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Old 01-25-2008, 01:53 PM   #1
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Tire Size for 4x4

I am purchasing a 2004 Ford/Sportsmobile, RB 50. It's a 2004, V10, 32,000 miles, 4.10 rear with limited slip, currently 2WD. I am considering taking it up to Salem Kroger for the 4 Wheel drive conversion (Sportsmobile wasn't too interested due to its age and mileage). Craig asked if I wanted to go with 33" or 35" tires on aftermarket wheels. I went through all the postings here and couldn't find much information on tire size. Is it worth paying the money to re gear the vehicle to original or near original gearing to get the speedometer somewhat correct and lower gearing, or leave it alone and get better gas mileage at the sacrafice of low low going? Thanks. oclv

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Old 01-26-2008, 01:12 AM   #2
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I did the same thing with my van and went with 33" and the 4" lift. now it drives better then when i got it.

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Old 01-26-2008, 02:12 PM   #3
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I also have 33" tires on my S-K conversion. I got F350 275/70-18 tires and 8" wheels from eBay. New takeoffs cost about $800 a set. Adaptors for the rear bring the rear track in line with the fronts.

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Old 01-26-2008, 08:49 PM   #4
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I went with larger (37" tires) and I'm happy I went with the taller (4.88) gears. This was a recommendation of Craig's as well. The speedo matches, and we cruise at 65mph turning over 2100 rpm in the diesel.

Some people have expressed a concern that the taller gears might be more fragile, and I'll just have to wait and see if I ever have a problem.
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Old 01-26-2008, 08:59 PM   #5
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I am guessing that you went with the 4.88 front and rear ratio to get the vehicle back to what was a stock ratio with 4.10 and OEM tires, now that your tire diameter was so large. This puts the transmission shifts and speedo back within spec with this correct?? Thanks. oclv
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Old 01-26-2008, 09:22 PM   #6
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Originally Posted by Buji
Some people have expressed a concern that the taller gears might be more fragile, and I'll just have to wait and see if I ever have a problem.
The taller the gear, the thinner the ring gear. That said, 4.88's are hardly at the extreme edge of ring and pinion ratios. I'd be VERY surprised if you ever had any issues with those.

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Old 01-27-2008, 12:31 AM   #7
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Feel a little strong about 4.88 gears!

Heres a few of my comments about tires.

If you do fairly hardcore off-roading max tire size should be 35 inches. The cons outweigh the pros for a 37 inch tire.

Pros.. Better flotation in sand. Higher ground clearance. Look cool.

Cons.. Cost more. Higher unsprung weight... Hurts performance.... Body clearance issues... The weight and higher diameter put stress on all the front and rear suspension componets.... steering box....tie rod...draglink....springs rear axles...Those big tires just hammer the steering box off-road.. Fully loaded theses vans weigh 10000lbs and were designed for the highway. Most off-roaders are at 4000 to 7000lbs off-road

Keep in mind, that my goal is always reliability and you might be not as much of a nut about it as I am.

4.88s have a much smaller pinion gear than 4.10s. Is it too small ? If it breaks in the middle of nowhere you will wish it was bigger. Think about this situation. bouncing up a steep hill van weighs 10000lbs you begin to roll back you hit the gas.... the 37 inch 4.88 gears are not your friend....

Land Rovers, famous for being off- road worthy have 3.50 gears.. They lower the overall gear ratio by putting lower gears in the transfer case ,, like jeep Rubicons...

Please these are just my comments, they are not meant to be argumentative. I know you do alot of off-roading...


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Old 01-27-2008, 04:29 AM   #8
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Hope this helps.. Here is what I'm planning:

I was going to spend the cash on new bigger fancy tires(Toyo's 315s) and wheels(MT's) but I realized I have to fix the 3.73 gear ratio that I have now with a 285 75 16 tire setup if I want to go with bigger tires..

At 75mph, I'm turing 2000 RPM.. So with bigger tires, it's really going to be even TALLER and hard on the tranny.. especially up hills, running start, etc.. I'm on my second tranny(this time its a bulletproof rebuild compared to stock E40D Tranny) and I believe the first failure of the tranny was due to this tall gearing! Heard it happens alot to users with more torque(Diesel) and the weak stock E4OD Tranny..

I'm at 3.73 gear ratio front and back with 285' 75 16 tire size currently.
So I'm planning to upgrade to 4.10's or 4.56 Gears (front and back).

My other proven Rig, the FJ80 (Landcruiser) is at a gear ratio of 4.10 with 285' 75 16 tire size also and I feel it's the right size and combo for 4x4ing on most terrain with the SMB and I'm not rock crawling. It's a balanced setup.. and I want the same for the Sporto..

Since I'll be upgrading the ring and pinion and they have it opened them up, I'm might as well add a Detroit Locker in the front. I'd go with the Powertrax which the FJ80 has but I don't think they make it for the Dana 60s..

The Powertrax has worked out for me on the FJ. The same automatic manual locking technology will work with the SMB.

The Detroit Lockers in the front only will be in play and engaged when I'm in 4H or 4Low only.. ... 191&page=1

Lock-Right looks promising: ... 217&page=1

Why not the ARB AirLocker? I'd go with Airlockers but I'm trying to keep the costs down and the EZ-locker/Powertrac setup has been working for me to provide good 4x4 Traction...and it's a lot less expensive..

After all that, I'll be ready to upgrade the tires to bigger sizes beyond 285's.
I'll have to figure out what backspacing and negative offset is all about with the new wheels... gotta make sure it's the right wheel with the right backspacing and offset so it matches the weight spread on the axle ends I hear..

Complicated but crucial to get the balance right.. or I'm asking for it.
I could just be happy with what I have now and do nothing.

Anyone have a opinion on Detroit Lockers and Powertrax? I have yet to confirm if there is a weak point for heavier vehicles..
I would think the torque needed for a heavier application is equivalent to people who use the Detroit Lockers for racing applications..
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Old 01-27-2008, 08:45 AM   #9
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I have been strongly considering switching from my 33" tires to a 35" to smooth out the ride. I continually come up with a host of cons to making this change yet only a couple pros (hopefully smoother ride and looks).

As for gears, I have 4.10.

A somewhat related question to the original:

For those that have made the switch from 33" to 35", are you glad you did or wish you had stayed with 33"?


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Old 01-27-2008, 09:32 AM   #10
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Interesting thread, thanks.

I'm curious about this tire stuff as well, but don't have a clue about the basics. I've got BFG 285s on my SMB. Are these 33 or 35 inch tires? I'm happy with the ride on my 285s and do not want anything bigger.

When it is time for new tires, I'm thinking about the Toyo's or the E rated BFGs. Not sure about the BFG Es because of the "not rated for snow" designation. I have no idea what that means exactly, but I do almost all of my 4wd driving in snow, some of it more than a foot deep.


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