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Old 03-05-2023, 10:01 AM   #1
evan love
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How many rpms does a 454 like on the highway?

Hello,

I have a 1979 single cab C30 dually with a rebuilt, but stock, 454 with and edelbrock carb and headers...

...th400 trans and 4.56 rear diff gears

And I'd like to drive it 1100 miles from my parents house to mine on major highways.

Any idea what rpm range these engines can do comfortably without any damage to the engine for long days on the highway?

Any thoughts on expected fuel consumption and top speed at a safe rpm for the engine?

Thanks,

Evan
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Old 03-05-2023, 10:35 AM   #2
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Re: How many rpms does a 454 like on the highway?

Assuming 235/85r16 tire size with that combo of parts --

70 mph -- 3500 rpm
65 mph -- 3300 rpm
60 mph -- 3000 rpm
55 mph -- 2800 rpm

Larger tires would drop these rpms somewhat. These are all spinning the engine faster than ideal for fuel economy, but it's what you have to work with. It's not worth regearing just to save on fuel for that one-time trip. Someone else can answer as to whether continuous 3000+ rpm is okay for a 454 or not.

To check out other tire sizes, punch your numbers into the calculator at grimmjeeper.com. It's the best gear ratio calculator I've seen.
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Old 03-05-2023, 11:12 AM   #3
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Re: How many rpms does a 454 like on the highway?

Those RPM's are not really hard on the engine, just the fuel mileage! I have both a 1978 C20 Camper Special with 4.10's and TH400 and a 1979 C35 Dooley Camper Special with 4.10's & TH400 - both get around 10-10.5 MPG on the highway. I am considering replacing the TH400 on the 1978 with a 4L80E - the 1979 I am trading for body restoration on my 1975 Laguna Type S3!
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Old 03-05-2023, 12:35 PM   #4
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Re: How many rpms does a 454 like on the highway?

If you have the 235/85r-16s on it (which are 31'' tall) A 3.73 ratio would be accommodating. My big blocks like to live around 21-2200rpm on the highway. But my flat bed has a t400 with 4.10s and a gear vendor with 31'' tires, then my 68 has an SM465 with a ranger OD with 3.73s with almost a 33'' tall tire, so..... if you do any heavy towing with the 3.73s you'll see an increase in transmission temps and a somewhat struggling 454 unless you bump up the compression a little and go with a mild cam.
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Old 03-05-2023, 02:02 PM   #5
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Re: How many rpms does a 454 like on the highway?

Won’t hurt it a bit doing 70mph all day long . I’ve put 35k miles on my 350 with 4.10 gears with about 70 % of that on the highway
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Old 03-05-2023, 03:03 PM   #6
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Re: How many rpms does a 454 like on the highway?

My 350 runs between 3500-4000 when at speed on the highway. Yea, I get about 7 MPG.
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Old 03-05-2023, 07:51 PM   #7
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Re: How many rpms does a 454 like on the highway?

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My 350 runs between 3500-4000 when at speed on the highway. Yea, I get about 7 MPG.
Haha. I feel ya. I’ve had lead foot disease since I learned how to drive.
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Old 03-05-2023, 10:22 PM   #8
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Re: How many rpms does a 454 like on the highway?

Cruising at 3500 rpm will be like driving 2200 miles in 1100 miles lol. You gotta do what you gotta do. Personally I don't like running over 2500 for hours on end.
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Old 03-05-2023, 11:18 PM   #9
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Re: How many rpms does a 454 like on the highway?

$440 in gas each way. Ouch
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Old 03-06-2023, 12:14 AM   #10
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Re: How many rpms does a 454 like on the highway?

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Haha. I feel ya. Iíve had lead foot disease since I learned how to drive.
That's pulling a trailer in 3rd gear at 75. Not a whole lot I can do about it.
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Old 03-06-2023, 01:17 PM   #11
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Re: How many rpms does a 454 like on the highway?

I really think people have gotten so used to the new OD transmission where the engine turns at 2200 at 75 that they think they are doing harm to the motor when it revs . Cars and trucks put 100s of millions of miles on the highways doing 75-80 @ 3000 + RPS . My Chevelle turns 2900 and change at 60 mph . I've driven it for hours at 70-75 going to Carslisle ,Syracuse nationals and Goodguys shows. Little noisy yes did it damage the motor? Hardly
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Old 03-10-2023, 05:51 PM   #12
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Re: How many rpms does a 454 like on the highway?

RPM won't hurt anything. Mileage will be low 6 to 8 MPG range. Put a 4L80E in it and reap the rewards of 20-25% better mileage. Close to a Bolt in swap for a TH400. I have a 85 C20 Suburban that was 454/TH400 3.73 235/85/16 changed to 4L80E and really like it.
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Old 03-15-2023, 09:38 AM   #13
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Re: How many rpms does a 454 like on the highway?

I've always been told 2200 was the sweet spot for mileage
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Old 03-17-2023, 01:59 PM   #14
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Re: How many rpms does a 454 like on the highway?

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I've always been told 2200 was the sweet spot for mileage
Depends on the vehicle/drivetrain/situation.

For SBC's & BBC's that 2200 number is great if you can pull it off on the highway. It's likely that 2200rpm would need a stock or mild cam combo as well. But, even w/a BBC & correct cam, it might be under target for a heavier vehicle that's working/pulling a load.
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Old 03-19-2023, 04:32 PM   #15
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Re: How many rpms does a 454 like on the highway?

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Originally Posted by 72c20customcamper View Post
I really think people have gotten so used to the new OD transmission where the engine turns at 2200 at 75 that they think they are doing harm to the motor when it revs . Cars and trucks put 100s of millions of miles on the highways doing 75-80 @ 3000 + RPS . My Chevelle turns 2900 and change at 60 mph . I've driven it for hours at 70-75 going to Carslisle ,Syracuse nationals and Goodguys shows. Little noisy yes did it damage the motor? Hardly
I agree completely. If it worked when new, it should work now too.

However, an engine in poor shape can live a much longer life at lower RPMs than it can at higher rpm. So if the engine is a little tired, it may be wise to keep the speeds down.
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Old 03-19-2023, 05:40 PM   #16
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Re: How many rpms does a 454 like on the highway?

I have a 1983 C20 camper special with 454, SM465 granny 4 speed, and 4:10 rear axle. When I travel at 55-60 the engine is screaming. I don't know the rpm because I don't have a tach. When my Gear Vendor overdrive was still working it was not so bad. GV says they reduce the final drive ratio by about 20%, which is probably correct in my case. I was getting 11 city and 13 highway with the GV working, but now that it is not working I get about 9 city and 11 highway, but I almost never take it on the highway.

The local Gear Vendor mechanic recommended replacing the granny 4 speed with an NV4500, removing the GV and drive shaft, and installing a new factory length drive shaft, and possibly a highway rear diff (maybe 3.23 or 3.73.) The cost of all that would be about $6-7K, so not practical since I only drive my truck about 800 miles per year to pick up long items from the home centers or go to the dump. Just swapping the rear diff for a 3.23 is probably the most cost effective for me, but that reduces the CGWR significantly, and is not justified based on how little I drive the truck.
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