If you take care of them by not abusing them, changing the fluid regularly and adding a good transmission cooler then these things can last for a very long time. I have seen several that are over 250,000 miles and I had one like that in my 96 Thunderbird myself.
Mine went 140k until it died...but the previous owner never did a fluid/filter change on it in the first 130k and it had pretty low line pressure when it puked. About half of those miles were pulling a 16' horse trailer or 22' travel trailer, so it had a semi-hard life before I got it. Line pressure was a little better after I did a fluid/filter change but not enough to save it.
I've seen several 4R70W equipped trucks around here with 200k+ on them and they're still shifting just fine. Pretty sure the key is regular scheduled maintenance and not abusing them.
IMO, the 4R70W is too light of a tranny for an off-road or tow rig half-ton but it will last a long time if you take care of it.
I run 305/70/16 tires with 4.10 gears and change the fluid every 25k or so. I was getting a slight slip going into overdrive, but I added some lucas trans fix the last fluid change, and it went away. Now it is back to normal shifts. I on occasion tow an 8000lb trailer with it, but not very far, given where I live.
My 4r70 has 195 miles ans still kicking. The first 160,000 were 7-8000 pound cattle trailors behind a 5th wheel. The last 35000 were with me behind the wheel and I am full bolt on right now (intake, LT headers, exhaust, UDP's, e-fans, and SCT). I rag on the old girl quick hard but it has been maintained very well in its life with regular flushes.
Not sure if its still original since I'm the third owner of mine but I've got 246000Kms on mine, (about 145000 miles). Changed the tranny fluid twice since I had the truck. Got it with 219000Kms.
You know, it's cool to see these trucks with this kind of mileage. I wondered sometimes after I bought mine, how many miles I'd get out of it. Rock on guys..
I got 178k out of my original, which I thought was damn good considering it was used on the farm and drove into the ground. I also never changed the fluid in it except for the time i overheated it and it dumped about 5 qts on the ground outside of memphis pulling a trailer... And even then I used some fluid from a gas station that cost less than a $1/qt, to fill it back up (it was Sunday and I couldn't find any Mercon V), trans. lasted another 34k miles after that. However it came back with a vengance because when it finally went it split the casing, destroyed the transfer case, and even broke the bell housing connecting the trans. and the transfer case. If I had done some mantainance i'd bet money that it would probably still be in the truck. Those things are bulletproof
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