Shepherd57

Will 315s make my rear end ratio to high I have 3.73 with a 6 speed trans or should I get 285s. Is there anyone making 295s seems that there is not a lot of choices for 17 inch tires

RamWheelsBy4

I am running 315s with auto and 48re. The 315s make the effective ratio about 3.43. I like the combo. With the 6 speed it should be even better, much more control of shifting. I dont tow anything heavy or I would probably not go with 315s. Just my .02.

4x4not

I have 4.10s now and I'm thinking 315s will be almost perfect with my 6spd. RPMs will be a bit high on the highway, but still much better than with the 245s I have now!

Nuttymopar

Nittro makes a 295 x 70 x 17 that look pretty nice.

I have the 4.10's and looking to get 315's real soon and hopefully it will bring down my rpms a little.

RamWheelsBy4

With 48re, 3.73 gear and 315s my highway rpms are:

1800rpm=70mph
1900rpm=74mph
2000rpm=79mph

All are nice cruising speeds.

6 2 Carl

I have had 315's on my 3.73 six speed since day one. It is a perfect combination. I don't know why anyone would want it geared any lower. I have pulled up to 10,000# and even going into a strong headwind, the truck had no problem pulling in sixth gear. If I had to pull heavier loads I would just leave it in fifth and have all the power I need. I could still cruise comfortably at 65 mph turning 2400 rpm.

Carl

CETANE

Alright...

Maybe I am just really confuzzzd about this ...

but I thought if you increase your tire size you will run higer RPM's on the highway and would get poor fuel mileage?

RamWheelsBy4 if you are there ...I would like to hear how your MPG is doing

Nuttymopar

Larger tire will lower your RPMs down which could result in better mpg depending upon your rear gear ratio. My truck has 4.10 so going from a 265 (like 30" tall) to a 315 (about 34" tall) will bring my gear ratio down to maybe a 3.73 or there abouts, not really sure. That is one reason that if you do tow, you may not want a larger tire.

CETANE

Nuttymopar:

Thanks for your assistance.

So just to make sure I have this down right. I have 3.73 six speed and am running stock 265/70/R17's right now. I would love to go up to a larger tire...i assume I will need to get my rig re-calibrated after changing to a larger tire....

Do you have any recommendations on tires and a larger size that will help out the MPG? I drive about 1000 miles/week...

I appreciate your feedback!

Nuttymopar

You will need to re-calibrate your speedo if you want it to be exact. I know I will be going to larger tires as I like the look and I do not haul or tow anything other then my butt. I have been writing down numbers of what speed at what rpm and what rpm at what speed. Kind of like double duty but doing it to compare after I get the larger tires to see what the difference is. I use to have a GPS on my old truck that would give me a constant MPH while driving. Probable more accurate then the truck's speedo since it has nothing to do with gears moving around, just signal bouncing off the sats in the sky. So it never bothered me going to larger tires as I went by that instead of speedo.

With a 6 spd, not really sure. But I would assume you would want the motor turning around 1700 - 2100 for the best mpg. I know when I cruise around 50 at 1800 rpm or so, I get the best mileage. Once I start going over 2100, mpg starts going down.

I think there must be charts on the net to show what size tire (larger or smaller) and what it might do to the RPM of the motor.

hope this helps.

adamkn

Just thought Id make a couple comments regarding the gear ratio and mileage issues...

As far as gear ratio, to figure out what your new effective gear ratio would be after increasing tire size, do the following simple calc:

(T1/T2) * G = New gear ratio
T1 = old tire diameter in inches
T2 = new (bigger) tire diameter in inches
G = gear ratio

For example, to go from 30" stock tires to 35" BFG's, with a 3.73 gear ratio, new ratio would be 3.2. So yes, RPMs will be decreased considerably at any given speed.

However, I doubt that any of you will get better gas mileage by going to a bigger tire size. If you're lucky, your mileage may stay the same. Problem is, with bigger tires, you have 1) more friction to overcome (larger pavement/tire contact area), 2) the bigger, heavier tire requires more power to get it spinning (moment of inertia), and 3) the biggest downfall is that it changes your effective gear ratio so that you'll burn additional fuel to get the truck moving from a stop ( or even to accelerate), relative to how much fuel it took with the smaller tires. So the 3 issues I just described will pretty much cancel out any benefit you get from the lower RPMs, and then some.

If you do a lot of flat highway driving, you might be in luck, since your avg RPM will decrease. But if you do a lot of stop and go, and/or hills, dont get your hopes up!

RamWheelsBy4

CETANE

I agree with adamkn. My highway mileage seemed to stay about the same on long flat cruising trips, my city mileage dropped 1-1.5mpg with the 315s.

Rpms down, more tire resistance on the road, tires weigh more and the truck is higher profile. Any gain because of lower rpm is lost with these differences. If there is a head/cross wind they are going to cost you mpg.

BigBlueDodge

IMO, 315's with 4.10 is a good combination. This will get you close to the same ratio has having 3.73 and the stock 265 tires. 3.73 with 315's will cost you some grunt on city driving, but will give a nice highway gear ratio. Beware, that newer trucks with the G56 tranny will be lower geared, so 315's with a 3.73 rear end and the G56 may be a good combination as well (similar to 4.10's and 315 with NV5600)

CETANE

How do I know if I have the G56 Tranny six speed?

I would like to increase my tire size on the stock rims...but I travel about 1000 miles/week on the Interstate (unloaded) and am afraid that if I increase my tire size I will loose any MPG.

My michelin's 265/70/17 should get 55,000 - 60,000 miles of wear which isnt that shabby....

Any recommendations in tire sizes and brands that will wear good and are larger that will work with factory rims????

Ramtough

Yeah id agree with that if you had stock ratios. However us guys with 4.10s will really see a little improvment. The problem (if you want to call it that) with these trucks is they make so much torque and dont spin so high. On a lowered power vehicle, gear changes are much more needed. but the principal still works the same. the leverage on the tires with a lower gear (higher numerically) will be enough to offset the drag and rolling resistance of the bigger tire.. now am i saying im going to get as good mileage as a stock 3.73 truck? no probabbly not. will i get better mileage than a truck with 3.73 and 315s? yes.