Tire Diameter and Width Advice

sgltrk

Observer
Hello,

I'm new to off-roading and I'm building an overland/expedition rig. I have purchased an early Toyota mini-truck 4x4 which is set up to be a rock crawler and currently runs 37" tires so the drive train is beefed up to handle large tires. I need advice on tire diameter that best fits the needs of overland use as that will dictate axle gear ratios among other things. I live in Southern Calif. and will spend most of my time in the Southwestern deserts and mountains. That said I would like to travel other areas as well.

What tire diameter do you consider optimal for overland/expedition driving and why? Also what tire width would you recommend? Is a narrower or wider tire better for my intended use?

Thanks,
Lyle
 

ADVW/Liam

Adventurer
http://expeditionportal.com/the-10-commandments-of-modifying-an-overland-vehicle/

Keep it simple... overlanding and hardcore rock crawling are pretty different build paths, but there is some overlap. You do want it to be durable, for instance... that said, a ton of unsprung weight can make a bigger difference in a 400+ mile trek over unimproved roads versus a 10 mile trip up a craggy trail.

Great Read! This makes me happy that I did not spend thousands on a 6 inch lift for my Tacoma! I did'nt think it needed to be that high anyway, and who cares about shoving 35 inch tires under a Tacoma? I admit that it looks cool, but it adds a lot of stress on the drive train, and other componets. I'm happy with the ride of the OME suspension upgrade, and 31x10.5 tires. Although I must admit, I would like to have 33x12.5... but that would be the max tire size for me.

be safe
Rick
 

4x4junkie

Explorer
There's considerable overlap if you build the rig right (of course this almost for sure means forgetting about the leaf springs up front... long trips on the freeway and over graded (washboarded) roads will become painfully arduous (not to mention, boring) if the suspension and tires are rattling your guts and teeth out, bias-ply tires will have to go out the window as well).

I run 35x12.50 radials on my rig (BFG MTs currently)... It seems about the perfect size if you want to include some moderate rock crawling in your trips (likes of Dusy, Rubicon, etc). I'm not sure there exists any trails that require tires bigger than that that you can actually camp anywhere on (in a practical sense, anyway), so anything bigger is likely to be overkill unless you have a real long wheelbase (fullsize truck, etc.).
 

sgltrk

Observer
What type of motor, transmission, and axle ratio does the truck currently have?

Hi Clandr1,

The current eng. is a Toyota 20R, trans is a W56 5spd and the axle ratios are 5.29:1. All of that is changing as I convert from rock crawling to overland.

New eng. is a Cummins B3.3t, trans is a Toyota R150F and axle ratio will be determined based on tire diameter and the diesel's power band, which is peak torque at 1600 rpm and redline at 3200 rpm.

Lyle
 

sgltrk

Observer
http://expeditionportal.com/the-10-commandments-of-modifying-an-overland-vehicle/

Keep it simple... overlanding and hardcore rock crawling are pretty different build paths, but there is some overlap. You do want it to be durable, for instance... that said, a ton of unsprung weight can make a bigger difference in a 400+ mile trek over unimproved roads versus a 10 mile trip up a craggy trail.

Great advice Tarditi. This will be a complete vehicle makeover including eng., trans., axle ratios, tires, etc.
 

sgltrk

Observer
Great Read! This makes me happy that I did not spend thousands on a 6 inch lift for my Tacoma! I did'nt think it needed to be that high anyway, and who cares about shoving 35 inch tires under a Tacoma? I admit that it looks cool, but it adds a lot of stress on the drive train, and other componets. I'm happy with the ride of the OME suspension upgrade, and 31x10.5 tires. Although I must admit, I would like to have 33x12.5... but that would be the max tire size for me.

be safe
Rick

Hello Liam,

My concern regarding tire sizes has to do with the trails out here where you can find ledges of 12" on lots of trails and 24" is not that unusual. I'm not opposed to 31's if they will do the job but, I suspect 33's to 35's may be more appropriate for my area. I don't know that for sure hence my request for the opinions of those more experienced than me.

Lyle
 

rxinhed

Dirt Guy
With the diesel your rig 'may' be slower than with the gas engine. Do you expect mileage gains? As the 20R was last used in the early 80's, we presume the trucks to be an 81-84?

Gear ratio vs tire size = engine rpm in the table below, calc'd at 65 mph without OD:
gear_ratio_chart1.gif
 
Last edited:

sgltrk

Observer
There's considerable overlap if you build the rig right (of course this almost for sure means forgetting about the leaf springs up front... long trips on the freeway and over graded (washboarded) roads will become painfully arduous (not to mention, boring) if the suspension and tires are rattling your guts and teeth out, bias-ply tires will have to go out the window as well).

I run 35x12.50 radials on my rig (BFG MTs currently)... It seems about the perfect size if you want to include some moderate rock crawling in your trips (likes of Dusy, Rubicon, etc). I'm not sure there exists any trails that require tires bigger than that that you can actually camp anywhere on (in a practical sense, anyway), so anything bigger is likely to be overkill unless you have a real long wheelbase (fullsize truck, etc.).

Thanks 4x4 Junkie,

I'm concerned about the front leafs for the long term. I'm picking up about 200 lbs. additional weight with the new engine so I hope that will help provide a little more supple ride. Regardless I will work with them for now as there are many other mods planned. I have been leaning toward the 33" to 35" range so your experience is moving me toward the taller tire. With the 37's currently on the truck it is quite a step up to the cab, something I want to reduce as much as I reasonably can. How are your MT's on the road in terms of handling, stopping, ride and noise?

Lyle
 

sgltrk

Observer
With the diesel your rig 'may' be slower than with the gas engine. Do you expect mileage gains? As the 20R was last used in the early 80's, we presume the trucks to be an 81-84?

Gear ratio vs tire size = engine rpm in the table below, calc'd at 65 mph without OD:
_35422.jpg

Great to hear from you rxinhed,

Mostly going diesel for torque, reliability, durability and simplicity. The B3.3T I have is an early mechanically injected model. I expect it to accelerate more slowly than a gas engine but with 220 lb/ft of torque stock I can pull a taller gear than with the 20R so normal highway speeds will not be an issue. I am hoping/expecting better fuel economy. The truck is a '79. Thanks for the gear ratio chart.

Lyle
 

east_tn_81

Adventurer
This is great info. I often think there is a limit to tire size for overland vechiles. You have to have a balance between whats good for the trail and whats good for the road. for the OP what size gears are you thinking of? 5.92 for an overlander is really big.
 

verdesardog

Explorer
I'm running 315-70-17's on my Dodge CTD. I know it's not even the same class of truck as yours but these are over 32" tall and work well for me on the highway and in the dirt and rock and occasional sandy washes in central Arizona...
 

clandr1

Adventurer
You'll need to find the target RPM range for the motor you're using and take into account the output ratio of the transmission it'll be hooked to. Then, once you know your target RPM range, you can match the axle ratio to the tire size to achieve your ideal RPM.

If someone here says "I'm running XX tires with XX ratio gears", it won't necessarily mean it's right for your rig.
 

sgltrk

Observer
This is great info. I often think there is a limit to tire size for overland vechiles. You have to have a balance between whats good for the trail and whats good for the road. for the OP what size gears are you thinking of? 5.92 for an overlander is really big.

Hi East_TN,

The truck currently has 5.29's which, as you point out, are way too low. That's why I'm asking about what size tire works best in my area for an overland type rig. Once I decide on a tire diameter I can then figure the gears I will need.

Lyle
 

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