Revving 4000 rpms on the highway

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trivium91

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Hey Guys, pickuped up a 70 Dart swinger 340. It's supposed to be 3.73 gears which feels about right ( I had a 3.91, 360, TF 727 Auto before). For some reason the tach is reading 4000 RPMs at 62MPH, I verified the speedometer is correct with a GPS app. Should I not be revving around 3200 rpms? Is it possible that the tach ratio is wrong? The tach seems to work ok at idle, it's right around 900 RPMS. I suppose I can check it with one of those cheap little digital things that you wrap around the spark plug wire?
 
trivium91 said:
Hey Guys, pickuped up a 70 Dart swinger 340. It's supposed to be 3.73 gears which feels about right ( I had a 3.91, 360, TF 727 Auto before). For some reason the tach is reading 4000 RPMs at 62MPH, I verified the speedometer is correct with a GPS app. Should I not be revving around 3200 rpms? Is it possible that the tach ratio is wrong? The tach seems to work ok at idle, it's right around 900 RPMS. I suppose I can check it with one of those cheap little digital things that you wrap around the spark plug wire?
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tire diameter ?
4.30s maybe
slipping tranny
 
RogerRamRod said:
Tach have a 4-6-8 switch, set on 6?
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that would be 75% of the V8 readings, pretty close at 3200 with normal convertor slippage. Ive never come across an tacj with the 6 cyl loop/switch enabled, seem to all come off V8s......
 
I just swapped out 4.10 with a 29'' tire from my Belvedere. 60 mph was 3200 rpm.
 
pishta said:
that would be 75% of the V8 readings, pretty close at 3200 with normal convertor slippage. Ive never come across an tacj with the 6 cyl loop/switch enabled, seem to all come off V8s......
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yes! so its the tach? The RPM calculator said I should be around 3200 RPM. Should I pull the cluster and check the back?
 
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3159 rpm at 60mph with 3.73 gear and 25'' tire and 5% c slip on a wallace calculator if i filled it out right .
 
either the tach is wrong or you 4.56s then if trans is ok
 
Put it in neutral jack up the rear tires off the ground and rotate the tires one rotation and count how many revolutions and partial rev you have on the driveshaft. then there’s no question.
 
Are we talking stock tach, build into the dash, or are we talking aftermarket dash with aftermarket gauges ?
 
And you are shifting out of second into third, correct ?
 
Do you understand how to manually determine rear gear FOR CERTAIN?

If you don't know if it's limited slip, jack up one wheel with front safely blocked, and with transmission in neutral, see if you can turn the jacked up tire. If not you have limited slip and you must now jack up BOTH rear tires

With the one jacked up, assuming no limited slip, watch and count the driveshaft turns as you turn the tire TWO turns. This will give you the ratio

If it is limited slip, turn either tire ONE turn and count driveshaft turns. Same thing. You can easily estimate between 3.55/ 3.91/ 4.11 by being careful
 
somebody you know has a few old tachs in a box in the garage. Borrow one and hook it up with enough wire to reach through the window and see if they match...
 
big difference between 3150 and 4000 should be easy to tell driving down the road 3150 is a little buzzie 4000 is screaming .
 
trivium91 said:
yes! so its the tach? The RPM calculator said I should be around 3200 RPM. Should I pull the cluster and check the back?
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If its an aftermarket tach, they've usually had a 3 position switch for use on a 4,6,8 cyl engine. I've never been swanky enough to have an in-cluster aftermarket, but I would assume they would have the same arrangement. The newer stuff might have a soft key button rather than a physical switch, and/or maybe even a connection to set with a laptop or something. if you see a brand name anywhere Google to see what they offer even if you can't find a model #
 
3.73 is not a factory ratio.

Do you know or suspect the ring and pinion have been replaced with aftermarket?
 
RogerRamRod said:
If its an aftermarket tach, they've usually had a 3 position switch for use on a 4,6,8 cyl engine. I've never been swanky enough to have an in-cluster aftermarket, but I would assume they would have the same arrangement. The newer stuff might have a soft key button rather than a physical switch, and/or maybe even a connection to set with a laptop or something. if you see a brand name anywhere Google to see what they offer even if you can't find a model #
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I believe it’s an OEM reconditioned part as it was $1400, apparently i would have to drop the steering column to pull the cluster without scratching it accordingly to the previous owner.
 
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