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Maximum sustainable engine rpm


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What do you guys think the maximum safe sustainable engine rpm is. With my 4.10 gears and 28" tires it is at about 3000rpm at 65mph in 5th gear. Is it safe to sustain that rpm for a long period of times (about 6-7 miles each way, everyday in my commute)

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yes, your mpg might suffer a bit though

 

 

lol...not a y/n question...

 

 

james, you should be running 30's at least with those gears, 31's preferable. 3.73's would have been ideal for 28" tires.

 

that said, it won't kill your truck to run that for short bursts...but don't expect to be driving a 500 mile trip on it and keeping up with traffic.

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Your tach must be off because 65 mph on a 28" tire is 3198 rpm in 4th gear.

 

If you have 4.0 with a BA-10 or an AX-15 at 65mph in 5th gear is 2526 rpm. This is the ideal gear ratio for a 4.0. Feel free to run 75 mph because you won't hurt a thing.

 

A stock 4 cylinder with 4.10 gears at 65 mph is at 2718 rpm with a AX-5.

That would be a stock gear ratio that was used at the factory.

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What do you guys think the maximum safe sustainable engine rpm is. With my 4.10 gears and 28" tires it is at about 3000rpm at 65mph in 5th gear. Is it safe to sustain that rpm for a long period of times (about 6-7 miles each way, everyday in my commute)

Perfectly okay.

 

Remember, these engines date back to an engine that AMC introduced in 1964. Back then we didn't usually have overdrives. As an example, when I was in the Army I drove a 1966 Rambler American. I had a 3-speed manual tranny, 3.08 gears, and who knows what tire size. I put a tach in it. The speed to RPM worked out to exactly 24 MPH per 1000 RPM. That means a mere 60 MPH was a 2500 RPM cruise, and 72 MPH was 3000 RPM.

 

I sold my American when I was sent to Vietnam, but my brother had a 70 Gremlin with the exact same setup and it went well over 300,000 miles with no engine work. IMHO you're good to go.

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3000 rpm sustained will not hurt a thing for any period of time, at highway speed the load on the engine really isnt that much, just enough to keep up speed and combat aerodynamics. My Hyundai Tuscon has stock 5:37 gearing with the 2.0 4 cyl and runs around 3500 at 70 MPH which I run across country on a regular basis. I would be more worried it the rpms was in the 4-5 range. 3000 will not hurt anything.

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yes, your mpg might suffer a bit though

 

 

lol...not a y/n question...

 

 

james, you should be running 30's at least with those gears, 31's preferable. 3.73's would have been ideal for 28" tires.

 

that said, it won't kill your truck to run that for short bursts...but don't expect to be driving a 500 mile trip on it and keeping up with traffic.

 

"is it safe to sustain that rpm for a long period of time"

 

sure seems like a yes or no question to me :smart:

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yes, your mpg might suffer a bit though

 

 

lol...not a y/n question...

 

 

james, you should be running 30's at least with those gears, 31's preferable. 3.73's would have been ideal for 28" tires.

 

that said, it won't kill your truck to run that for short bursts...but don't expect to be driving a 500 mile trip on it and keeping up with traffic.

 

"is it safe to sustain that rpm for a long period of time"

 

sure seems like a yes or no question to me :smart:

 

YES

 

IT

 

IS

 

SAFE

 

But you can't be at 3000 rpm with 28" tires and 4.10 gears in fifth gear at 65 mph.

 

Do the math...

 

See my earlier post.

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I've taken my 2.5 on 1000 mile round trip 3 times, 205/75/R15s, 3.55, keeping it in 4th most of the way, averaging about 3400 RPM most of the time. Went 3-4 hours with keeping it at least that high and if anything, I think it's helped to work out whatever bug that truck has.

 

Oh, and I just did the math too... you should be running a hair above 2500 RPM at 65mph/5th/4.10/28" tires.

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