Tires vs gearing.

[bucket]

Something to mention, 31's are almost NEVER actually 31 inches in diameter, usually they are in the 29"-30" inch range.

 

[Mattchu60]

When I got my truck it had the stock 350 TBI, 3.42 gears and 33" tires. It was a very bad combo and I really did not enjoy driving the truck much. Any headwinds and it would constantly shift between D and OD. Couldnt do it. I considered going down to 31" tires (stock is around 29" I believe). Instead I switch everything over to 8 lug by getting a 14 bolt and matching 10 bolt for the front. The new axles were 4.10s. When I got new wheels/tires, I settled on a 32" tire. It looks good, fills up the wheel wells but not too much.

The point, i deterimined through a lot of calculations that my 4.10 with 32" rubber combo was very close to the 3.42 with stock 29" rubber for engine RPM and speedometer. Its a whole different truck, I can drive in OD without issue, tow heavy trailers (still pulls very hard, but much better) but in general its really improved the driveability.

 

[MadOgre]

The 700r4 is only better off in first gear over the th350. Gears 2, 3 are exactly the same gear ratio's as the th350. So you only see a difference in the first 20 kmh of acceration.

SO in gears 1 and 2 you are multiplying your engines power with the increased gear ratio's. 3rd gear is 1 to 1 so there is no multiplying force factor in OD you are now going the other way and it now takes 30% more engine power to accomplish the same as in 1 to 1.

Remember your transmission is the force multiplier in both ways. So when it becomes difficult to exert power multiplying force it is your transmission that takes the stress. Stress = heat.

Its just like trying to ride a bicycle up hill in top gear. It now takes much more power and effort to go the same speed, and you get hot.

Well the transmission becomes hot too because of the added slippage in your tourqe converter while the engine is working much harder to compensate for the loss in gear ratio and this is why it is important to have a functioning lock up tourqe converter when operating in OD. To eliminate slippage thus eliminating heat build up.

In gears 1-3 its not much of an issue, but because of the now 30% OD, or loss of force multiplication, it becomes a real issue.


Its not about lugging the engine its about having the power to keep going down the road at set speed.

With my 350 and worn 33s and 3.42s to get my self into the 2000-2200 rpm range for OD it means im doing about 130-5 kmh/ 82-84 mph. Most times eeven on the highway I just leave it in 3rd.

Same idea as the bicycle again. In top gear it takes considerable effort to travel at walking speed, but increase your speed to 25mph and now it becomes much less difficult to pedal.

So traveling in OD at 1500 rpm its going to create more heat then traveling at 2000 rpm



I honestly believe the 31.5s and a set of 4.10s will produce the most efficient and comfortable drive with a 700r4

 

[flyboy1100]

Something to mention, 31's are almost NEVER actually 31 inches in diameter, usually they are in the 29"-30" inch range.

Tirerack lists the general grabber at2 at 30.8 for the 31. The other are all sub 30 for 30x9.5 or 255/70r15 I would consider.

Is there a gearing/rpm calculator out there to figure out a good compromise combo?

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[Driver4r]

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And that's @60mph

 

[rich weyand]

For 700R4, multiply your rpms from this chart by .7 to get the value in OD.

For example, 31s with 3.42 is 2409 in 3rd from the chart, times .7 gives 1686 in OD.

 

[MadOgre]

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And that's @60mph

Im assuming that is with a transmission ratio of 1:1 ???

 

[flyboy1100]

this is all assuming my TC locks like it is supposed too. i really really need to get my tach hooked up. can someone add like 4 hours to each day for me?

also looking at tires in the stock size for my truck, 235/75/15 they all look to be about 29" in diameter. so according to that diagram, I am never in the middle "good" area, always in the yellow because even stock tires are 29"

255/70/15 is 29.1"
265/70/15 is 29.6"
30x9.5 is 29.8
31x10.5 is 30.8

 

[flyboy1100]

found this site:
http://www.ringpinion.com/Calculators/Calc_RPM.aspx

for transmission ratio RPM calculator I entered:
3.42 30.8 75 0.7 = 1959
3.42 29.8 75 0.7 = 2024
3.42 29.1 75 0.7 = 2073

so wouldn't any of those really be ok? with 4.11 gearing, pretty much add 400 to any of the above combos

not trying to be argumentative, just trying to learn, and obviously there is something I am still missing
should add after some more reading (this thread again twice!) I am highly considering the 265/70/15 now. price is about the same at the 31x10.5, rpms will be slightly higher at cruise, and a treadlife warranty (i will probably run out of time before mileage though) should give me 1766rpm @ 65 and 2038rpm @ 75mph

 

[rich weyand]

You probably won't catch overdrive until 65, and it may downshift if you catch a headwind, but at 75 you should get great gas mileage.

 

[flyboy1100]

You probably won't catch overdrive until 65, and it may downshift if you catch a headwind, but at 75 you should get great gas mileage.

kinda sounds like what it does already

 

[flyboy1100]

Been nearly a month since I have really driven my truck. Drove it last night. With the current old 265/65r15 installed at 75mph I am turning between 2100-2200rpm. Should be 1920ish

So to me this means:
1. My gearing is different than the RPO code of 3.42
2. My tach is inaccurate, sunpro super tach ii
3. My torque converter isn't locking

Any other ideas? I plan to drain the diff's this evening and I will count gears.

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[flyboy1100]

Well front for sure is 3.42, I haven't counted the rear yet, but I hope its the same!

At this point I think I am going to trouble shoot tcc circuit. Only reason I can think of for tcc not locking up and keeping rpm higher then should be.

Maybe also TV cable?


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[350runner]

Try bypassing the vac switch

 

[flyboy1100]

just a toggle switch between the 2 wires?