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ENGINE RPM @ VEHICLE SPEEDS

1978 7
Hello,

I am currently working out some kinks in our 90 Plymouth Sundance, 2.5 liter TBI, with the A413, 3 speed automatic transmission. The tachometer isn't currently working and my previous attempts to repair have failed (so far).

I have been told that while traveling with my 3 speed automatic transmission in 3rd gear @ 65 mph, the engine is turning "approximately" 2,800 rpm, near or at maximum torque.

What I would like a bit of help with is the following:

1) At what vehicle speed in 1st gear, is the engine making 2,800 rpm? Lets call this vehicle speed A.
2) At vehicle speed "A", if I shift manually into 2nd gear (without increasing the vehicle speed), what rpm is the engine turning (in second gear)?
3) At what vehicle speed in 2nd gear, is the engine speed 2,800 rpm? Lets call this vehicle speed B.
4) At vehicle speed "B", when I shift manually into 3rd gear (without increasing the vehicle speed), what rpm is the engine turning?
5) At what vehicle speed is the engine rotating 2,800 rpm in 3rd gear?

I don't expect anyone to know these numbers off the top of your heads, so I am actually asking you to, perhaps, today or tomorrow, while running a local errand or driving to work, if you (whomever doesn't mind) don't mind doing a stranger a small, seemingly ridiculous favor, to check these vehicle and engine rpm speed for me.
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· Super Moderator
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You're in luck. Awhile back, I made a table of RPM vs speed vs gear, based on published gear ratios for various cars I owned or considered buying. So here you go:

A-413:
1st gear: 2.69:1
2nd gear: 1.55:1
3rd gear: 1.00:1
Reverse: 2.10:1
Axle: 3.05:1

1st gear at 2800 RPM is about 25 mph
shifting into 2nd at 25 mph, RPM drops to about 1700 RPM
2nd gear at 2800 RPM is about 42 mph
shifting into 3rd at 42 mph, RPM drops to about 1800 RPM
2800 RPM in 3rd gear should be about 65 mph

This will vary a little with tire size, I think this is based on 15" (849 tire revs per mile).

If you want to swap your tach board with my known good one, let me know. I have a partial schematic and can test it functionally, maybe find a bad component if there is one. Make sure the Chrysler P/N is 4375410, that's what I have.
 

· Super Moderator
Joined
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12,300 Posts
How is the tach behaving? Is it just dead, or does it flutter/fluctuate all over the scale, or read too high or low? Or work intermittently?
 
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