Tires vs gearing.

[flyboy1100]

i can't seem to find what I was reading before, so here is the question.

currently running mismatched tires (not in 4wd mode!) fronts are 31x10.5r15 and are completely shot. rears are 265/75r15 and are very badly weather cracked.

looking to replace the tires, thinking General AT's in 31x10.5 as I really like how it looks now with this tire size. Engine is the 305, 700r4, np208, gearing is supposed to be 3.42, not sure on open or lockers, guessing open (need to verify)

will be a good combo to keep running? mostly will be highway use, snow in winter, maybe some mudding/offroad if i get other drivetrain issues sorted out. I'm guessing 3.42 gearing is minimal for 31's, but not planning to replace diffs or gears yet (unless it turns out my rear is bad, then I will!)

I also don't understand the gearing vs tire size vs power, can someone explain that to me?

 

[skysurfer]

Are you familiar with the term "tolerance stacking"? Tolerance stack-up calculations, also known as worst-case scenarios, represent the cumulative effect of part tolerance with respect to an assembly requirement.

In your case, the truck represents the assembly. The stacking is the collection of components that make up your driveline. Each one by itself would not be an issue, but when combined with other less-than-ideal components, you end up with a problem.

305 engine: The weak sister to the 350 (which is the weak brother to the big block).

3.42 rear end: More of a passenger car ratio than a truck. Think 3.73, or better yet, 4.10.

31" tires: The taller the tire, the more torque required to get things moving. Also drops engine rpm's.

700r4 trans: Equipped with a .70 overdrive, it needs power to keep it going down the highway. You don't have it.

K-series truck: Extra static weight from 4x4 gear, plus increased parasitic loss of power through the transfer case.


Add all that together and you have a recipe for a truck that will disappoint. I know because I had a C-10 with that combo. You didn't mention towing and shouldn't. I tried towing a 16' travel trailer and every hill was a nightmare. Every headwind required near full throttle. Sure, it was still able to haul home the groceries, but always fell on it's face when I really needed it.

 

[bucket]

My '88 Burb was purchased with a similar combo, the only difference is I got the 350. 31's with 3.42's was less than ideal but it did work fine normal driving, at it was possible to tow with it (I did) but don't bother trying to run it in OD.

The burb is heavier, so that should make up for the added power of the 350. You should be OK with 31's. It beats the hell out of the 3.08's and 33's a lot of guys run.

 

[Driver4r]

3.42's are a great gear ratio for all around driving....

Hell, Mine currently has 3.08's in it and I can still break them loose on 32"s, but I had a reason for going with 3.08's for the time being. I wouldn't do it on a 305, but a hopped up 350 like mine yes.

31"s are a good all around tire, most tire stores carry them.

 

[MadOgre]

I think you will find that setup a little tall for the 700r4 and that it will take a little bit more speed to shift from 1-2 which I think really makes it a little bit of an unpleasant drive around town. but the rest of the gears will do fine, although not so much with the 305.

I have a similar set up, 350,700r4,3.42s on worn 33s and it does good in OD. Even so I really feel the need for 3.73 gears or better. Im thinking im gonna end up with 4.10s

 

[flyboy1100]

No towing, will probably get a load of crushed concrete this fall, but that is only 35 miles away one time. Just normal driving, mostly highway 65-75mph. New gears are not in the budget at the moment unless I find after ujoints that my rear end is making noise. I would like a 350 or some other engine in the future, maybe next year this time.

As long as 31s won't blow anything up, and it doesn't sound like they should, I will probably get them and worry about extra power next year. Really I just need good rubber that holds air and is driveable in the winter if I need or want to drive it.

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[flyboy1100]

How does the taller tire affect the torque?

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[Driver4r]

Yes, get tires, then engine. After the engine swap, see if you still need more git-up, if you do, then re-gear, if not, then don't

 

[MadOgre]

How does the taller tire affect the torque?

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Taller the tire the less tourqe you will have.

You shouldn't burn it up with 31s and 3.42s but a must for any 700r4 is a good tranny cooler up front.

 

[MadOgre]

No towing, will probably get a load of crushed concrete this fall, but that is only 35 miles away one time. Just normal driving, mostly highway 65-75mph. New gears are not in the budget at the moment unless I find after ujoints that my rear end is making noise. I would like a 350 or some other engine in the future, maybe next year this time.

As long as 31s won't blow anything up, and it doesn't sound like they should, I will probably get them and worry about extra power next year. Really I just need good rubber that holds air and is driveable in the winter if I need or want to drive it.

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If your axles are on there way out then try and find a set of higher geared axles. Usually the 3/4 ton stuff is geared a little higher, usually starting at 3.73 and up.

Higher gearing will make it seem like your engine has more power as it will take less power to do the same job.

If you regear maybe youll be happy with the 305 and 700r4 combo and burn less fuel.

The other option would be to go with smaller tires, which are usually cheaper.

 

[rich weyand]

One thing no one has mentioned. The 3.06:1 first gear in the 700R4, through a 3.42:1 rear end, has the same engine revolutions per tire revolution as the 2.52:1 first gear in a TH350 through a 4.11:1 rear end.

Even with a 305, a TH350 with 4.11:1 rear end should be fine to get her moving with 31s, so a 700R4 with 3.42:1 rear end should be fine to get her moving as well. RPMs at 60 in OD will be 1550, at 70 will be 1800, which is a touch low. 2000 would be better, I think.

A 29 inch tire (like the 265x70R15) would give you about 1925 rpm at 70 in OD. I think that's probably a better pick. Also, as others have mentioned, 29s will have more go out of the hole.

If you do change gears down the road, you could then go to 31s on the next set of tires. Tires aren't permanent.

 

[flyboy1100]

Well when I only drive this truck maybe 3000/yr the tires will be permanent.

I don't understand why low rpm in od is bad for the transmission, I would think making it constantly spin faster would create more heat than lower rpm.

I could do 265/70s or 30x9.5 too I guess. Looking at the specs they aren't much smaller than 31x10.5

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[flyboy1100]

Taller the tire the less tourqe you will have.

You shouldn't burn it up with 31s and 3.42s but a must for any 700r4 is a good tranny cooler up front.

I think I have one, mounted on the passenger side?

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[flyboy1100]

Stupid phone app double posting

 

[rich weyand]

Well when I only drive this truck maybe 3000/yr the tires will be permanent.

I don't understand why low rpm in od is bad for the transmission, I would think making it constantly spin faster would create more heat than lower rpm.

I could do 265/70s or 30x9.5 too I guess. Looking at the specs they aren't much smaller than 31x10.5

You don't want to be lugging the engine driving on the highway. There is such a thing as too low an rpm. You don't want to be driving 60 mph in third all the time for example, and have to do 70 mph to even get into overdrive.

Maybe some other people driving 700R4s can chime in here about what their rpms are at 60 mph, and give some feedback on how it drives for them.