High rpm at 60?

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doogievlg

doogievlg

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Took my dart out on the interstate for the first time since my rebuild and rear end swap. The car did great, never overheated or cut out and it drove straight as an arrow.

I am thinking 3200 rpm is a little high for 3:55 gears at 60 mph. Tires are 235/60/15 by the way. Motor is a aluminum Head small block mild build with 727 transmission.
 
Yeah something is not right.
Those tires are about 26.1 tall, and 82 inch roll-out. so the formula spits out 60 = 2750rpm with zero-slip. Most autos will slip downwards a bit at cruise;perhaps as much as 200rpm. So I would expect the tach to read 2550 to 2600 @ 60mph.
3200 maths out to 70mph and adding at least 3% for slip, I would expect 3200 to be close to 72 mph.
So first thing to do is to prove your Speed-o and your Tach.One of them (or both)is probably in error.

Here is the formula;
Rpm x TC / 1056 x R1 x R2 = mph .
Where TC is tire circumference, and R1 is rear gear, R2 is tranny gear

This is for Zero-slip. Expect the Tc to be slipping some 5% at 60 under cruise conditions, so the tach will be reading some 4% to 6% less than Zero-slip. You can measure your slip by applying Some throttle at 60 mph (not enough to kick-down,and only for a split second) from a steady cruise. The amount of tach pop-up divided by the rpm before pop-up is the % slip of your TC at that mph/speed It will vary at higher speeds, eventually becoming Zero or less. And it may slip more at lower speeds.
 
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what type of tach is in your car? Factory or aftermarket? I know that many years ago I had an aftermarket one on my 68 Barracuda and that it had a switch it something which could be changed from 4 to 6 to 8 cylinders. If yours is similar, make sure the setting is right.
 
jimt1971rr said:
what type of tach is in your car? Factory or aftermarket? I know that many years ago I had an aftermarket one on my 68 Barracuda and that it had a switch it something which could be changed from 4 to 6 to 8 cylinders. If yours is similar, make sure the setting is right.
Click to expand...

It is an aftermarket tach. I'll check it but it seems accurate, I can tell the car is revving high without looking at the tach. I'll check it out tomorrow when I go back over to the garage though.
 
doogievlg said:
Took my dart out on the interstate for the first time since my rebuild and rear end swap. The car did great, never overheated or cut out and it drove straight as an arrow.

I am thinking 3200 rpm is a little high for 3:55 gears at 60 mph. Tires are 235/60/15 by the way. Motor is a aluminum Head small block mild build with 727 transmission.
Click to expand...
You need to check your speedo with mile markers, a Garmen, or a cell phone app. I went from a 2:87 to a 3:23 and put 255-60x15's on and was 10mph slow. Like AJ said you're probably at 2500 r's. You need a different speedo gear.
 
Torque converter needs to be known. Years ago I had a GER, and it was so "spongy" it revved just like that
 
318willrun said:
Torque converter needs to be known. Years ago I had a GER, and it was so "spongy" it revved just like that
Click to expand...

Junk jegs 3000 rpm stall I got about six years ago. When I raced I went through the lights at 5700 rpm at 99 mph.
 
273 Commando with 26.1" tires and 3.23 gears I'm just a hair over 3000 rpm at 65 mph. My speedo is right on using GPS.

Maybe measure the tires to double-check the height. Perhaps they are shorter than 26".
 
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AJ/FormS said:
Oh Gee, I shoulda guessed that :(
I dunno what I was thinking.
I guess you called it:
junk
It is slipping alrighty, but the wrong way!
Click to expand...


Even if this issue wasn't going on I was planning on putting a quality converter in it this winter.
 
In this day and age there is no excuse for not having a GPS. I have 3 or 4 I've bought in thrift stores, or on CL. I think the highest I paid was 40, and the lowest was FIVE BUCKS. It had no bracket nor charger. I found a car charger cord (mini USB) in another local thrift store for a buck, and adapted a bracket off some Tom-Tom thing that I got didn't work.

There are dozens of online gear/ speed calculators, all you need do if an automatic is allow a little 'fudge' factor for the converter slip.
 
Keep us posted with what cures the problem.
 
Similar issues in Dads 67 fb. Turns out when factory tach was rebuilt, calibration is off.
Inst bezel has to come out to fix tach so I'll live with it for now. Damn thing
 
with a 28'' tall tire and 3.91 gears im at 3000 at 60 mph on level ground in a incline it can go to 3300 at 60mph my converter is an ultimate 3600 stall .
 
4spdragtop said:
Similar issues in Dads 67 fb. Turns out when factory tach was rebuilt, calibration is off.
Inst bezel has to come out to fix tach so I'll live with it for now. Damn thing
Click to expand...

I'm hoping it is just the tach. I have a couple others laying around I can throw on the car and see if they read the same.
 
jimjimjimmy said:
with a 28'' tall tire and 3.91 gears im at 3000 at 60 mph on level ground in a incline it can go to 3300 at 60mph my converter is an ultimate 3600 stall .
Click to expand...
And the direct-drive equivalent for that is 2816 rpm.(zero-slip)

Doog
If you have a timing lite with a tach, those are real handy. For this exercise you can clamp any convenient plugwire; the tach won't care. And you can steal power any convenient place too, making it possible to bring the lite/tach into the cabin.
 
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AJ/FormS said:
And the direct-drive equivalent for that is 2816 rpm.(zero-slip)

Doog
If you have a timing lite with a tach, those are real handy. For this exercise you can clamp any convenient plugwire; the tach won't care. And you can steal power any convenient place too, making it possible to bring the lite into the cabin.
Click to expand...

Excuse my ignorance here but how is this done and what does the timing light do for the rpm?
 
He's saying that some timing lights have a built -in tach. This would be a way of checking yours

What have you done so far? I suggested a GPS. In fact this thread reminded, me, and I spotted one locally for 20 bucks. I may check on it later.
 
67Dart273 said:
He's saying that some timing lights have a built -in tach. This would be a way of checking yours

What have you done so far? I suggested a GPS. In fact this thread reminded, me, and I spotted one locally for 20 bucks. I may check on it later.
Click to expand...

I plan on taking the car for a quick spin with the gps tonight. I will keep you posted.
 
Just ran it up to the grocery store with a gps in the car. Speedometer is about 5 mph off. So my 60 mph at 3200 rpm is 65 mph. At 60 I was just below 3000. Probably 2850.
 
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