Cruising at 80mph with turbo 400

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Smoothk9

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Want to find a way to modify the turbo400 to cruise at 80mph and save some gas….

Sorry maybe it’s impossible just thought I’d ask

My plan is doing a top end kit on the 454 and will be lifted on 35s at least.

Should I stay with the turbo 400 or swap to another transmission.
 

kennythewelder

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That's all on the gearing, and tire size. What rear end gears do you have, and how big are you tires? As for a trans swap, yes, a 4L80E is what you need, but I'm sure this will involve installing a transmission controller for the 4L80E. I see this same issue with the guys at the car shows all the time. Gear the car or truck to run good, and without an overdrive transmission, it's screaming down the high way. I have 373 gears with 275/60/15 tires on my 97. My tucks runs grate. Pulls strong from a dead stop. With out the 4L60E trans, it would scream doing down the HWY. With the OD trans, it's not bad at all. The OE gears, we're 342. So basically I gained about 300 RPMs at speed. It's been almost 1 year since I rebuilt my rear end. I am very happy with the results. I also installed an Eaton trutrack when I did my rebuild. Man I love the results.
 
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618 Syndicate

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These trucks were never designed to run that fast, especially the early ones without overdrive. Even with a 4L80 80 mph turns too many rpm for decent mileage.
Don't get me wrong, they'll do it, but you're gonna have to figure out your gearing.
 

thegawd

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I'm not going to suggest cruising at that speed but if you want more gears there is an option but I wouldnt plan on going fast in a lifted truck. lowered trucks can go a bit faster but man, you need to stop!

One of my good friends was run over by a speeding lifted truck that ran a stop light, drove over his minivan and he took the front axle to his head. he is now paralyzed. think about it.

....but if you want more gears there is an option!

gearvenders overdrive. turn your 3 speed into a 6 speed.


 
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df2x4

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Here's a useful tool for calculating RPM at any given MPH. You can play with the variables (transmission, gearing, tire size) to get the RPM where you want it.

http://chevellestuff.net/tools/rpm_calculator.htm

You're probably going to have to swap to super tall gears and giant tires to get a TH400 to cruise below 3K RPM at 80MPH. Either that or add something like the Gearvendors OD unit mentioned above.

If this were my truck, I'd either 4L80E swap it or just not worry about the MPG. Most likely the latter. :biggrin:
 

GoToGuy

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You have blowout or shred a tire at 80 , ya got good medical? 35's, 4.11 ( ? ), 80 mph, should be 3200 rpm
4.56 would be 3500
And you'll probably get 6 mpg.
Oh what fun.
 

b454rat

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Go with the 4L80. The 454s are torquey, but still a dog. I had an 88 with a 454/TH400, had 3.42s and 35s, it would light the tires up around a corner, but was a dog on the highway at 70s. It would do it, but foot was always mashed, so defeating any better gas mileage. Have to realize these trucks have the aerodynamics of a small barn, are heavy, and will never get good mileage with a 454.
 

Smoothk9

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That's all on the gearing, and tire size. What rear end gears do you have, and how big are you tires? As for a trans swap, yes, a 4L80E is what you need, but I'm sure this will involve installing a transmission controller for the 4L80E. I see this same issue with the guys at the car shows all the time. Gear the car or truck to run good, and without an overdrive transmission, it's screaming down the high way. I have 373 gears with 275/60/15 tires on my 97. My tucks runs grate. Pulls strong from a dead stop. With out the 4L60E trans, it would scream doing down the HWY. With the OD trans, it's not bad at all. The OE gears, we're 342. So basically I gained about 300 RPMs at speed. It's been almost 1 year since I rebuilt my rear end. I am very happy with the results. I also installed an Eaton trutrack when I did my rebuild. Man I love the results.
Actually my trucks got 4.11 gears ⚙️ front and back. And it’s on 265/75/16 planning on lifting and putting 305/75/16
 
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