Big tires on back and skinny tires on front (2wd)

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Big tires on back and skinny tires on front (2wd)

Postby Begle1 » Sun Aug 02, 2020 11:03 am

Right now I have 265/75R16's on all four corners, with good "speed rated" P's on the front and crappy "load rated" LT's on the rear. (I don't think I've had four identical tires for longer than a week over the last 15 years.)

At the time it seemed that 265/75R16 was about as large as I could go on the front without rubbing and I wanted the same size on all four corners.

Now I'm on Maui, have no spare tire any more, don't have opportunities to drive 100+ MPH any more, but I need to haul heavy loads all the time. So I need to reshuffle things and plan ahead.

I'm considering going back to stock-sized LT235/85R16's on the front and going to thicker LT305/70R16's on the rear. And then carrying a LT235/85R16 as a spare. Any reason I don't want to do this?


If a 305/70R16 is 32.8" diameter, and a 235/85R16 is 31.7", would that be too big a diameter difference to use together on a limited slip rear end, in a flat-tire type situation (no more than 100 miles, probably closer to 20?) (I have an open diff right now but I want to fix that soon... I'd like a 4.11 LSD.)
1990 D-250 Regular Cab: Tweaked injection pump, built transmission, a cataclysmic charlie foxtrot of electronics, the most intense street-ran water injection system in the country, and some more unique stuff.
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Re: Big tires on back and skinny tires on front (2wd)

Postby Richie O » Sun Aug 02, 2020 7:06 pm

I would not run different diameter on the rear with a limited slip for even short distances. An open rear would be ok for a short emergency run, but the clutches in the posi would be working ( slipping ) continuously while driving.
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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Re: Big tires on back and skinny tires on front (2wd)

Postby Begle1 » Sun Aug 02, 2020 9:58 pm

Richie O wrote:I would not run different diameter on the rear with a limited slip for even short distances. An open rear would be ok for a short emergency run, but the clutches in the posi would be working ( slipping ) continuously while driving.


Clutches only do work after a point, right? So they don't burn themselves up going around a corner? But where is that point? Is an 1.1" difference enough to go past that point? Or is it just like constantly driving in a circle?
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Re: Big tires on back and skinny tires on front (2wd)

Postby Richie O » Mon Aug 03, 2020 4:57 am

Don't mistake me for someone that has a clue, but the way the posi works is torque. The torque it takes to move the truck, also forces the clutch pack together. So with forward torque being applied, the clutches will have some pressure on them depending on throttle input. If they have pressure and the wheels are spinning at different speeds there is some clutch slippage. So ya, you could say it would be like driving in a big circle forever. Even with an open rear end excessive driving with mis match size tires will over time eat it self to death. I would just be more concerned with a mis match on a posi, then an open, even for short distances. Thats just me, others may say run it.
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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Re: Big tires on back and skinny tires on front (2wd)

Postby Begle1 » Tue Aug 04, 2020 4:17 pm

I don't reckon many have a pause putting a fresh 31" tire on one side and a worn out 31" tire on the other, and that's damn near a 1" difference already.

So where's the point where it really makes a difference?
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Re: Big tires on back and skinny tires on front (2wd)

Postby PToombs » Tue Aug 04, 2020 4:35 pm

I don't know what you are trying to gain by running bigger tires on the rear. I would just buy 4 matching tires and a spare. (either new or used spare) Then you have matching tires and no worries when you go to limited slip.
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Re: Big tires on back and skinny tires on front (2wd)

Postby Begle1 » Tue Aug 04, 2020 6:43 pm

Just going for that drag racer traction. As far as I can tell, 305's are the widest (12") "commonly available" tire I can get that is anywhere near to stock diameter. I think that 265/75's are the widest I can fit on the front without rubbing and they're only 10.4" wide. Stocker 235/85's are 9.3" wide.

So I figure I ought as well go as skinny as I can in the front and as wide as I can get in the back. (In reality I'm going to continue to use whatever I find on the side of the road or that I can steal from the airport parking lot, but at least thinking it out gives me something to aim for, you know?)
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Re: Big tires on back and skinny tires on front (2wd)

Postby PToombs » Wed Aug 05, 2020 4:15 am

Gotcha! Tried to do that on an S10 years ago, couldn't find the right tires as I was trying to keep same height tires.
Good luck!
pete

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Re: Big tires on back and skinny tires on front (2wd)

Postby Richie O » Wed Aug 05, 2020 3:30 pm

Begle1 wrote:I don't reckon many have a pause putting a fresh 31" tire on one side and a worn out 31" tire on the other, and that's damn near a 1" difference already.

So where's the point where it really makes a difference?



I don't know the threshold if there even is a spec of allowed difference. To me in a pinch with open rear I would not hesitate to run a mismatch. Even the automobile industry must think its ok to do it temporarily because they give out the donuts for spares. I suspect with l/s any mismatch equals unnecessary clutch wear. I have been known to drive with no tire, a blown tire, I have dragged the rear tires ( blown rear end ) with the front axle. If it runs and moves under its own power, I drive it. :D
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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