I can't drive 65
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Joined: Mar 2007
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I can't drive 65
Not to long ago I got a 1984 f-250 with the 6.9l diesel. It has the C6 auto.
I noticed when I drove it home that at 55mph it sounded like the engine was reved fairly high. I talked to a local rear axle mechanic and he said I most likely had 4.10's in it and if I put in 3.53's in it I would be able to cruse at 70mph with no problem. So I went to a salvage yard and picked up a dana 60 with 3.55 gear and threw it in, and when I was done, took it for a drive and found that I can only go 65 befor it sound like I'm taching out the motor. I have 265/75/16 rear tirers. It dose not have a tach so I cant say for sure whats the rpms are. Also there it a pulsating vibration at speed. Could it be a u-joint or a tc problem?
I noticed when I drove it home that at 55mph it sounded like the engine was reved fairly high. I talked to a local rear axle mechanic and he said I most likely had 4.10's in it and if I put in 3.53's in it I would be able to cruse at 70mph with no problem. So I went to a salvage yard and picked up a dana 60 with 3.55 gear and threw it in, and when I was done, took it for a drive and found that I can only go 65 befor it sound like I'm taching out the motor. I have 265/75/16 rear tirers. It dose not have a tach so I cant say for sure whats the rpms are. Also there it a pulsating vibration at speed. Could it be a u-joint or a tc problem?
Elder User
Joined: Sep 2002
Posts: 519
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From: Iowa
Well, your tires are about 31.5 inches in diameter, so using this calculator:
http://www.csgnetwork.com/rpmcalc.html
one can see that with 3.55's you are theoretically turning about 2460 rpm at 70 mph, plus maybe 200 rpm for slippage of the torque converter. So you'd probably be running somewhere around 2650 rpm. With the 4.10's at 55, the engine would have been spinning almost the same speed.
http://www.csgnetwork.com/rpmcalc.html
one can see that with 3.55's you are theoretically turning about 2460 rpm at 70 mph, plus maybe 200 rpm for slippage of the torque converter. So you'd probably be running somewhere around 2650 rpm. With the 4.10's at 55, the engine would have been spinning almost the same speed.
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