How much does milage go down w/ 315's on an otherwise stock 6-speed 3.73 CTD?

Also, do the RPM's drop too much where performance suffers? Do 315's fit on stock rims? Is a suspension lift required?

 

You'll probably need a leveling kit. Your mileage will go down probably 1-2 mpg depending on your type of driving. Theory would say that since your RPM's drop while cruising (probably 300 at most), you'd gain better highway mileage but it hasn't happened yet to me. You'll need to add in ~15% to your odometer and speedometer. Or have the dealer recalibrate the computer for 600 revs/min.

 

I have been running 315s from the day I drove her home. No lift needed, I checked clearance by going over curbs at full steering lock and no rubbing. If the front sags at a later date, a leveling kit may be needed.

They will fit on stock rims, but my tire guy recommends not due to increased tread wear in the center. The narrower rim pushes the center of the tire up, you can't just lower pressure to compensate due to needing the load rating at normal PSIs.

I used Hummer II wheels as they fit with minor machine shop mods (I have a lathe and a mill) and are the right price as well as the correct width/offset. Aftermarket wheels would work great with the proper offset but would cost a bunch more.

As for power: I think it would depend on how muck you tow. I tow kind of light at 7k to 10k, and at 6000ft it holds cruse in 6th at 75-80 MPH no problem.

As for MPG: As I put them on when I got home and only have about 2200 miles on her I can't say but I am getting 15.5 city/hwy and 17.8 hwy. And it's getting better as the miles go on. As good MPGs as many are getting with 04.5s and stock tires.

Speedo is off by 10% as verified with a GPS, dealer wants $35 to change it and I just have not had time to take it in yet I am having to much of a good time driving it.

Go for the 315s they fill the wheelwells very well and look great, drive great, and lower hwy RPMs

 

Speedo is off by 9%. Mileage went down from about 17 to 15.5. No lift needed with stock wheels and 25k on 315's so far. Performance will suffer slightly but top end is up to about 117

 

hialtitude,

Must be a Cheyenne thing. I will keep a lookout for your truck. What brand of flares are you running?

 

Mine also lost 1.5 mpg. I have stock rims and suspension with no problems. The tires don't even stick out of the wheelwells so you really don't need flares. That's what I like.........

 

At 65mph, with 315's installed, 6spd 4x4 & 3.73 rear, I'm turning 1700-1800 rpms and getting 19.5 to 20 mpg. Yes, I had the dealer recalibrate the computer for me for $30.

I mounted them on the stock rims, no lift or leveling kit. They do rub on a full-turn, like when going into a tight parking spot. I rotate the tires every 6000 miles and have not seen any abnormal wear patterns. I keep them at 40-45psi all the way around, except when going off road, then drop them to 30-35psi.

Only complaint is they through rocks and I'm constantly using touch-up paint. And, here in Texas they slip on the Caliche Roads, so once you get going, don't stop..or you'll slide off the top of the road into a ditch.

 

I put mine on at 2300 miles, so dont have a good comparison for milage change. I honestly didnt see much difference either way. Probably a little better on highway and little lower in town. I have a 2" front leveling kit. I cant make them touch on or off road.

They seem to be the perfect size as far as looks go. I have ran BFG All-Terrain T/As on all my trucks for the last 20 years or so and have always had good service and decent ride on and off road. I did upgrade shocks and stabilizer to Rancho when I put the bigger tires on. This is just what you do when you put bigger tires on a 4x4.

As far as acceleration with or without load, the Cummins Torque seems to take care of that up to the 10000lbs I have pulled.

 

I did upgrade shocks and stabilizer to Rancho when I put the bigger tires on.

I didn't know Rancho makes shocks yet for the 04 2500 4x4's?

Can you please post the shock and stabilizer numbers?

Thanks

 

I have Rancho 9000's on my 2004.5 with 4.5 " lift. They came with the kit. They are much longer then the stock ones.

 

Yes, you can get them with a LIFT KIT.

As of a month ago neither Rancho or Bilstein had a shock for the 04 2500 4x4 with Cummins, in stock-height yet or with the 2 inch leveling kit.

That's why I asked for the part numbers so I can call Rancho and see/ verify the correct part numbers.

 

These are the part numbers installed on my 4.5 2500 with 2" leveling kit. They are the listed numbers thru 2003 for 4x4 2500. The dealer called someone, don't know who, and verified they would work on 2004.

Rancho Part Numbers

RS99221 Front Shock
RS99269 Rear Shock
RS5412 Steering Stabilizer

One note: I have had them on for 5500 miles now. Up until now I have been very pleased with the ride and front end feel. I have noticed a little more bounce at high speed over bumps lately. Not bad yet, just starting to act like shocks more than anything else. I have my 9000x set 7 front 5 rear. I will play with settings and see if this helps.

 

I have an 03 Cummins with 3.73 axle and six speed. I ran about 15k miles on the truck before I changed from stock 265's to 315's. I also had my speedo and computer recalibrated at the dealer.

what I saw as an immediate drop in mileage with the change to bigger tires. I was very disappointed. However I have it back to where it was on the 265's.

I had to learn to drive the truck all over again to recover the fuel mileage. I now shift at different rpm than when the truck had the 265's on it.

it took a few weeks to work it out.

so my advice, go for the 315's, be patient and feel the truck, your mileage will be ok. Also I tow sometimes doubles probably about 10k-12k in weight. it tows just fine, but again, the shift points are differnet from what I learned while driving the truck new. now with 10k miles on the 315's that is the only size that will be on my truck from here out.

I hope that makes sense.

ps, no lift kit or leveling kit on mine, you can see pictures in my gallery.

 

Your truck looks good! Did you need to put on fender flares? I saw the Mud Guards - do they help w/ stone chips w/ the larger tires?

 

I know the question is not directed towards me but I did not put any flares on mine. I just have some mudflaps and I have had no problems with stone chips. No more than I had before. The BFGs pick up lots of small rocks and throw them but they must all hit under the truck because I have no chips. The tires don't stick out past the wheel wells so for me I decided that flares would not look very good. I think lots of guys here who drive in lots of dirt or gravel really like their flares.
I noticed that Grizzly has the chrome wheel lips. Grizzly, do those stick out any further than the stock wheel lip?