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1989 v10 blazer axle ratio swap

The highest geared vehicle I owned was a 1980 El-Camino "Royal Knight" that had a 267 V8,and a TH350,with a 2:56 posi in it..

The thing was lucky to scratch the tires on sand if you floored it,and you'd need ice if you wanted to spin the tires much!..(and it would go sideways without moving an inch forward on slick surfaces thanks to being so light in the rear,and the posi!)...
It cruised nicely at 80 mph though,smooth as glass,and would deliver 20+ mpg on long trips at highway speeds..though it wasn't much good as far as doing burnouts,it did have brisk acceleration,and wasn't a dog on hills,probably due to its light weight..
 
The OP wants the ability to have his cake and eat it too. Nothing against the stated speed limits, we got 75mph limits around here in CO and the locals tend to run 10-15 over it normally. Should you be towing with a stock 700r4 in a brick shaped object at that speed or lower in OD? Even with 1500 pounds you don't think it's causing any issues on flat ground. But it is.

Speaking from experience I killed OD in my 4l60e (which is the same trans with electric controls and a few upgraded bits) in my Trailblazer by towing in OD 1 Time. The payload? A rented, enclosed 4x6 U-haul trailer with a 8.1L big block in it. It might have been 2000 pounds total. Shortly after the trans had issues staying in OD. It was just out of warranty but I had one of my dealers at the time check it out and had to go through it completely. Got it snuck in as goodwill. Still, if you want to tow in OD at highway speeds, go for it. We haven't already been through the same thing you are just starting out doing now.

I don't mean to have that come of snotty or anything but multiple knowledgeable guys are giving sound advice and it's getting blown off. Take a step back and re-read those posts. Do some other searches online about that trans and you find the same EXACT thing being pointed out as problems and the fix and how to deal with it. Nobody is trying to steer the OP wrong here. Just keep that in mind.
 
Well said, and I agree. I'll add that having a K5 with tallish gears, taller tires I never used OD in my '90. Sure it'd hold OD, but it was lugging, and on grades of any size it'd hunt. That was with 3.42's and 35's with a true height of 33 1/4". You don't need something hoked to the hitch to hurt a trans, and the 700R-4 isn't a paragon of toughness anyways.
 
One guy who runs a local transmission shop we sold a lot of cores to at the junkyard always said the 700R4's in stock form were rather frail when it came to using it as a "real" truck,especially the first few years they had them..marginal for everyday driving commuting cars,when used to tow,or with tall tires, or heavy loads,they didn't hold up long..the fact most trucks are highway geared for fuel economy doesn't help..

He always upgrades all the known weak parts,if someone doesn't want to spend that much ,he tells them pay me now,or pay me later..or for low incomers looking for the cheapest way out,he'll put in a salvage yard transmission that the yard will stand behind for 30 or 90 days ,that way he doesn't have to worry if it turns out to be a dud..
You pay him twice for R&R labor,and the salvage yard supplies another tranny..

He once told me when he heard I had a 6.2 Suburban with a stock '85 700R4 that I should "drive it like the transmission is made of glass" and it might last awhile in its present condition..he also said "I'd get AAA if I were you!"...so far its never been registered yet..:blush:


I often wonder,if you can only use overdrive when cruising with a light load (not towing,but "stuff" packed in the truck like camping gear,etc)--are you any better off with an overdrive transmission than an older TH350 or TH400 if you do want to tow a trailer or heavy loads--seeing you'd be in 3rd gear (direct drive ) anyways ?..if you have to leave the overdrive tranny in "D" instead of "OD" ,I think I'd rather have the TH400 which is beefier..
 
My 83 GMC C1500 with the 6.2, 700-R4 3.42's and 235/75's on it...has nearly 200,000 miles on it now. It has the original tranny (weakest year allegedly) and when Iparked it the trans was still A-OK. I wouldn't say made of glass, but I do know the possibility is there. A good cooler, don't tow in OD, correctly set TV cable are the big three...

TH400, nice trans if you have an abundance of horsepower...it eats a ton of hp, and doesn't have the most ideal first gear. Behind a stock small block it'd feel like you're dragging an anchor IMO.
 
Compare that blue C10 I mentioned earlier to my Blazer, That C10 had a 305, a 700R4, 2.73 rear gear, and 28 inch tires. My Blazer has a 305, a TH400, 3.42's and 31 inch tires. That C10 would run circles around my K5, it felt like it had maybe 3 or 4 times the power of my truck. And that 305 in the C10 was basically one huge vacuum leak that would diesel for 30 seconds solid when you shut it off.
 
No doubt the lower first gear of the 700R4 provides an advantage in take offs compared to a Th350 or Th400..and the overdrive is good,provided you can use it...

I agree the TH400 does take more HP away to run it,it isn't the greatest behind a 6.2 with high gearing..the higher first gear does make a noticeable difference in take offs..I know my K10 Suburban would beat it in a stoplight race,same engine with a 700R4 ,but it also has 3:73's in it,the pickup has higher gearing and may weigh about the same,just over 3 tons empty..
I know when I had the '79 C10 ,250 six,TH350,3:08's,that thing felt like it took off twice as fast as either of my 4x4 diesels..
 
My '90 K5 with a 6.2 and 700r4 came factory with 3.73 gears. It had 32's on it when I bought it and it would easily cruise 70-75 mph down the freeway (keep in mind 65 mph was the highest speed limit around here at that time).....I got a couple of speeding tickets on the freeway to prove it. Later ran 4.10's and 33's and still felt comfortable at those speeds even after putting on glass packs.

When I was a kid remember one of my uncle's friends that had an early to mid-80's K10 shortbed with a lift and 35's.....with 2.73 gears and 700r4. He would run around with the shifter in "2" and only go up to "D" at 55 mph. OD was completely useless.
 
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