1965 Dodge Dart high RPM issue

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shawn65

shawn65

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Ok....so I’m new to the Mopar world have i have been able to figure out a lot of things on my own and some manuals...So I bought a 65 dart 270 and I have rebuilt the motor 225 /6 this baby runs great.....my issue.... I took it out for a spin the other day and I was very happy...the 3 speed auto trans shifts real smooth no slippage I can feel.....But in third gear at about 45 MPH my RPM's are crazy high I don’t have a tachometer yet but I’m guessing like 4000 RPM’s……I’m lost on this one and could use some much needed help…..

Thanks in advance

:banghead::banghead::banghead:
 
Who knows, maybe somebody slipped a low gear into the rear axle?

This is EASY to chase down. Just google and find an online speed calculator, like this:

http://www.kabamus.com/garage/gears.html

http://vexer.com/automotive-tools/speed-rpm-calculator

etc etc. You can Google

You need your tire size, and Google around until you find a tire chart that shows the diameter of your rear tire size

Then figure your rear axle ratio.

First, block the front tires, and jack up ONE rear tire, be sure the parking brake is released, and that the trans is in neutral.

See if you can turn the jacked up wheel. If you can, you don't have Sure-Grip (limited slip) rear axle.

Now, watching the driveshaft, turn the tire EXACTLY 2, that's right 2 full turns, and count the driveshaft turns. A 3.55 will turn the shaft just a tiny bit past 3 1/2 turns, a 2.76 about 2 3/4, etc

Now, if you can NOT turn the one wheel, (AND CHECK that the parking brake is off, and in neutral) this means that you have a Sure Grip/ limited slip rear axle.

So now you will have to jack up BOTH rear tires, and turn one of them (the other will follow) just ONE turn, while watching the driveshaft. As before, the driveshaft turns will give you the axle ratio

WITHOUT doing this, and WITHOUT an engine tach, you are just guessing.

Another good cross check nowadays is if you have or can borrow a GPS which will give you just about the most accurate speed reading you can get.
 
Thanks I will try that in the morning... If by chance PO did slip low gears... Is replacing the ring and pinion a hard task? And where could i find the correct OEM replacement? I would like to drive this beaut farther than the grocery store. Pluse with the up coming spring fling I would like to go on the Cruse with other Mopars.:D
 
Ok so I don't have a sure grip... Turn the wheel 2 time... Drive shaft spun 2 3/4.... Which would be 2.76 gear ratio... Something else is wrong I'm thinking with 2.76 gears my RPM should not be so high I don't think.... But hey I'm not an exspert by any means.
 
this may sound a little uuughhh, but are you sure it is drive and that it has actually shifted into third? if you are running 4000 + rpms at 45 mph with a 2.76 gear in 3rd gear, something is certianly not right.
 
2.76 should be at about 2500 at 60, pretty fast gear back there. Count the shifts, manually cycle the gears from a stop, 1-2-3 and see if there is a 3rd gear. If you have no kickdown, youll blow through first in about 10 feet from a stop and itll burn up the trans in short order.
 
Ok so I decided to stop guessing and it's not as bad as I thought. So I got a tachometer and used a gps to get actual speed. I'm at 55MPH at 2800RPM. Still seems high to me but what do I know.....
 
Your original rear gear ratio was 3.23. As said before, make sure you are actually in 3rd at 45, sounds like you are in 2nd. As for changed gears, it would take 4.88 or 5.13 gears to get your revs that high in 3rd, neither of which are available for the 7 1/4 as far as I know.
 
Thanks everyone I appreciate the help... I love this car. I will get some pictures up soon. Oh this is funny when it's cold out my guages don't work but when it's warm out everything works great.... I guess it does not want to do anything when it's cold... Very odd.....
 
shawn65 said:
Ok so I decided to stop guessing and it's not as bad as I thought. So I got a tachometer and used a gps to get actual speed. I'm at 55MPH at 2800RPM. Still seems high to me but what do I know.....
Click to expand...

That's still high for a "cruiser"

What size are your tires?
 
Funny you bring that up. I was thinking the same thing... 175 65 r13. I'm trying to see what the Original size was.
 
Original size was about 6.50 X 13, optional 7.00 X 13 - modern equivalent about 185-80x13 or so.
 
Original rear axle ratio was in fact 2.93 with 225 engine and automatic transmission. You may still have that ratio, or someone might have swapped the rear axle assembly for a later 2.76 unit. Since both 2.76 and 2.93 are between 2¾ and 3 turns, you may want to count quarter-turns of the driveshaft (easy, just count each rear U-joint cap as it comes parallel to the ground while you turn the lifted tire through exactly two revolutions). If you count exactly eleven quarter-turns, it's 2.76. If you count closer to (but not quite) twelve quarter-turns, it's 2.93.

2.76 instead of 2.93 will make the engine run slower at any given road speed, but smaller-than-stock tires will make the engine run faster at any given road speed.

Tired/sagged engine and transmission mounts will make the engine noise a great deal more noticeable.
 
shawn65 said:
Ok so I decided to stop guessing and it's not as bad as I thought. So I got a tachometer and used a gps to get actual speed. I'm at 55MPH at 2800RPM. Still seems high to me but what do I know.....
Click to expand...

shawn65 said:
Funny you bring that up. I was thinking the same thing... 175 65 r13. I'm trying to see what the Original size was.
Click to expand...

OK, 2800, 55mph, 175/65 R13 which shows up as 22" diameter

According to what I come up with, that's somewhere between a 3.23 and a 3.55

A 3.23 gear, 22" tires yields 56.7 mph at 2800

A 3.55 gear, 22" tires yields 51.6 mph at 2800
 
I’m going to try an slap on some 180/85R13 and see if that helps any...I’m also going to spin the wheel again and count 1/4 turns and see what number i come up with... I fear that i need to find and replace the ring and pinion gears. But i want to do all avenues before i break down and do the rear-end. I have only done 1 in the past and it was on a Chevy and that was years ago. I can pay someone to do it but they want an arm and a leg. So next is to put close to original size tires on and see what happens. I’m also going to do a tranny flush for ***** and giggles.
 
Those tires are AWFULLY short. You might try and scare up some 14" wheels, some of the guys on here that have converted to BBP wheels might have some
 
Ya I agree I will look around and see what I can find.

This is interesting. If I change to a 185/85r13 from 175/65r13i will gain 10MPH at the same RPM.... I will change the tires tomorrow and also try to find 14 inch rims to change to... I really like my original hub caps though...oh well. Thank again guys very helpful much appreciated. I will let you know what happens tomorrow…
 
Shawn, where are you at? Bring your rear to my house and Ill do it up. Eww, that sounded BAD!!! Ventura? Bring it, a six pack, and some new gears down in a truck, take us 2 hours.
 
Your first measurement strongly suggests a 2.76 or 2.93 ratio. I can't figure out why you'd want to change that, and I don't see you talking about symptoms of a failing ring and pinion, so…why even thinking about replacing the ring and pinion?
 
At this moment I'm not. I'm taking the car tomorrow and get close to original tires put on it. The tires now are just to small I believe. Don't know why I did not think of it earlier. So different tires may be my remedy. I hope...
 
Look for a ratio tag on one of the cover bolts. If it's still there it should solve the mystery (I'm betting 3.23). 185/80-13 is a really common size (cheap) and fits an early A pretty well.

A slant will run all day at 3000 RPM, and shouldn't be especially noisy doing it. Make sure your exhaust system isn't touching the unibody anywhere.
 
Ok well got different tires and a world of difference. Now I'm at 60mph at about 2600 2700 RPM's. Glad it was just tires. Again thank you all for helping me out with this.
 
Oh ya.... I'm going to check that out now... It just seems no matter what I do I always come up with a different number.
 
You may want to check that the correct speedometer gear is installed for your tire size and axel ratio. 20 bucks for the right gear, if needed is a lot cheaper than a speeding ticket.
 
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