37's vs 40's for Fuso FG Build

[chapelheel]

My truck currently has 35 inch tires (315/75 R17), but having seen insta_canter's rig on IG with 40's I'm tempted to built mine around them as well seeing as I'll need custom springs either way as a part of my buildout.

It seems like the cost difference between the two sizes is fairly small, but I've seen that companies like EarthCruiser spec 37's for their rigs.

Is there is a particular reason for this? Why would I NOT want to go with 40's?

 

[Hodor]

Tire choice
Availability
Weight capability
Speed index

Carrying lots of weight on a rig can max out components, and tires can quickly become your weakest link if you're not careful. I assume you're talking about a super single setup because 40s won't fit on duallies, or you won't have much choice in rubber if they do.

I wanted a tire that could handle as much as possible and a 37" size with a 131 load index rating (4k lbs per tire) only gave me one option - toyo MT 37x13.5x17

 

[Howard70]

Depending which engine, transmission and final drive ratio you have 40"s or even 37"s could leave you with pretty tall gearing and reduced performance. I suspect that's of greatest concern with the 3.0 liter/duonic/single speed transfercase trucks.

Howard

 

[rruff]

It seems like the cost difference between the two sizes is fairly small.

Currently looks like ~$500/ea vs $750 going from 37s to 40s. And with 40s you are stuck with mud tires... which is fine if that's what you want.

The 40s (typically 15.5" wide) have a higher load rating at 50 psi than the 37s at 65 psi, so that isn't a problem.

 

[chapelheel]

@Hodor Yes, I have super singles, and you're right about load rating being a concern!

@Howard70 I have a 2006 so no Duonic/single speed to worry about.

It looks like the Nitto Mud Grappler and Gladiator X Comp MT both come in a 131 Load Index for a 40/13.5 R17 tire, but like @rruff said, they are going to be a fair amount more per tire and availability doesn't look good on the Gladiator at the moment.

The Toyo MT 37/13.5 R17 in a 131 load index does look to be the best option for price and availability.

Would the 40's provide enough benefit in terms of clearance/offroad performance to justify the extra cost?

 

[rruff]

Would the 40's provide enough benefit in terms of clearance/offroad performance to justify the extra cost?

37s give you very good clearance. 40s would be more... depends on how important that is. There are tradeoffs to raising it... harder to get in and out, higher CG, etc. If you are going with 17s, that is a lot of tire... probably not so great for on-road handling, but nice cush offroad.

 

[SkiFreak]

My 2c worth...
It all comes down to what you want to do with your truck and where you want to take it. It does not matter what others do; it's what YOU want to do!

If you are putting a fully built out camper on the back you would probably not be doing a lot of really rough/extreme tracks, unless you are the type of person that likes rearranging all of their possessions onto the floor.
Basically, if you have a house on the back, things are different, as are the types of terrain you are likely to tackle.

If you want a vehicle that can get you into the most extreme terrain, then big squooshie tyres will definitely be advantageous. Bare in mind, these trucks are not what can be called a powerhouse, so unless you increase the HP/torque as well, you may find that you actually have less offroad capability when it comes to the truck's crawling ability.
Also, as mentioned, the 40's would be more vague on the highway.

 

[yabanja]

The 37's on my truck required minor modifications to make them fit. Had to move the a/c condenser in the right front. Despite having a larger suspension lift than I would like (3"). Gearing for serious off road use will be compromised. Low range with the 37's is on the verge of not low enough in some situations. I never run out of gear on the freeway. Current max speed is 83 mph.(downhill). Honestly, the truck is capable enough with 37's to go just about anywhere there are tire tracks.

A.

 

[chapelheel]

@SkiFreak That's a good point. It's radical what insta_canter is doing with his rig off road (see his latest video here) but I recognize that I will fall pretty much on the opposite end of the spectrum since I'm putting on a 16' x 8' x 8' box for full-time living capability (as opposed to his low-profile lightweight camping setup). That size and bulk will definitely keep me off of the extreme tracks by default, I reckon. My plans will involve lots of highway miles as well, and given what @yabanja says about his gearing with 37's maybe that's the way to go.

 

[rruff]

I watched part of that video... I kept wondering... why? Is there some incredibly awesome campsite at the end of this?

 

[chapelheel]

I watched part of that video... I kept wondering... why? Is there some incredibly awesome campsite at the end of this?

Sometimes the journey is the destination!

 

[SkiFreak]

I watched part of that video... I kept wondering... why?

In Australia there are some well known "extreme" tracks and many believe that you are not a true 4x4 driver unless you have completed them. I do not fall into that camp, but many do.
As I said... if you are building a truck to go where others fear to tread, that's perfectly fine, but if you are building a camper, those types of tracks and the vehicle modifications required to do them, are probably at different ends of the scales.

Having said all of that... over the years I have had many discussions with people regarding the "best" tyre size for a 4x4 Canter.
Many have given strong arguments for 35's, as this is a good compromise between having a decent highway speed while retaining excellent low range crawling ability.

The biggest problem, as I see it, is the lack of choice in suitable tyres. You need a tyre that is rated high enough to carry the load and you want something that can air down for sand or mud.
Sadly, there does not seem to be a perfect choice here, no matter what tyre size you go for. Basically, you just get to choose the best of a bad bunch.

There is no right and wrong here, it just comes down to what you want to do with your truck.

 

[Peter_n_Margaret]

A rather rambly response.
Our OKA motorhome is up to 6.5T on the road. When I built it 17 years ago, the tyre choices were more limited than they are now. I fitted Michelin 305/70R19.5 XDE2s which have plenty of load capacity, are a fabulous highway tyre, have long life and air down well enough to take the vehicle pretty much anywhere, including the Simpson and the Carson Track. One of their main down sides is that the rim plus tyre weigh 80kg each (x 6 = half a ton).
This Canter runs the same size tyres in Bridgestone.

On the Carson Track.

I am now building another OKA motorhome that will be lighter. It will be powered by the original Perkins Phaser 110 (as is the current vehicle), so will be under powered according to many drivers.
The new build will get alloy wheels (probably Method) and 12.5/37R17 tyres as part of the weight reduction programme. Alloy rim plus tyre weight is about 50kg, a weight saving of 180kg for the set. I do want to avoid mud terrains and that limits tyre choice, but there are still some acceptable AT alternatives. Load ratings for these are typically 124, but there are some greater including 131, any of which will be adequate.
Keeping tyres to a "reasonable" size keeps weight under control which helps reduce unsuspended weight which is useful to maintain suspension performance.
Cheers,
Peter
OKA196 motorhome

 

[yabanja]

Great points Peter. Weight reduction both rotating and unsprung will have a profound effect on performance, efficiency, and ride quality.

Another thing not mentioned yet is that the bigger tire you run, the more strain you are putting on the drivetrain as evidenced with the broken axle in the mentioned video. I was flabbergasted when I saw this. The axles on these trucks are extremely robust. I once read a quote from camel trophy that the reason land rovers never have axle/drivetrain issues is because people always retain the original tire diameter.

As an aside, I am very happy with the yokohama geolander x-at in the 37x12.5x17 size. They are a bit more aggressive than most all terrains but still completely silent on the highway.

Allan

 

[11overlandram]

Hey guys I am in the process of purchasing a 1990s fuso fg. It needs all new tires and I don’t want to buy a set that I will just replace right away. Really want the higher gearing for long distance travel so the 37s sound great. My problem is my budget I can’t afford the lift and the tires. Can you get away with 37s on stock suspension if you cut more away? Thanks for letting me Hijack the thread.