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Discussion Starter · #1 · (Edited)
I recently bought my 2012 2500 LBCC and had 35" Cooper STT's put on at the dealership. My in-dash read-out says I'm only getting around 8 MPG. I knew that bigger tires would affect the fuel mileage but does this sound right?

Also is there any way I can adjust the spedometer? The dealer said that it is calibrated to the best of their ability but it is still off a considerable amount.

Another thing...where do I find out what gearing I have?

Any comments and help would be appreciated!
 

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Gas or deisel? Deisel shouldnt change but gas doesnt like bigger tires.
They shouldn't have put the tires and wheels on w/o calibration anyway. It's against the law. That's knowingly putting a defective vehicle on the road. Take it back. It's done through programing I believe.
You should have metal tags hung on the front and rear end cover bolts. Call an auto parts store with the numbers. They should be able to tell you or look it up on the net.
 

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I dont think dealer can recal beyond 33s is what I have read, someone confirm that? If thats the case by a programmer and reprogram for tires. Also in the mean time hand calculate mpg, also it should increase after its broking in
 

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best place to check for gearing and everything would either be call your dealership and read them the vin and they should be able to pull up your build sheet. if not the dealership try chrysler.com my buddie did that with his 01
 

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I have a difference of opinion than previously stated. Here's why:

The larger the tire, the better the gas mileage, and I don't care what anyone else says. Your dash readout isn't accurate and here's why also:

Take a pencil and glue a dime to the eraser. turn the pencil one time and measure the distance the dime travels.

NOW: Take a pencil and glue a 50 cent piece to the eraser. Again, turn the pencil one time and measure the distance the 50 cent piece traveled.

The pencil is your axle. The dime, smaller tire, the 50 cent piece larger tire. Your truck would have to travel twice the distance with smaller (dime) tires than it would have to with larger (50 cent piece) tires, ad your axle would spin 3 or 4 times to cover the same distance. Care to venture a guess why all the automotive manufacturers stopped making 12/13/14 inch rims and tires? 16 inch tires are about the smallest you'll find. It's because increasing the tire size saves fuel and helps vehicles comply with federal EPA regulations and MPG requirements, because they use less fuel to travel farther=MPG increase.

Your speedometer is the giveaway. It might say 35 mph while you're doing 70. That's proof enough that you're getting better mileage because your truck thinks a mile is much smaller than you're actually traveling. Now if you want to believe what you can demonstrate yourself on the kitchen table, go right ahead. I encourage it. However, this is just a proven fact and my opinion and you can listen to reason and proof, or just opinions. What people "think" and what they know are two vastly different things.

You can have someone travel with you and at 10mph intervals let you know, Then you can just get a dial cover and correct it for yourself. Sounds easy enough to me.
 

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Warlock, I agree to a point. The problem isn't with the tire size, but rather the HUGE lugs guys put on them. Causes wind resistance and friction.

That's why our trucks now come with car tires from the factory...it's how they get their MPG claims!
 

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When is the last time you saw a 15 inch tire on a Ram from the factory? Granted the larger lug design may have a negative effect but not enough to cancel out the gains from a larger tire. What size are the factory rims now? 20? 22? that's a huge increase in 10 years from 93-03, a 7 inch larger rim.

But I stand by what I originally posted, with the exception that larger treads will have a negative effect, but not overall canceling out the increased mileage of the larger tires.
 

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Well, airflow tests have proven that the rake that everyone (myself included) cannot stand actually helps reduce wind reduction. And the generic tread factory tires are another benefactor in increasing MPG claims. Lifting a truck in the front increases wind drag, and larger diameter tires take more energy to get moving, weigh more..thus increasing rolling resistance, and the deeper tread design all take part in lowering ones MPG as a sum. It may not be much, but it is a factor...

I would say possibly 1-2, maybe 3 MPG loss...buts that's all just SWAG (scientific, wild ass guess):thk:
 

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There's definitely a lot of good points on here already. Warlock, you're definitely not WRONG, but I do think that you are discrediting these external factors way too much. The wind resistance, deep lugs (friction), etc. are what reduces mpg in the first place, why wouldn't they be expressed even more largely as you grow in size.

One of the main reasons that we don't have the 15 inch tires is because people don't want them, they aren't asthetically (however the hell you spell that dam word) pleasing, so we've gone to larger wheels, which is what everybody did aftermarket anyway. You can re-gear it to get pretty much the exact same results. Honestly, I believe they get better results because it doesn't require as much loss of power through gearing to turn them to get the vehicle moving, ever notice that in dirt track racing they aren't using tractor tires?
 

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Tire size

Yes, a larger tire will roll further than a smaller one with one revolution, but to say that has to equal better MPG is a very long stretch. I agree that most of our trucks come geared lower than optimal for MPG, and a larger tire (circumference, but same drag coefficient) would net better MPG in a empty or lightly loaded truck. The 1/2 tons tend to have taller gearing and better MPG than the 3/4 tons, because they have to post the MPGs. The more they are designed to pull, typically the lower the gearing. The lower gearing is actually better at MPG once you put a big enough load on them. Even with the on-board computer in my 2500, you can see the MPG drop drastically every time you hit a hill (load) and have to put a bit more throttle in it. The taller the gearing, the less mechanical advantage you have on that load. I don't think the gearing is the biggest factor for bad MPG when you slap on those 35" M/T tires, it's a lot of factors, but mainly rolling resistance from a much larger contact patch with the ground. It's like going from a Lance Armstrong road bike with razor thin tires that are as hard as a rock, to a Mountain bike tire thats almost flat. Push the two around and you can feel how much that MTB tire takes energy to conform to the ground and roll. Other smaller issues the high flotation tires have are tire weight, flex, wind resistance, lug depth, usually a softer compound etc etc). Here's a article that explains some of the issues, and keep in mind, this article is comparing a road tire to a A/T tire, the rolling resistance is a lot worse when your talking a high flotation tire.

http://www.fourwheeler.com/techarticles/wheels/129_1104_tires_and_fuel_economy/viewall.html
 

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Well, airflow tests have proven that the rake that everyone (myself included) cannot stand actually helps reduce wind reduction. And the generic tread factory tires are another benefactor in increasing MPG claims. Lifting a truck in the front increases wind drag, and larger diameter tires take more energy to get moving, weigh more..thus increasing rolling resistance, and the deeper tread design all take part in lowering ones MPG as a sum. It may not be much, but it is a factor...

I would say possibly 1-2, maybe 3 MPG loss...buts that's all just SWAG (scientific, wild ass guess):thk:
Yep, raised the front with Bilstiens and lost 1 MPG straight out the window.
 

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its not getting 8 mpg, the dtc is going to be off due to the fact that the speedometer/odometer is off. going from a 33" to a 35" tire makes a considerable differnce.
the speedometer differnece between a 285 70 17 and a 35 12.50 17 is 6.318% at 100 km/h,
if you travel 106.318 km the dtc thinks you only went 100 km. get a tuner and change the tire size you will see the mpg increase without even doing a tune, or at least i did on mine.
 

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for gearing, on my 2011 there is a sticker on the bottom of the rear axle towards the driver's side tire. the sticker is on the underside of the axle and lists the gearing in bold. not sure if the 2012s are the same, but I would think so
 

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How do you know its off alot? I check mine against my GPS which has MPH and seems pretty accurate. They probably cannot recal past what is available on the trucks stock. (uneducated guess)
 

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35's are actually only an inch bigger than 33's. One inch goes on top, one inch on bottom...which is another popular misconception with tires. Technically they are 2 inches bigger except in the respect of clearance.
 

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On my 98 cummins with 3.55 gears once I put the 35x12.50r20 toyo mud terrains I saw a decent gain in mpgs of the 295/75/16 bfg a/t's that I had on it before. The reason for the increase is that the 24v cummins made peak torque at around 1800 rpms, my cruising rpm after bumping up the tire size was 1800 rpm's so my motor was operating at it's optimum rpm while under a steady load.

That truck got 20mpg with 35" mudders and a 6" lift. I can't imagine that the newer motors can't do better especially after a dpf/egr delete and a tuner.
 

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My buddy put 35 TA/KOs on his. They look awesome. If he didnt live 200 miles away I would go get a picture ...lol
 

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35's are actually only an inch bigger than 33's. One inch goes on top, one inch on bottom...which is another popular misconception with tires. Technically they are 2 inches bigger except in the respect of clearance.
35's are 2 inches bigger that 33's. one inch on top and one inch on bottom = 2 inches. you will however only get 1 inch lift going from a 33 to a 35
 
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