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Big tires, Stock gears? Tranny question!

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k5blazinredneck

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I have just recently completed my 3/4 ton axle swap on my 1990 blazer. The gears that are in the new axle are 3.73s and the tires I have on it are 37x12.50r16.5 Goodyear MT/R's. It has a 350 4 bolt main and a 700r4 transmission. As expected, when I started going down the road, the trans took a while to shift into O/D. And when it did, it kept switching between drive and O/D because the transmission is under too much load because of the tires. My question is, would it be ok to drive it in drive instead of O/D? Using only gears 1-3? Just until I can afford to regear to a lower ratio. As of now, the truck is 90% street driven. And never exceeds 60-65 mph. The trans was rebuilt ~12000 miles ago. Thanks in advance! -Aaron
 
I'd go smaller tire or more gear really. Or that not being able to afford a gear swap is going to turn into not being able to afford a tranny rebuild and a gear swap :whistle:
 
With those gears and tires, O/D is almost useless. My 5.7/700r4, 4.56:1 and 35s doesn't like 65mph. Constantly hunting for gears. If it's not dead flat road, I run it in D.
 
as said here lots and lots of times .

overdrive and more than 35" tires = DEEP gears .

or don't use the overdrive function . as hunting from 3rd to 4th to 3rd to 4th will kill the tranny real fast .

most guys like min of 4.88 gears . but if all that work is done just go 5.13 .

now with your combo @65mph should be around 1550rpm this is not in the sweet spot of a good small block power band . of 2000rpm or more.

swap up to 5.13 @ 65mph will get you 2120rpm and a lot better of a gear for lower power stock motor and not lugging the tranny down and keep it in over drive better and a lot less hunting .
 
My old k5 w/3.73's and 33s held OD for the most part. Until you had to push a mean headwind.

I usually just ran 3rd, that's what saved my trans for a couple years me thinks.
 
I think it really depends on the overall situation; altitude might have a bit to do with it.

My '89 Suburban has 35" tires and 3.73's, with 700R4. Where I live near sea level, that size tire is about max for my setup; it holds 4th no problem on the freeway, although I do take it down to 3rd for bigger hills and strong headwinds. I don't tow anything, but have been known to haul around 1,000 pounds of firewood in the back, which down here at sea level it has handled just fine.

Up at Lake Tahoe, however, 4th is out of the question. Absolutely useless. Having said that, I would think that even at sea level, if I wanted bigger tires I would certainly go with deeper gears.
 
It won't hurt the trans to drive in third and at 70 mph you should be running about 2400 rpm (if the converter is locked up) which is a good place for the engine to be pushing that kind of load. You may notice a bit of a bump coming to a stop that you didn't notice before but it will not hurt the trans.
 
Also the 700 is longer than a 3 speed, so driveshaft, particularly front ones, are easier to deal with.

I would highly recommend gears asap, and deep ones as well, whenever you get the money. But driving it in D wont hurt anything.
 
Greg Ducato said:
3.06 to 1 first gear ratio, locking torque converter, wide low sprag design, ( 87-up), no intermediate sprag to fail,etc. make it attractive even as a 3 speed.
Click to expand...

I guess I look at it as the strength and cost of rebuild.


It seems 700s are much more complicated than a 400 from what I've been told. If I wanted 3 gears, I'd want a 400 for strength. To each his own
 
He's not asking what transmission to swap to or spend money on. He's asking if it's ok not using OD, you're over complicating this.
 
700s are not complicated at all.

Yes run it in 3rd ( D ) it won't hurt anything.

Then when you get deeper gears you got overdrive.

My Jimmy had 3.08s and 35s for a while, never put it in overdrive, now with 4.56s I do.

Although I would go deep when you regear, like 5.13 range. With the overdrive it makes it possible. That coupled with the 241 ( I assume you have a 241) and the 700s first gear make for a great crawling truck
 
Just make sure the converter is actually locking in 3rd. Sometimes it only locks under certain load/speed conditions in 3rd, so make sure it is for most of your cruising. You can test it by tapping the brake pedal just enough to unlock the converter (not actually apply the brakes) and see if the engine RPM goes up. Then if you wheel it, use low range.

A cooler is a good idea, as is a temp gauge. Both are still good to have even when you get your real gears in. In the meantime they could save your trans.
 
I've owned my '90 since 1994. With 3.73 gears and only 32" tires I would always shift directly into D no matter what and then only shift into OD once cruising on a flat road going at least 50-55 mph. Later ran 4.10's and 33" tires...point being the gears were fine for the tire size but still kept it out of OD most of the time. As said above the worst thing is the shifting in and out OD, and the constant locking and unlocking of the torque converter. If the truck is struggling to maintain OD and downshifts then it is also not locking the converter.

Once the truck became a mainly off-road toy it got 38's and then 40's with the same 4.10 axles. Haven't even tried OD since the 38's went on over 10 years ago though the truck is not drive much, but was regularly driven on 200 mile round trips to the trail and back prior to getting a trailer. The trans lasted about 120k before going out which included years of being abused off-road and driven low on fluid (long story but leak + stored at parent's house).
 
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