Rear End Gear Ratio

[Hubert]

I have purchased a 1974 Roadrunner about 2 years ago. It was an original automatic and had been switched to a 4 speed. An agressive cam had been installed as well as a dual double pumper carburetor. It has a 318 engine. When i got the car it had 323 rear end gears and was extremely sluggish when taking off in 1st gear, sometimes even stalling out. Moving today, i had 373 gears installed and was still a little sluggish. Future investigation it was thought the carb was way to big for the engine, so i installed a new holly 650. All issues resolved. Problem now is at 45 mph in 4th gear the engine is running at about 3100 rpm. Sounds like it going to come out from under the hood. Any suggestions to lowering the rpm. Should i go back to 323 or even mayby 273?

 

[Don Frelier]

Nice car.
But 3100 rpm at 45 just can't be correct with those gears. Are you sure your tach is set up correctly?

 

[Jerry Hall]

3:23 and 3:55's are what most run for a all around gear. Taller rear tires also help give a lower rpm at today's speeds.

 

[VANDAN]

Check out some 3:55's. They are a Great All-round gear...

 

[dadsbee]

Me and my tires love my 2.94's... cops hated those gears though. Today I wish I had of threw in some stock 3.23's during my resto, but it is what it is.. my son's problem now!

 

[Darter6]

Mr.Hubert, Finding the middle of the road when it comes to gearing,sometimes you have to compromise.
First what are the types of driving you normally do ? The 3.73 will keep it fun unless you drive mostly highway and not many secondary roads.You can try to tame it down a bit by adjusting the rear tire height.Looking at your photo I will guess a height of 26'' You may want to try 28'' or my preference a 30'' tire height.
You may not be happy with the 3.23 with the cam you have. Something you may have to deal with.2.73 is great for highway speeds of 65/75 mph but you may be riding the clutch allot with that cam.
I recently went from 4.10 to 3.23 in my car and it felt like I put a Slant Six under the hood at first but now I am used to it.
4.10's were fun but now I live where any direction 3 miles from home the speed limits are 65 to 75 and most of the time you will be going 80 to 90 to keep the tailgaters from riding the back bumper and cussing you out. I used to get the finger allot.Dang Vegas drivers....

 

[davek]

Go with the 3:23 I have 3:23 in my 440-4spd Bee and they work just fine.

 

[Hubert]

Nice car.
But 3100 rpm at 45 just can't be correct with those gears. Are you sure your tach is set up correctly?

Yes it is set up correct. I forgot to mention it has an agressive cam installed as well. Tires are correct size. I also had been reading that mayby a change in the transmission gear ratio would help. Per Passons they believe that 4th gear is to high and suggest a complete over haul to get gear ratios more in sinc if you will to have 4th gear more like an overdrive with .80 ratio.

 

[Hubert]

Mr.Hubert, Finding the middle of the road when it comes to gearing,sometimes you have to compromise.
First what are the types of driving you normally do ? The 3.73 will keep it fun unless you drive mostly highway and not many secondary roads.You can try to tame it down a bit by adjusting the rear tire height.Looking at your photo I will guess a height of 26'' You may want to try 28'' or my preference a 30'' tire height.
You may not be happy with the 3.23 with the cam you have. Something you may have to deal with.2.73 is great for highway speeds of 65/75 mph but you may be riding the clutch allot with that cam.
I recently went from 4.10 to 3.23 in my car and it felt like I put a Slant Six under the hood at first but now I am used to it.
4.10's were fun but now I live where any direction 3 miles from home the speed limits are 65 to 75 and most of the time you will be going 80 to 90 to keep the tailgaters from riding the back bumper and cussing you out. I used to get the finger allot.Dang Vegas drivers....

Very good info. I do hardly any xpressway driving at this time but might in the future. The tires are what they are which is a standard size for that year car. Most of my driving is local with a little 55-60 speed limits.

 

[BSB67]

3100 rpm at 45 is not correct. Maybe 65 mph @ 3100, or maybe 2200 rpm at 45.

Either the speedo is wrong, or the tach is wrong, or maybe both.

 

[Don Frelier]

And the cam has nothing to do with engine rpm.

 

[coloradodave]

Measure the height of the rear tires. Then we can tell you what the speed and rpm should be. Are you sure the transmission is shifting into 3rd gear?

 

[Cranky]

Aggressive cam....how aggressive? Do you know? And a 650 Holley isn't exactly small for a teen. Is it a DP too? What was the other carb? Aggressive cams and big carbs on a small engine generally do not like 'street' gears behind a small block. With a small engine, it's best to think small as in small tube headers (if it has headers), small cam, smallish carb.....you get the idea. Now a small race engine is a bit different because they usually see lots of rpm but don't think that's what you have here. Also would like to know how aggressive that cam is because if you're looking for a street cruiser, the cam might be too big.

 

[BSB67]

...........Aggressive cams and big carbs on a small engine generally do not like 'street' gears behind a small block in a big car....

 

[440 4 speed]

Would changing the speedometer at the trans get him closer to correct rpm and speed?

 

[440 4 speed]

Oops! Meant to say speedometer gear, in the above post,

 

[Ron H]

In my built poly I went from 3:23 to 3:55's. With a lot of the driving I do, living in the sticks, would a preferred staying with the 23's. At around 65-70 it's turning 3 grand. You can run it all day long this way; but is a waste and irritating. I didn't find any thrilling difference btw the two off the line but don't get into burnouts so should have left this alone saving me some cash.

 

[Don Frelier]

Would changing the speedometer at the trans get him closer to correct rpm and speed?

If he went up with the rear ratio the speedometer would read low.
Maybe his clutch is slipping.

 

[Cranky]

Even a light weight car matters lol

 

[Challenger340]

Yes it is set up correct. I forgot to mention it has an agressive cam installed as well. Tires are correct size. I also had been reading that mayby a change in the transmission gear ratio would help. Per Passons they believe that 4th gear is to high and suggest a complete over haul to get gear ratios more in sinc if you will to have 4th gear more like an overdrive with .80 ratio.

Tire Diameter can make a huge difference in Engine rpm at highway speeds/cruising, IMO, try a taller 27"-28" Tire with those 3.73 Gears