where and how it's being performed.

Both options are E-Rated, and while it doesn't look to change the GVWR or payload, the load indexes should be 119/116 for the smaller and 125/122 for the larger. The higher load index for the larger tires would generally be better when heavily loaded.

Gearing wise: 3.73 with the smaller is similar to 4.1 with the larger, while 3.73 with the larger is like 3.45 with the smaller. Which is better or worse than which other when all loaded up is open to debate, and may likely be situationally dependent.

I'd definitely avoid the all-terrains and stick with the highway all-seasons. I don't see this as a clear cut answer, but I'd probably lean towards the larger tires for continuous heavily laden commercial use and might not count out getting the 4.1 axle with them.

Wow Driven5 thank you for the well-researched answer.  We are 100% going with the highway tread tires as thats how these vehicles get used.  

I read through the specs on the configurator and was somewhat surprised the aspect ratio stayed the same between sizes. The delta between 235 or 275 for tire replacements isn't the end of the world, we spend more than that on lunch each day.

The next question is what it does to the GVWR, I didnt see any info on that online if there is a change or not.  

The biggest things are safety, comfort, longevity, in that order.  My gut tells me the 18s would be better for that criteria.  

In reply to 93gsxturbo :

Why no locker to go with the 4x4? Dealer stock or something?

Loaded up for that many miles, I wonder if you might not also still come out ahead going diesel.

93gsxturbo said:

Wow Driven5 thank you for the well-researched answer.  We are 100% going with the highway tread tires as thats how these vehicles get used.  

I read through the specs on the configurator and was somewhat surprised the aspect ratio stayed the same between sizes. The delta between 235 or 275 for tire replacements isn't the end of the world, we spend more than that on lunch each day.

The next question is what it does to the GVWR, I didnt see any info on that online if there is a change or not.  

The biggest things are safety, comfort, longevity, in that order.  My gut tells me the 18s would be better for that criteria.  

 

The GVWR won't change between the packages.  Gross ratings are cut off at DOT ranges for truck classes.  You'll likely notice that 3/4 ton trucks from chevy, dodge, and ford are all 8800 lbs gross or really close to it.  It's more of a registration/licensing/class thing than an actual "this truck can safely haul X lbs."

Tire and wheel packages on trucks are often notably higher than the GVWR as a buffer.  In your case, going with a higher-load-rated tire will likely pay off in longevity.

You'll also be squarely in E-range tire territory, which is good.  E range tires have stiff sidewalls... not because the sidewall carries weight, but it makes them more able to retain higher pressures.  You'll find that E-range tires can often maintain their contact patch over a wider range of pressures, so you can just set a pressure for the max load and leave it there without wearing the center tread faster when empty like you might on a passenger tire.  My F250 never had anything but 80 psi in the E-range tires and they were just fine.

I think you'll find that H/T tires are made for the long haul.  I had a set of Firestone R4S on my F250 in 215/85-16 and they aged out before they wore out.  Like nearly 100k from a set of tires.  Super quiet, great in the rain, awful in the snow/ice.

Driven5 said:

In reply to 93gsxturbo :

Why no locker to go with the 4x4? Dealer stock or something?

Loaded up for that many miles, I wonder if you might not also still come out ahead going diesel.

These trucks will all be ordered, we are standardizing our fleet for better pricing vs just taking what the "closest to" thing is on the lot.

Discussed the locker option, we don't feel its needed for what these trucks do.  They are never run off road, worst case is a gravel lot, and they are 4x4.  They do get run in snow but are always loaded down so traction isnt a huge issue.

The diesel option was on the table, few things keep us from wanting to go that direction:

• Its a $10k swing for diesel right out of the box
• The fuel is generally more expensive in recent history (currently in the midwest its ~$1/gal difference but it does move around)
• Maintenance costs and powertrain repairs are more expensive, and we are out of warranty in 1-2 years based on miles.  
• The rest of the truck is worn out and becomes a liability at 200k miles.  Even if the engine and trans goes forever, they still have the same everything else.  We get rid of trucks every 3-4 years.  
• We do zero towing (none, nada, nilch)
• We are not crossing the rockies or running grades all the time, these trucks are primarily used in the midwest and the south. 
• We don't need a standardized fuel option where we are filling semi trucks, medium duties, and service trucks at the same tanks.

In short, the maths dont math on a diesel for this use case. 

I disagree a bit on the non-diesel.  The fuel is more expensive, but the MPGs more than offset it.  Sure, it's a $10k swing now, but it's also pretty much a $10k swing when you liquidate.

Buy a gas truck and run it for 300k and you sell it for $2500.  Buy a diesel truck and run it for 300k and you can still get over 10k for it.

BUT... the caveat is that diesels since the early teens have been so crushed with emissions crap that they're not quite as reliable as they used to be.

Well thats exactly it, Curtis.  The ROI on diesel just doesn't math.  

These are company owned so we cant to any of the lightweight mods to them.  An out-of-warranty DPF, high pressure pump, any of the DEF stuff, has to be fixed out of pocket.  

Fuelly shows 10-12 MPG on gas or 14-16 MPG on diesel for a 2023 RAM 2500.

Most favorable case (10 MPG gas, 16 MPG diesel) is 5.3 cents/mile difference between the two powertrain options using current midwestern regular gas and diesel.  Just to cover the delta in purchase price is 187xxx miles.  Thats not counting known kickers to the diesel like DEF purchases, high capacity oil changes, decreased payload or accelerated wear on front end components due to the heavier engine.  

We could investigate the resale deltas, keep in mind these are completely worn out tradesman trim trucks by the time they go away.  

The value to us is having our technicians on-site, on-time.  The trucks are consumables in the process.  If the simpler gas engine meets the goals, its the preferred way to go.