33's or 35's with 3.7L?

[Brad]

I just bought a 2008 H3. It's a base model with the 3.7L. I've been seeing online that a lot of people are putting on 35's but they say it robs a significant amount of power from the engine. The truck has the 33's (size from Adventure trim) on it now but they are snow tires in a normal car tire tread pattern. They are also showing some dry rotting between the tread so they definitely have to go. So here is my dilemma. I would love to put on 35's to have for these Class VI "roads" up in NH and the trails in NW Mass but I do a lot of driving in the mountains and a lot driving in general and am worried about the power loss and a little about the fuel economy loss. Should I be? The 3.7L H3 with the 33's is definitely faster and feels more powerful than my last truck, a 1996 V6 4x4 Explorer with 30 inch tires, which although no race car seemed to be sufficient. I seem to remember a time when a chevy 305 V8 only had like 150 stock horses too. Some people are making it sound like if I put the 35's on the H3, it will have trouble even maintaining 75mph on the highway heading into the White Mountains though and some mentioned average MPG down as low as 12. Is it really that bad? Should I just buy new All Terrain 33's or will I be alright if I go with the 35's?

 

[alrock]

Just about every significant mod I've done on the H3 has dropped me about 1 mpg each. Including in that was going to 35s. So, I'm saying I lost about 1 mpg with that mod. (Also 1 mpg with roof rack, front bumper, leveling, lift, etc.) You'll definitely spend more time out of overdrive when on the highway with headwinds and hills. But you can make it a bit better with a tune from PCM of NC (a vendor on here).

If you're just doing bad roads and forest roads, I'm sure 33s would be sufficient. But 35s are great too and I've run 35s for at least 100K miles with my 3.7

 

[Guerrero]

I just bought a 2008 H3. It's a base model with the 3.7L. I've been seeing online that a lot of people are putting on 35's but they say it robs a significant amount of power from the engine. The truck has the 33's (size from Adventure trim) on it now but they are snow tires in a normal car tire tread pattern. They are also showing some dry rotting between the tread so they definitely have to go. So here is my dilemma. I would love to put on 35's to have for these Class VI "roads" up in NH and the trails in NW Mass but I do a lot of driving in the mountains and a lot driving in general and am worried about the power loss and a little about the fuel economy loss. Should I be? The 3.7L H3 with the 33's is definitely faster and feels more powerful than my last truck, a 1996 V6 4x4 Explorer with 30 inch tires, which although no race car seemed to be sufficient. I seem to remember a time when a chevy 305 V8 only had like 150 stock horses too. Some people are making it sound like if I put the 35's on the H3, it will have trouble even maintaining 75mph on the highway heading into the White Mountains though and some mentioned average MPG down as low as 12. Is it really that bad? Should I just buy new All Terrain 33's or will I be alright if I go with the 35's?

Hello Brad!

I am going to tell you my opinion in my experience:

H3s in stock form and not used for heavy rock climbing, is more than enough with a set of 33s...35s makes sense if you are going to do a lot of rock crawling or very difficult trails, BUT you are going to suffer a significant drop in MPG and power (depends on your driving style, highway specs in your area (is not the same flat than highway hills) and tyre choose.

In my opinion, 90% of people just put 35s because esthetic, and not because improvement for offroad.

So here goes my understanding of perfect balance (super important concept that people forget) for a Hummer H3:

-Stay with 33s: It is the diameter designed for the vehicle from manufacturer, is the tyre designed for all the transmission components, brakes, axles, suspension, steering, etc, so is going to be the best balance between efficiency, strength and capability, but if you want to enhanced them, you can go a step further that is 305 70 R16 (33s but wider) I mounted them in my alpha, a set of Toyo Tyres Open Country MT and the result is awesome, on road and offroad (lot better) and seems like is a little bigger in diameter because of the tread of the MT.

-Spend some money on engine mods: This is by far the best way to enhance efficiency and power. You have the 3.7 i5, so you have the most efficient Hummer built, i recommend you this list:
_Airdoc Intake and AFE PRO Dry S filter
_Supermodulation Ported Throttle Body + supersparkz (still selling them on ebay)
_PCM of NC tune, adjusting after all the mods, for quicker shift points and quicker downshift under less throttle

With this, the H3 of my friend with BFs allterrain 33s is doing 8'9 seconds in the 0 to 60 mph (stock is 9'7 second with 32s), and achieve a stable 20 MPG in highway trips at 65 mph (3.7 are far better than 3.5 in mpg), so is aprox 270 hp at the flywheel

If you can go further and have the money I recommend you this next two mods for the engine:
_Orion Headers with a non fouler o2 in the second hole
_Dual efan setup (you can check my post here: https://www.hummer4x4offroad.com/forum/showthread.php/14490-Guerrero-s-take-on-H3-cooling)
_Retune for this mods with PCM of Nc

With all of this, you will achieve a super balance fast (is not about fast, is about efficiency, more power equals to less mpg if well done) and capable truck. If you go the route of 33s bigger (305 70 R16) in MT you will see a super slightly less MPG, thats all

P.d: Of course I recommend you install some other important mods, crucial for h3s, like Tranny cooler bypassing stock tranny cooler in the stock radiator, Outfitter design kit for steering rack and front differential, and a good maintenance

 

[Brad]

These class 6 "roads" in NH were never really roads as we think of roads today. They might have been horse trails or something some time around America's founding. They are littered with giant boulders, are sometimes muddy with water holes and stream crossings, usually go up a mountain, and sometimes turn into pure granite bedrock with cracks and crevices on a cliffside. Many of them haven't been traveled in years and eventually come to a point where they are completly impassable no matter what you are driving. They are definitely no joke and there has to be hundreds if not thousands of them in NH if you know how to find them (I do). That being said, the H3 cost me alot more money than I had into the old explorer and although more capable, I don't want to wreck it or have to leave it on a mountain side so I will probably avoid the ones that I know are real crazy. Also, I'm from RI and I head up there 2 and a half hours away to my friends house every other weekend or so and often drive another 2 hours or so from his house, so fuel economy and highway driveability are also quite important to me. I also do a lot of hiking/mountain climbing and fishing up there so It's not always crazy off roading. If the MPG difference isn't too bad and the H3 will have no problem maintaining highway speed uphill I would like to go with the 35's otherwise I will stick with 33's. Does your H3 just not like to be in overdrive uphill on the highway or does it struggle to even maintain highway speed uphill at all?

 

[Brad]

Hello Brad!

I am going to tell you my opinion in my experience:

H3s in stock form and not used for heavy rock climbing, is more than enough with a set of 33s...35s makes sense if you are going to do a lot of rock crawling or very difficult trails, BUT you are going to suffer a significant drop in MPG and power (depends on your driving style, highway specs in your area (is not the same flat than highway hills) and tyre choose.

In my opinion, 90% of people just put 35s because esthetic, and not because improvement for offroad.

So here goes my understanding of perfect balance (super important concept that people forget) for a Hummer H3:

-Stay with 33s: It is the diameter designed for the vehicle from manufacturer, is the tyre designed for all the transmission components, brakes, axles, suspension, steering, etc, so is going to be the best balance between efficiency, strength and capability, but if you want to enhanced them, you can go a step further that is 305 70 R16 (33s but wider) I mounted them in my alpha, a set of Toyo Tyres Open Country MT and the result is awesome, on road and offroad (lot better) and seems like is a little bigger in diameter because of the tread of the MT.

-Spend some money on engine mods: This is by far the best way to enhance efficiency and power. You have the 3.7 i5, so you have the most efficient Hummer built, i recommend you this list:
_Airdoc Intake and AFE PRO Dry S filter
_Supermodulation Ported Throttle Body + supersparkz (still selling them on ebay)
_PCM of NC tune, adjusting after all the mods, for quicker shift points and quicker downshift under less throttle

With this, the H3 of my friend with BFs allterrain 33s is doing 8'9 seconds in the 0 to 60 mph (stock is 9'7 second with 32s), and achieve a stable 20 MPG in highway trips at 65 mph (3.7 are far better than 3.5 in mpg), so is aprox 270 hp at the flywheel

If you can go further and have the money I recommend you this next two mods for the engine:
_Orion Headers with a non fouler o2 in the second hole
_Dual efan setup (you can check my post here: https://www.hummer4x4offroad.com/forum/showthread.php/14490-Guerrero-s-take-on-H3-cooling)
_Retune for this mods with PCM of Nc

With all of this, you will achieve a super balance fast (is not about fast, is about efficiency, more power equals to less mpg if well done) and capable truck. If you go the route of 33s bigger (305 70 R16) in MT you will see a super slightly less MPG, thats all

P.d: Of course I recommend you install some other important mods, crucial for h3s, like Tranny cooler bypassing stock tranny cooler in the stock radiator, Outfitter design kit for steering rack and front differential, and a good maintenance

I wasn't really planning on doing a whole bunch of engine mods but will definitely be adding a snorkel and could potentially do the canned ECU tune at a later date. Already doing a shackle and torsion lift and after buying tires I will be almost out of funds to sink into this for awhile though. I like the idea of the wider 33's but that Toyo tire seems to be the only choice in that size and I'm told MT tires wear down way faster than AT tires.

 

[alrock]

Does your H3 just not like to be in overdrive uphill on the highway or does it struggle to even maintain highway speed uphill at all?

I have no problem with hills except for the long, steep ones. I can't think of an east coast equivalent, but climbing the Grapevine in CA or up I-17 from Phx to Flagstaff it isn't happy if I lose momentum. But also note that my H3 is heavy too with rock rails, undercarriage protection, off road gear, roof rack, etc. In NE, I think you'd be fine unless you're regularly driving up Mt. Washington.

Guerrero is right on target for the 33 vs 35 discussion, though it seems like you're pointing out that you'd likely fall on the beneficial side of having 35s. You'll see a difference but you'll be fine.

 

[gearhead1161]

I have a k&n cold air intake and filter ( on it when I bought it) and 35" MTR's ( on it when I bought it) I don't have any real hills here or mountains. I get 13mpg. Mostly highway miles or in town. No mudding no climbing. It is an 08 adventure model so the gearing is different. It doesn't ever downshift on the road at highway speed but uses a reliable 13mpg no matter what kind of driving I do. I think 33's and a tune are what I personally need. Driving up long grades or rolling hills, I can't imagine it would get any better gas mileage. I would go with factory size 33s like an adventure model came with.

Sent from my SM-A205U1 using Tapatalk

 

[Brad]

I like the idea of the wider 33's but that Toyo tire seems to be the only choice in that size and I'm told MT tires wear down way faster than AT tires.

I was mixed up with 305 75 R16 which there seems to be almost no selection in. In 305 70 R16 there does seem to be a pretty good selection though. The General Grabber ATX like I had on the Explorer can be had for 275 each in that size. If I went with those would I be able to keep one of the snow tires I have now (there is one newer one with no dry rot) as a spare or would a slight height difference from different tread and it being a 285 75 R16 mess up the 4wd?

 

[Happy Hummer]

265/75r16. 31.6"

305/70r16. 32.8"

That's a pretty good difference.


315/75r16. 34.6"

 

[Brad]

265/75r16. 31.6"

305/70r16. 32.8"

That's a pretty good difference.


315/75r16. 34.6"

I have 285/75R16 now though NOT 265's. I know there is no way I should use one of my current tires as a spare if I go to the 315/75R16 but the height should be pretty close to the 305/70R16.

 

[rascole]

Go with 35's. I have an 06 and with A LOT of extra weight I would only struggle on a very long climbs but could always keep it at 65-70. I feel that as soon as you see it with 35's and then hit a few trails you will be glad with the decision. PCMofNC is great at retuning and can get some of that power back for you. I am on 37's with a SAS and can still maintain 70 on cruise control at @ 2400 RPM.

 

[Bigunit]

Damn, our rigs will never get the gas mileage of a Musk mobile. The remaining life span of our Hummers is not that long, so I say, WTF, go with 35s, pay for the gas and enjoy driving your beast for as long as you can. Go big or go home with no regrets. [emoji6][emoji16] BTW, I can't afford the gas to go to Moab this year! LMAO.

Sent from my moto z4 using Tapatalk

 

[Brad]

Sounds like I am probably going to go with the 35's and maybe try to pick up a used 35 as a spare. I just saw that 295/75r16 might be a middle of the road option though but as long as the truck can still cruise 70-75 uphill on the highway I will be happy I think. There is a highway here going into the White Mountains that has a 75mph speed limit. That being said, is there any way with 35s anything close to 16 or 17mpg would be possible on a normal highway with a light foot? The guy I keep seeing on a sticker on the gas pumps that says "I did that" is killing me.

 

[alrock]

It is an 08 adventure model so the gearing is different.
Sent from my SM-A205U1 using Tapatalk

All I5s have identical gearing (transmission and diffs @4.56), regardless of year, except when in 4 low and then the Adventure models have lower gearing due to the 4:1 transfer case. All V8s have 4.10 gearing from the factory.

is there any way with 35s anything close to 16 or 17mpg would be possible on a normal highway with a light foot?

Downhill and with a tail wind? Yes.

What are you getting now? I'd expect about a 1 mpg change, maybe 2 mpg, but likely just ~1 when going to 35s.

 

[H3Hummer]

Hy Brad, I'm running 35 with an manual transmission on my I5 too and just don't care about the gaz-mileage.

 

[H3Hummer]

All I5s have identical gearing (transmission and diffs @4.56), regardless of year, except when in 4 low and then the Adventure models have lower gearing due to the 4:1 transfer case. All V8s have 4.10 gearing from the factory.


Downhill and with a tail wind? Yes.

What are you getting now? I'd expect about a 1 mpg change, maybe 2 mpg, but likely just ~1 when going to 35s.

The H3 3.5 2006 Adventure with off road pac and manual transmission like mine came OEM with 285/75R16 , 4.1 T case and 4.88 at the rear diff :wink:

 

[gearhead1161]

Hy Brad, I'm running 35 with an manual transmission on my I5 too and just don't care about the gaz-mileage.

I too figured out what mine is getting but I don't really care about mileage. I gotta buy gas for anything that drives. How much, is dependent on what I am driving.


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[SlcHummer]

I've got an 06 3.5l with several mods, including 35" tires, ARB front bumper w/steel winch, PCMofNC tune, etc. and my obd2 scanner claims I average 15-17mpg depending on the distance and aggressiveness of my driving. I drive like a grandpa in my h3 (no offense to anyone here) which might be why I get so good of gas mileage. I will say 65mph+ that my rig can struggle to pick up speed on moderate inclines but on a flat road I don't seem to notice any issues. Good luck.

 

[alrock]

The H3 3.5 2006 Adventure with off road pac and manual transmission like mine came OEM with 285/75R16 , 4.1 T case and 4.88 at the rear diff :wink:

Well, I can't account for you crazy Europeans.

 

[Brad]

Ok I'm definitely going with the 35's. Might leave the tires I got now for a couple months if I can get away with the small amount of dry rot on the inspection though. The trails won't be open for another couple months anyway. They are closed to wheeled vehicles in the winter and aren't open again until after the "mud season" from the snow melt is over. Just found a bunch of stuff wrong with the Hummer too which are cutting into my funds. Biggest problem was frame rot around drivers side rear sway bar link mount. The mount had actually broken off the frame and there were some holes there. Luckily I know a guy who does an awesome job on that type of thing. It cost me 800 bucks to have him cut that all out and rebuild it with new metal in addition to some other work. The rest of the frame isn't bad luckily but I also had him beef up the mount on the other side with extra metal anyway and I also had him beef up the rear upper shock mounts. I don't want anything breaking on the trail. After I got it back and started driving it more, (still needs no be inspected) I noticed front passenger wheel bearing is humming, thermostat isn't closing all the way causing it to run cold and sometimes causing AC off message on the dash and flatline 0 on temp gauge and causing pending p0128 code. Then on top of all that I'm getting an intermittent missfire on cylinder 5 at idle (pending p0300 but misfire counts only on cylinder 5) but it only happens sometimes at idle and other times at idle missfire counts will all be at 0. The H3 has 126k miles on it. Based on the history I have on it the only service ever done was oil changes, brakes, and tires. So I'm assuming it needs plugs, TB cleaned, primary o2 sensor, and possible fuel injector service. I already installed a new OEM MAF. I already bought all of the other parts too (all OEM) and will be trying to get most of it done this weekend. Also planned on coating the entire frame in Rust Bullet too if I have time this weekend. I could still buy the tires now on credit if they fail my inspection but I have started looking for a used set instead of buying new. None that size on Facebook Marketplace or on Craigslist though. I tried calling used tire places and junkyards too. I usually can't even finish telling them the size and they say "315? I can already tell you we don't have those". Lol, looks like I won't have a choice but to buy new.

 

[alrock]

Sounds like a good plan. Sorry to hear about all of the repairs and maintenance but at least you'll be a lot more comfortable with your H3 knowing these things are done. It's also a good time to replace transfer case and diff fluids. Easy work and cheap. Under 2 qts Dexron VI in the t-case, ~4 qts total (maybe a touch more) 75W-90 synthetic in the diffs.

 

[H3Hummer]

Well, I can't account for you crazy Europeans.

:giggle::horns::cheers: the worst was, I had to pay extra cash for it, to have the I5 in manual transmission with the adventure off road pac and UCP. That costs me a leg :asoff:

 

[Brad]

So I decided I want to go with either the Goodyear Wrangler Duratrac (cheapest), General Grabber ATX(what I had on the Explorer), or BF Goodrich T/A Ko2s (probably the most I want to pay per tire) in the 315 size. I usually get my tires from Town Fair Tire but is says these are all unavailable and they don't even have any in their warehouse. So I continued looking and the website for Monroe showed they had them and I made an appointment and everything only to get a call that they can't get their hands on any of those three and the cheapest ones they can get are Falkens for 350 a piece. Pepboys website says they can have them in two days but it wouldn't let me make an appointment because it says they don't fit my vehicle so I gave them a call and they refused to mount anything but a factory size tire. This is very frustrating.

 

[Brad]

So the guy who did the welding for me said he would mount and balence them for me no problem. So I ordered tires online and am having them shipped to his shop. I ended up buying the Ironman M/Ts in 315/75r16 and got 5 of them for an incredible 965 dollars shipped AFTER tax. That's an incredible 193 per tire! I hope I am happy with them.

 

[Jeepwalker]

For reference, my '07 with 285's on stock wheels, 3.7L get's 17.2 mpg tankful averaged out over 6 tankfuls. Mix of city and highway driving.