Search around some of the older threads, many of the early buyers have changed tires and/or lifted their trucks. There is a really good thread breaking down tires sizes. Going to a slightly larger tire has not hurt these trucks too much - most are loosing about 1-2mpg with larger tires.

That being said, most owners are still pulling down 25+mpg and better on the highway - with 3.92 gears and slightly taller (non-stock) size tires. Some guys with mild lifts are getting that kind of mileage too. One of the members has 35's and is still getting 24+mpg.

 

I have 34.1" tires that are 10 ply with 3.92 gears. Most of my early mileage was towing getting 17 mpg. Not towing I avg 21 mpg with 21.6 being my best so far.

I think it is the best of both worlds having a truck with real tires, not the cheesy passenger tires the truck came with and I still get better mileage than my 2013 hemi did with passenger tires.

 

I went with 285 70R 17, getting 25 MPG when commuting through the Finger Lakes hill country for a 2 hr round trip. 23.5 religiously on my regular 1/2 hour commutes. Still holding off on the 2" leveling kit, maybe next month...

 

I searched tire rack and could not find 285-60-17 tires but did find 285-65-17 tires which should be a bit taller. They have an overall diameter of 31.6". The stock 17" tires, P265x70x17, are 30.7" overall diameter. The stock 20 tires have an overall diameter of 33" So with something like you are contemplating you will still be lower geared overall than the trucks with the 20" wheels. Ram offers both rearend ratios with both wheel sizes so given the the different tire diameters you can get 4 effective overall ratios. You have no need to go to the 3.92 due to the tire change you are proposing since it is still lower geared than the 3.55 with the 20" wheels and tires. I am very happy with the overall gear ratio of my truck with the 20" wheels and tires and the 3.55.

 

Correct me if I'm wrong but isn't the circumference of the tires very close between the 20 inch wheels and 17s? Just less sidewall on the 20's? Or are they that much different?

 

Good question... I would have thought the same. Here's what I found. That would explain why one reviewer mentioned tire rub at full wheel turn.

 

Therefore... ratio = (3.55 X 33/31.6) = 3.7:1 Pretty close to half way, Howie.

 

correct from my perspective. I guess that we are both on this site on a Saturday night says something too! Although I am enjoying my Vodka and tart cherry juice!

All the best,

 

Well Howie, you are 3 hours ahead of us, so I haven't had my Margarita for the evening yet... Cheers!

 

Thanks for the clarifications....I assume each different truck with different rear ends and different tire sizes are calibrated for accuracy on the speedometer? And if i were to switch between 17s with winter studded tires, and 20s with summer ones, i would in effect be throwing my MPH off as well as increasing my fuel mileage a tiny bit, all other things remaining equal?

 

yes the odometers and speedometers are calibrated as supplied by the factory. Since they are electronic they can be easily recalibrated by the dealer and maybe some other clever people. If you do not recalibrate them you will induce an error in the speedometer and the odometer based upon the change in tire circumference from original. It will indicate a change in mpg but most of that change will not be real since you will either be covering less or more miles per indicated mile. There may be a very small actual change due to overall ratio change but I believe most will not be a real change only an indicated change.

 

yes the odometers and speedometers are calibrated as supplied by the factory. Since they are electronic they can be easily recalibrated by the dealer and maybe some other clever people.

Well perhaps. But the dealer has some factory hard wired limitations. They can change to meet the following tires and wheel sizes only:

235/80/17
245/70/17
265/70/17
265/70/17

275/60/20

285/45/22

These sizes are off the factory software version as of 5/24/14. Perhaps a newer or a forthcoming version will add more options for the dealer to select.

 

Ok good information here.

I am perfectly capable of calculating the difference for myself even if the computer in the truck and speedo is not correct.

However if I did get a truck I could go with the 285 70 17 and they would be close to the factory offering on the 20" rim so they could utilize that tire size when calibrating. I went through all this back when I had my 06 2500 and they used the tire size from the power wagon

My biggest concern is can I avoid a serious hit to the fuel economy and still have it look like a truck. I know its tough to have it all but we can dream right?

now I would be curious to see like trucks with same gear ratios difference in mpg between stock 17s and 20s.

Or difference in mpg between gear ratios

 

Real world numbers- 3.92 gears, 275/60/20 (stock size) Falken Rocky MTN ATS tires, no lift/no level, and I consistently get 23-24 mpg highway in my hilly area at 65-70 mph. On long trips, I have averaged over 25 mpg on some stretches while doing 75 mph.

 

Same truck as azdztfox, except in Wyoming, still running factory 275/60/20, and getting 25-26 mpg on tank average for 128 mile daily commute driving 65+ depending on the snooze alarm abuse.

 

Ok so heres the test question

Can you get the 20" wheel option which inherently comes with a signifigantly larger tire diameter with the 355 gear ratio in a 4x4?

And if yes there is the answer I need to my question as to if I would need the lower gears or higher gears to run the tire size I desire and maintain fuel efficiency

Ram still hasn't updated their site to allow you to build a 2015 model

 

Ok apparently 3.55 is an option in 4x4 with the 20s just looked at one online here

 

I thought 3.55 gears and 20" wheels are standard with the EcoDiesel.

 

Tarpon, you are correct. There is a model (tradesman?) that 3.92 are standard.


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