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Old 12-28-2003, 08:50 PM   #1
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What is the correct RPM for 60 MPH?

Long story ... short question. What should my rpm be at 60 mph?
I have a 1988 Classic 345 with a 454 on a P30 chassis.

I just had my transmission rebuilt. The shift point is 35 mph from second to third at 3000 RPM. 60 MPH runs 3000 RPM according to my tach (which is another topic).
On the first trip out it seemed like the engine was really running too fast. I was running 65 MPH and the tach indicated about 3400 RPM. My gas mileage dropped to 4 MPG! on a 122 mile trip, mostly highway. I used to get 7 to 8 mpg before my transmission lost the ability to run more than 15 mph and had to be rebuilt.
On the second trip I got 8.8 mpg running 55 to 60 mph but only for 71 miles. The engine seems much quieter now.
I do not know what my cruising rpm used to be because the tach was all over the board so I just ignored it. (I still have a question about the tach on another post).
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Old 12-28-2003, 09:29 PM   #2
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The engine speed varies a bit.

The Xeppelin has a definite "sweet spot" dead on 63 mph. The 454 is turning over 3000 to 3100 rpm at 63, depending on how the tranny is feeling at that particular instant.

At 60, I turn a bit over 2900. Since the TH400 does not "lock", there is a certain amount of slippage at all times.

One thing I have observed, the tranny stalls at about 2500 rpm. i.e. the torque converter really starts to slip, and attempts to maintain the engine at 2500 rpm as the speed drops.

This really causes a lot of heat and wear and tear on the tranny, for that reason, if I'm losing speed, I drop it into second when the engine is pulled down to 2500 rpm. This happens at about 45 mph, and the engine turns a bit over 3100 rpm in second at 45.
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Old 12-31-2003, 10:59 AM   #3
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My MH turns 3000 rpm at 65 mph. At first, when getting up to speed, it sounds very whiny, but once it maintains at 3K, it purrs.

I was considering putting a Gear Vendor in it to give me overdrive, but I've gotten used to the 3 speed now.
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Old 12-31-2003, 11:12 AM   #4
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What kind of gas mileage do you get running 65 mph?
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Old 12-31-2003, 11:21 AM   #5
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Gas mileage

To be honest with you, I haven't really figured it out yet, but I will eventually just to satisfy my curiousity. I'm of the philosophy that if I feel the need to go a little faster, I will. However, I usually drive 55 - 60. I've gone as high as 70, but I didn't like it. I needed to do it though, to get out of the way of a big rig that was swaying way too much for my comfort, and I passed him and left him behind.

I think that my engine hums the best at about 3K rpm, which is what I mostly look at for long trips. I think someone called it the sweet spot.
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Old 12-31-2003, 01:09 PM   #6
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mine seems to run sweetest at about 60. I have cruised at 65, but the 58-60 range seems very smooth. I get 8-9mpg at that speed fairly consisitently, running the generator. I have gotten 7.5-8 doing that and pulling the Saturn, but I usually slow it down to 55-58mph. I also seem to be close to 3000 rpm at 60+.
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Old 12-31-2003, 02:33 PM   #7
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Alan,
Why do you run the generator while driving?
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Old 12-31-2003, 03:14 PM   #8
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We use it in the summer for the roof a/c. As with many of us the front a/c is not working well. The roof air is excellent, we used it running throught the mountains at 95 degree outside temps, and it did the job.
I use an inverter for the tv and kids videos while riding, and eiher shut off the fridge or run it when the gen is on. I don't like to run the lp gas stuff when moving.
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Old 12-31-2003, 04:06 PM   #9
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To answer your question:
1) Verify the accuracy of your tachometer. Any good garage.
2) Look at the torque curve for YOUR engine.
3) Identify the RPM corresponding to the peak torque.
That is your answer.
Dick
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Old 12-31-2003, 10:48 PM   #10
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4.11 gears-- 3000RPM=60MPH (+-2mph)
Terry
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