Berak

Doug,

Many are reporting no mpg loss or an actual increase with the 315 size tires, when corrected for the new size. Also you imply a larger radius is always a wider tire, which is usaully true (as in stock vs. 285 vs. 315) but there are certainly tires that are taller and not wider. That's one reason I don't like the 70 series. I'd like a 315/75R17 or so which is wider than stock but wouldn't be as much as most 35 - 36" tires.

I bought the 4.10 gears knowing I would get beigger tires someday. I'll probably go with the 315s if not a 37" tire eventually.

doug

yea, I'm aware that there are skinnier tires than the 70 series. my comments were specific to these. and roger on your dislike for the 70 series. Actually I agree with you that the 315/70-17 is too wide. They kept the same overall profile as the 315/75-16, loosing the opportunity to create a slightly skinnier tire in a 315/75-17. I would have prefered that too.

Back to my original comments. I was predicting "zero improvement" and possible loss in mpg when moving to a 315. to me the physics just didn't support a significant (worth mentioning) mpg improvement, unless perhaps under specific driving conditions as i mentioned which would maximize the rpm advantage. I should have emphasized one driving condition that will tend to maximize such an rpm advantage, and that is the use of a 4.10. This axle is known to net poor economy running empty at highway speeds esp with the 6 speed, due to high engine rpms. So I can definately see an "improvement" potential with 4.10s when going from 265 to 315. However, to say "improvement" is kind of a misnomer here because, for example, I would expect the 4.10/315 is likely to to yield slightly poorer economy than a 3.73/285. That would be interesting to study but probably not worth the effort...

also I did forget about something in the math, and that is the decreased rpms of the tire will act to cancel the friction loss associated with the wider profile. So that would allow for some marginal shift in my earlier estimate, perhaps up to a 1/2 mpg in theory, which modifies the "0-2 mpg loss" slightly to allow for the possibility of a small improvement.

Again, use of the 4.10 is one condition which favors the larger tires because without the larger tire the engine rpms are consistently placed in the "above 2100-2200" region at highway speeds, esp for the 6 speeds and depending on how fast you drive. so I guess what I'm saying here is that:

1. I'm not surprised that guys running 4.10 axles, highway speeds, and empty are seeing zero to marginal improvement in mpg with 315s. For this condition the theory suggests (to me) something like ".5 mpg improvement to 1 mpg loss" depending on driving conditions. in other words, not a factor because you don't choose 315s for mpg, you choose them for offroad traction and/or looks. also, and this is just an opinion, I'd expect the guys running such trucks would be the ones most likely to drive fast, which means that the high rpms are even more of a factor and the larger tire even more of an advantage ;D

2. most guys running 35" tires are probably not towing, but in case they do, the mpg advantage of the larger tires probably goes away because more engine HP is being used and at higher rpms and lower gears anyway.

Berak

With my 4.10s I'm right at 2150 rpms at 70 mph, which is where I typically drive on the freeway. This is a little higher than I'd like. I think if I could be at 1900 rpms at 70 mph I'd see an improvement overall. The difference in mpg going from 2150 to 1900 rpms is about 2 mpg it seems by my informal testing.

doug

That will be interesting to see what mpg you acheive with 315. You'll lose at least part of that 2mpg just because the tires are bigger. on my 02, I don't see 2 mpg for that rpm difference -- more like 1 mpg.

anyway, you should take some controlled measurements now and then after switching tires and tell us the results. the theory is fun, but what counts is what actually happens ;D. and your truck will get better mileage as time goes on too (break-in), so remember that.

Using published figures for the BFG 70 series tires, I'm showing that:

* with 265 tires you are at 2160 now doing 70. dern close to what your tach reads so thats good validation.

* with 315s you will be at 1980 rpm, pretty close to what a 3.73 axle and 285s will run.

* actual rpms will approach about 50 rpms higher as tires wear.

I'm going to run 285s with my 3.73 axle, so that will put me at 2014 rpm running 70 mph and 1870 at 65 mph. for towing I think thats about right. I agree with you that the CTD starts to suck up fuel when you go above 2000. At least thats what I find on my 02.

schu

I get about 1900rpm @70mph with my 315s and 3.73 gears. Now that I have the big tires I wish I would have gotten the 4.10 gears. When these wear out I'll likely replace them with the 285s. I just couldn't bring myself to spend $70 per tire more for the smaller ones.

doug

I would agree thats too tall for 3.73s. boy, its cool when real measurements coincide -- I'm showing 1906 rpm at 70 mph, using published figures from BFG. so yea, you and I agree that a 285 tire is much better and will give back about 100 rpm. the data shows 2014 when new of course. up to 50 rpm more when worn out.

Berak

Curious, what is wrong with 1900 rpms at 70 mph? That is what I'd like to be at but even with 315s I'll be at 1980 rpms.

Unless you feel your acceleration is too low with those tires?

doug

depends on driving conditions, habbits, and goals, but in my view 1900@70 itself is ok, but doesn't give you much of a margin if your speed drops. At 60, all of a sudden you're down near 1600 and pulling or "holding" power, if you will, drops (HP is only 169). also, the engine gets poor fuel economy if asked to pull hard at those low rpms, from my experiments anyway. If you shift down, 5th gear @ 60mph runs the engine at 2238 which puts you into the low fuel economy region. So that gearing is just not optimal in my opinion.

The 4.10 is much more attractive from that standpoint, yielding about 1800 at 60mph. the engine itself has 10-15% more HP capability at the higher rpm, and the axle puts 10% more torque to the ground via torque multiplication. double bonus.

SierraRam

I have BFG's 35x12.5.17 on my stock rims, added a 2" leveling kit, no rubbing even when doing some 4x4ing. I had the pinion factor set to the lowest (599) and my spedo is approx. 5% off. :