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What Better Gear Ratios For TH400 To Get To Just 70MPH In A 1983 C20 4 Door Dually 454 Carburated

Make sure the vacuum modulator is working.
Check the slip on the torque converter. Torque converter might be junk.
Put bigger tires on it. BFG 285x75r16 should still fit and will solve your RPM problems. Probably ride nicer also because it has more sidewall.
 
Bohemian said:
theres no reason why that truck with that engine, tranny and rear end ratio rolling stock with no mechanical issues can't peg that 85 mph speedometer all day long without the engine screaming...

Some Key things that can cause or contribute to the issue you describe is:
1. Torque Converter
2. Modulator is bad, out of adjustment or has a vacuum leak.
3. Valve body has issues.

You might want to consider taking the truck to a transmission shop for a transmission service, adjustment and diagnostic. Maybe its a simple as a bad modulator or getting the right torque converter for your combination.
Your transmission could also be so gummed up that the valve body is not functioning as it should.

peace.
Click to expand...

Here's some additional info that demostrates your listed tire diameter and ring and pinion and engine and transmission combination are not the reason you cant get past 65mph.

a optional factory configuration for your truck was the 4:10 ring & pinion, th400 tranny, 454 engine and 245-16 31.7" tall tires you itemized...
235-16 30.5" tall tires were another option.

The Spicer engine rpm calculator shows neither varation of tires with that combo you have listed will cause your engine to scream...
2600-2800rpm at 60mph is not screaming.
https://spicerparts.com/calculators/engine-rpm-calculator

Get a transmission service & diagnostic from a reputable transmission shop brother.
 
I drove my '67 with 454 with Q-Jet carb/TH400/4.10:1 (Dana 60 front/14BFF rear) gears on the highway all the time without any problems and I have a lead foot. Always ran with traffic fine. Sure your gears are 4.10?

ETA: I was running 35 inch tires though, along with a stock torque converter.
 
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Sounds like maybe the stall converter is your issue. Maybe try a stock one again or possibly the trans is just getting worn out and is slipping more than you think.
 
my stock th400 / 4.10 / 33.0 tires i pull 3k rpm at 70 mph no problem and if i push the pedal to hard she will go much faster . so as others have said your trans is the starting point of the problems .

also are you dead sure your getting in to 3rd gear ? and what rpm's does she shift for each gear ?

stock 235/85/16 tire size range would get you these rpm's @70mph with a th400 and 4.10 gears .

1st gear = 7970 rpm
2nd gear = 4750 rpm
3rd gear = 3210 rpm

if you jump to a 255/85/16 like i run super skinny 33" then you would drop around 170-180 rpm at 70 mph .
 
Maybe I missed it, but don't remember seeing the OP state an actual RPM the engine is running at. I have seen a lot of people's perception that cruising down the highway at 3,000 rpm is absolutely "screaming" the engine...and if you take an older truck with minimal interior insulation and a loud dual exhaust I understand where that is coming from. Especially if they are use to driving modern cars that are really quiet and may only run at 1,600 RPM down the freeway! I've had passengers about freak out when I let my 2500HD with the 6.0L gas engine downshift into 2nd and wind out to 4,000 RPM when pulling a heavy load up a hill.

Anyway, we need to know the actual RPM the engine is running out before making assumptions if anything is actually wrong with the truck.

Another comment is that the overdrive gear of a 4L80E is 0.75. That means that a truck with the 4L80E trans and 4.10 gears would be equivalent to a truck with 3.08 gears when just cruising down the highway. Somebody above said that going from 4.10 to 3.73 axle gears would be about the same as going to the 4L80E, but that would be nowhere close to the same decrease in RPMs.
 
sweetk30 said:
my stock th400 / 4.10 / 33.0 tires i pull 3k rpm at 70 mph no problem and if i push the pedal to hard she will go much faster . so as others have said your trans is the starting point of the problems .

also are you dead sure your getting in to 3rd gear ? and what rpm's does she shift for each gear ?

stock 235/85/16 tire size range would get you these rpm's @70mph with a th400 and 4.10 gears .

1st gear = 7970 rpm
2nd gear = 4750 rpm
3rd gear = 3210 rpm

if you jump to a 255/85/16 like i run super skinny 33" then you would drop around 170-180 rpm at 70 mph .
Click to expand...
It gets into 3rd gear clearly, the other day some guy who I know said about changing the gear ratio.
 
blackandgold51 said:
In that case, have the torque converter replaced first before anything else
Click to expand...
I would have it and the transmission checked.
There are a few things that could be wrong. Once you have that settled then you can judge the gearing. And definitely put a tachometer to see what rpms you're getting
 
I don't believe the TH400 ever had a lock-up torque converter, meaning it will always be slipping some. If it's slipping any significant amount it will fry the tranny fluid and then rest of the trans is short order.
 
chulisohombre said:
Sounds like maybe the stall converter is your issue. Maybe try a stock one again or possibly the trans is just getting worn out and is slipping more than you think.
Click to expand...

I'd agree. And with a 454 there's really no need for a high stall converter unless you're running a high rpm engine on something like a mud truck. The reason high stall converters exist is to get the engine in the power band quicker. For large heavy trucks, the power curve tends to be biased towards low-end torque. A loose converter is going to allow more slip (heat) and run at a higher rpm.
 
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