Full size wheelbase

[rruff]

Nice! Do you think 35s are necessary or would 33s or 34s be sufficient. Don't want to kill fuel economy too much

Tire size isn't what kills fuel economy... it's mostly the tough sidewalls. Bigger tires (wider and taller) actually have less rolling resistance all else being equal. Weight is a minor factor.

Get the lowest gearing you can so you don't have too many extra tall gears.

 

[mountainsports2002]

It depends a bit on overall weight, but I really like the 2000ish GM pickup rear foam bumpstops. GM 15712438

You can get them as generic replacements now too. The new OEM ones seem to be the best quality. They need to strike a smooth metal surface. Once they skin gets broken on them, they should be replaced. They will compress to 1" tall if you hit them hard enough. Pretty easy to mount also. View attachment 719824

2015 power wagon, with habitat and decked drawer system. In SLC, Ut.

 

[Todd780]

Nice! How do you find it off road?

Don't know yet. Only picked it up a couple weeks ago.

Last truck was a super crew 5.5 box. For what I do though, it'll be fine.

I guess it would depend on where you plan to take your truck. Seems to be a few people here with 3/4 ton and larger trucks and it seems to work for them.

 

[dstefan]

Maybe we need some more elaboration on tire size and weight. . .

I was a former bicycle racer where we lived and died on rolling resistance and weight, where you could feel minor differences immediately and much easier than in a vehicle. The best rolling resistance was always the skinniest highest pressure tire regardless of size. Larger wheels had a flat cruising and top speed advantage, but disadvantages (more effort) accelerating and climbing. Wheel/tire diameter also meaningfully affected the final gearing.

I agree that a larger tire diameter can have better rolling resistance on flat pavement when up to speed, but a wider one will have greater RR (as well as an aerodynamic disadvantage at higher speeds) for the same diameter and conditions. Off road the wider tire might have advantages if its wide enough for floatation in soft conditions, but as far as I can tell, it pretty much takes arctic truck size tires on really soft surfaces. I believe that an appropriately aired down larger diameter skinnier tire has been shown to have a larger contact patch than a wider tire, smaller tire.

I also agree that a heavier larger diameter tire is more not less mpg efficient than lighter weight tire of the same width at higher speeds, unless climbing or accelerating, or pushing the wind, but we’re rarely in those perfect conditions.

But in my experience heavier and larger diameter tires are definitely less fuel efficient in startup, acceleration, and climbing on pavement. Off-road is heavily startup, accelerating and climbing biased.

Practically speaking my mileage under the same conditions dropped ~2 mpgs going from stock size Michelin LTXs to ~2” larger Cooper AT3 XLTs which were 17lbs heavier per tire. All that rotational mass really hurt, though on flat highway conditions, not as bad.

There’s so many variables involved its always an exercise in tradeoffs. But, IMO bigger and heavier tires equals poorer mileage in general. I travel significant highway miles, with fairly tough wheeling when I get there, which for me means I’d rather run 34’s (actually 3.8s) with a 2.5” lift and skid plates than go to 35s for the slight, if any, off-road advantage they might offer. So far it works for me. YMMV … and it almost certainly will!

 

[rruff]

I was a former bicycle racer where we lived and died on rolling resistance and weight, where you could feel minor differences immediately and much easier than in a vehicle. The best rolling resistance was always the skinniest highest pressure tire regardless of size. ...
...I agree that a larger tire diameter can have better rolling resistance on flat pavement when up to speed, but a wider one will have greater RR (as well as an aerodynamic disadvantage at higher speeds) for the same diameter and conditions.

I've been a road bike racer since the 80s. It's only since the mid 2000s that I've seen anyone actually testing Crr of the tires. Jens Heicke was the first as I recall, using rollers to test them. I helped him interpret the data (rec.bicycles.tech and biketechreview). I've been testing them myself ever since I got a powermeter and rollers in 2009.

Larger (width and diameter) tires have lower rolling resistance, all else being equal. If you test tires of exactly the same model and design, but different sizes, this is 100% always the case. Even on a perfectly smooth surface. This is true because the larger tires have a shorter contact patch and less casing distortion. The compounds and construction of the tire are by far the most important factors, however. Some light tires that were thought to be fast proved to be absolutely horrible. Continental showed that you could make fast tires that were actually durable, but the lowest Crr tires currently are Veloflex Records in 23mm, Vittoria Pista Speed (or Oro) in 23mm; both are very fragile, though. With bicycles, aero drag is the greatest resistance to overcome in most conditions, but wheel manufacturers are now making rims optimized for wider tires, and 25-28mm tires dominate among the pros in road races and TTs, even on smooth roads... despite the slight aero and weight penalties. BTW, super high pressures were never optimal either (unless on a track), since the vibration losses were greater than any gains from lower tire friction. They just felt fast.

Trucks are less affected by the aero drag of the tires vs bikes, but the lift you get with larger tires, and sticking out past the body with width, aren't going to help! Larger and wider still tends to lower rolling resistance... also lower profile! There is a problem though in that no one is testing truck tires for Crr, and things like tough sidewalls and thick tread will be much bigger factors in the other direction. The weight is a minor issue, unless you tend to slam on the accelerator and brakes a lot. Gearing changes resulting can help or hurt depending on your ECU, mostly.

Practically speaking my mileage under the same conditions dropped ~2 mpgs going from stock size Michelin LTXs to ~2” larger Cooper AT3 XLTs which were 17lbs heavier per tire. All that rotational mass really hurt, though on flat highway conditions, not as bad.

My mpg actually improved overall (better in town, same on the freeway) from stock 32x10" street tires, to 35x13" E rated ATs, which weighed over twice as much! I think I lucked out with exceptionally good tires (Hankook ATMs) and the Bridgestones were probably exceptionally bad, but still...

Michelins tend to be good BTW. If you'd gotten those in a larger size there would have been less of a difference and maybe an improvement if they were the same load rating.

 

[dstefan]

@rruff - the bike tire findings you mention are interesting, and I have no doubt are true — my racing days ended about 12 years ago after a bad crash, so I’m a bit out of date.

I was attempting to make a practical generalization for the OP, maybe not the best analogy for overland trucks, but — kinda dont want to leave him with conflicting notions regarding his original wheelbase/tire size/mpg questions.

There are certainly so many complicating factors that its hard to pinpoint one thing, but it sure seems that tire size and weight changes affect both performance and mileage. Sure, so does tire design and compounds, PSI, driving style, aerodynamics, gear ratios and truck model, among other things too.

I don’t for a minute doubt your personal experience with tire size, but would you agree that the majority of people you know or whom you have interacted with on forums who go to larger, heavier, more aggressive off road tires without expensive regearing, tend to find they hurt their mileage in most cases? That’s overwhelmingly been my experience, as well as personally over 5 different vehicles.

For the OP — I think there’s a sweet spot if you want to travel in full size truck on the highway and then wheel it moderately to even heavily: The shortest wheel base you can handle for your needs (eg, camper, towing, payload needs, daily driver for work, etc), some lift to match your style of off roading (more if you want to rock crawl), better/larger/tougher tires without over or under doing the weight or size for your truck’s gearing, your anticipated payload, and style of off-road.

You might also want to play with a gearing calculator just to see how tire size affects your rpms at different speeds and potentially your mpg, though as you can see from @rruff ’s and my posts, there’s a lot of variables. This is a pretty good one:

Gear Ratio Calculator

 

[rruff]

I don’t for a minute doubt your personal experience with tire size, but would you agree that the majority of people you know or whom you have interacted with on forums who go to larger, heavier, more aggressive off road tires without expensive regearing, tend to find they hurt their mileage in most cases? That’s overwhelmingly been my experience, as well as personally over 5 different vehicles.

Absolutely... though I suspect the "aggressive off road" part of that is where the hit occurs, with "weight" having a minor direct effect, and "larger" actually tending in the opposite direction. Then there are the aero differences (+lift!), which will be particularly important at highway speeds. I think increasing tire width and height if you are already buying E rated ATs, is fine from an mpg standpoint. You probaly lose some on the highway, but gain some at slower speeds and especially in the rough.

Regarding the regear, I'm curious why that would help mpg. Usually I hear that taller gearing is better even with the new 10spds that have a very wide range. Isn't a larger diameter tire going to be the same as taller gearing?

 

[dstefan]

Regarding the regear, I'm curious why that would help mpg. Usually I hear that taller gearing is better even with the new 10spds that have a very wide range. Isn't a larger diameter tire going to be the same as taller gearing?

Yes, it makes the final gear inches greater. In a no load, no wind, no acceleration, no gravity world bigger covers more ground and is more efficient per revolution of the engine.

The problem is in actual conditions, it takes more torque to drive those bigger tire diameters. Add our over-weight trucks (mine at least) set up for off-road/camping, with heavy mods, heavy tires adding rotational mass, winds, hills, ledges and boulders, etc and the driver/truck combo has to input more energy in terms of rpms/gas to drive them, and that sucks the gas/$$ up.

Your truck and mine have tall-ish tranny gears and a very tall 6th. They come with 4.30s to compensate and be able to accelerate effectively. Problem is that at highway speeds a non-stock, weighted, and modified Tundra (or really any non-HD diesel truck) cant generate enough torque at the low rpm highway speeds (~ 2000 rpm +/-) to overcome gravity+weight+wind in the efficient 6th/top gear it was designed to run in purely stock and unweighted. So it hunts for gears on mild hills or headwinds. The net effect is the truck keeps unlocking the torque converter and shifting back and forth 6th to 5th and sometimes 4th or even 3rd and wasting gas.

The change to 4.88s from 4.30s for me meant my rig quit downshifting to 5th and 4th for every rise in the road. It’s running about 100-150 rpm higher at highway cruising speeds (65-70), but doing a lot less downshifting to lower gears at even higher rpms. At lower speeds, or off-road, I don’t have to put my foot into it as deeply to get the torque needed. It‘s a big difference in driving performance and a small but definite improvement in mpg on the highway, and a bigger improvement off-road and in town.

Apparently, there’s also some ECU/timing changes that happen too, but that stretches my understanding too much to try to articulate.

Without the mods and the camper, I could have gotten by without a regear. As it sits I’m about 400lbs over GVWR, which isnt too bad, but it’s still enough to affect things negatively.

 

[rruff]

The problem is in actual conditions, it takes more torque to drive those bigger tire diameters. Add our over-weight trucks (mine at least) set up for off-road/camping, with heavy mods, heavy tires adding rotational mass, winds, hills, ledges and boulders, etc and the driver/truck combo has to input more energy in terms of rpms/gas to drive them, and that sucks the gas/$$ up.

All those things increase what the engine needs to put out for sure, but a regear just changes the gear... it doesn't increase power, or necessarily change the rpm even... since the transmission can select a lower gear (unless you are in 1st) or you can select it yourself.

For instance, going from 4.30 to 5.29 is about the same as going 6th to 5th. Without the regear I could just put it in 5th and the truck would not know the difference... and I'd still have 6th for flat, downhills, tailwinds etc. With a regear I'd lose the high gear. I'm not seeing how there would be a mpg advantage on the highway.

I haven't noticed my truck having shifting problems, but I'm eventually going to have a large camper on it and I'll be flirting with the axle ratings, so that is another story. I can imagine that if the ECU is having a hard time deciding what gear to be in that could suck some gas. But... we also have S mode where we can select the high gear. It would be easy to just put it in 5th. Or maybe push the tow-haul button which makes it hold gears longer and changes the throttle mapping. You can also buy programmable throttle mapping devices for $150 or so.

Regarding big tires, the amount of extra torque needed to accelerate the heavier tires isn't a lot. If all the weight was concentrated at the tread the 160 lbs I added is doubled when accelerating and braking (and only then). So it's equivalent to <320 lbs that isn't rotating, but that's only ~5% of my total weight.

Again, the problem is that my gearing is biased higher than optimal for my load. But how much difference does that make? Mostly this effects 1st gear, and getting off the line. If I floor it from a stop, the 5.29 would take off quicker initially, and also shift to 2nd sooner while the 4.30 is still winding out 1st. After that initial launch the engine will be toggling through its optimal powerband in both cases. It's just like the difference between a 6spd vs a 5spd... which isn't a lot.

I'm not arguing against a regear, I'm sure it would be more responsive and accelerate better off the line. But I'm skeptical of mpg improvements... or if there are issues with stock gearing that hurt mpg, they can be fixed by other means. The price of the regear, extra noise, and potential reliability issues have detered me so far. I may change my mind when I get loaded up though. I'm curious which gears you got, and what you paid... ?

 

[dstefan]

You are right to be skeptical. I’m definitely not trying to convince anyone to regear. In fact I’d say it makes zero sense for just mpg purposes — the breakeven takes a long time. In my case the drivability issue was a big deal. The clincher was the drive last spring from Bluff to Phoenix with 50mph head winds the whole way and having the tranny hunt constantly (even using the select mode). I have some right side back and hip issues that are really aggravated on long drives if I cant use cruise periodically — and cruise was very poor without the regear and with my full buildout due to the hunting and surging.

You’ll just have to see how it goes with 35s and the weight and aero drag of the upcoming camper to decide.

I will say, that the notion you’d lose 6th is just the opposite of my on the road experience. 6th is now much more useable consistently and I run at a lower rpm that I would be in 5th and almost never downshift into 4th at highway speeds, absent real hills. IMO thats the bulk of the slight, but real mpg improvement I’m getting.

Using 5.29s is probably overkill for 35s, if you’re doing highway travel. 4.88s with 35s bring you very close to stock final gears and is what I did and would do again, though I’m on nominal 34s.

As for cost, part of my consideration was I wanted lockers too, so doing both at the same time was a labor savings. The gear portion alone was $3,600 at SDHQ with labor and REM polishing for Nitrogear 4.88s.

 

[rruff]

The "losing 6th gear" comment just meant that with 5th in 4.3 being the same as 6th in 5.29, I'd have another tall gear with 4.3, but I'm maxed out with 5.29. Granted that extra tall gear would have limited usefulness... but some in low power highway cruising, like downhills and tailwinds.

If I was regearing I'd definitely get 5.29s. I'll be ~40% above curb weight, and though the camper is aero, it's tall enough to stand in, so pushing a lot more air out of the way. The larger tires are a comparatively minor effect! And then I could get 37s... ?

That's not a bad price. I've definitely seen higher. I like to do things myself, but considering how much the labor is for a full regear, I suspect it's pretty involved and takes special tools.

Another question... if I was living in my truck, do you think I could I pull into SDHQ and have this done in a timely manner? I mean a couple days... with an appointment of course.

 

[dstefan]

If I was regearing I'd definitely get 5.29s. I'll be ~40% above curb weight, and though the camper is aero, it's tall enough to stand in, so pushing a lot more air out of the way. The larger tires are a comparatively minor effect! And then I could get 37s... ?

Yeah, given that situation, I think 5.29s are the better choice. I may regret the 4.88s in the future if/when I want a less minimalist camper on the same truck.

Another question... if I was living in my truck, do you think I could I pull into SDHQ and have this done in a timely manner? I mean a couple days... with an appointment of course.

I really doubt it. They wanted a full week for me for gears + lockers, and it actually took a couple extra days. I assume you’d do at least a rear locker too? They are insanely busy and booked out 4-6 months, or at least they were when mine were done in January.

 

[rruff]

I think everyone will get a lot less busy this year... the government fueled boom is running out.

 

[dstefan]

I think everyone will get a lot less busy this year... the government fueled boom is running out.

Let’s hope!

 

[Regcabguy]

Tire size isn't what kills fuel economy... it's mostly the tough sidewalls. Bigger tires (wider and taller) actually have less rolling resistance all else being equal. Weight is a minor factor.

Get the lowest gearing you can so you don't have too many extra tall gears.

I lost 1.5 mpg going from 285-75-17 to 35-12.50-17's. More mass to get going and a wider track with more tread drag with the same tread pattern and manufacturer.