• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Rear End Gear Ratio

Looking at his car he probably has a tire around 26". The rpm that he claims he's running means he has an 18" tire.

If he goes from a 26" tire to a 28" tire he'll drop 200 rpm at 60 mph.
 
Our stock 440 4 gear 3:54 with 275x15 x60 and original speedo drive gear shows 3300 at 65 mph. Actually it is going faster. When I did check with mileage markers on the 10 mile check, I was clocking 9 miles.
These are approximate because that was in 2016, and the records are put away
 
There is more that is not being told. I will wait til he comes clean. Can't be what he is telling us....
 
I have 3.91 in my car with a 440 and a lot of cam and I can still drive down the interstate. Not very far, but I can drive. :lol:

Something doesn't sound right about his explanation. He needs to clarify.
 
I had 2.71 open diff on my '70 Satellite. Now it's a 3.55 Yukon DuraGrip. I don't spend much time on the highway, but if I did I would have gone 3.23. I'm running a 440/727, and 28" tall tires. That's about 2750rpm at 60mph. I'm happy with my 3.55's. Best of luck.
 
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.
Mistake on my part. The carb is a 600 not 650 and not a DP. The carb that was on it was a DP 750. I also have the original carb which was a 2 barrel. Tks. for t It has the original dual exhaust manifolds, so no headers. Not sure how aggressive the cam is but here is but here is a photo of the crane cam with part # that was installed. I will admit i do not know much of anything about cams. Tks. for the help and info.

IMG_20190103_134030476.jpg
 
If this is the right cam card for what you posted, this is not a good cam for you. If you want to make life better, get a new cam

Even at 0.005' or 0.006" lift, this cam will still have a comparable advertised duration of 300°/310°. And even on a 114° LSA it will still have +70° overlap. Couldn't think of a worse cam for your application, with exhaust manifolds making it worse yet.

At this point I think your next step is to change the cam. After the cam change, revisit the gearing. At the end, you might go back to the 3.23, but I would not continue chasing gear ratio changes at this time.

Crane SSH-320.jpg
 
Last edited:
Agree, even with the correct matching components the card says recommended RPM range is 3200 to 6200 RPM. Terrible for a stock 318. Stay closer to a stock 340 cam if you must. Even then...
Still have the Hi-way speed RPM issue. Here is a list of most calculators available to determine a whole list of things you might want to know now and in the future. Have fun.
http://www.wallaceracing.com/Calculators.htm
 
Had a similar cam in 318 using a 600 DP. The whole combination was wrong but it went 13.51 @ 102. The thing was dead until 3000 rpm with a stock converter and 4.88 gears on 29" tall tires but it really lit off hard once it crossed that 3000 rpm point and stayed 'on the pipe' until 6200.
 
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?

4 Speed and sluggish, stalling off the line? I would bet the linkage is set up wrong or has wrong levers or rods. You may be starting off in a higher gear. May be possible.
 
Last edited:
Too many different questions being thrown at this post. Need to get ALL info from the poster on his car. Otherwise just a lot of guesses and opinions being thrown around, not really answering the problem(s).
He says that at 45 mph the engine speed is 3100 rpm. How did he determine the road speed of 45 mph ? GPS ? Any gear/tire changes would also change the speedometer calibrations. What size speedo gear is in the trans now ?
Stock size tire should have been an F78-14 wich is approximately 26.5"
Look on the driver door jam, unless it has been painted over it will give the OE tire/wheel size.
 
Last edited:
Yeah this truly IS a huge mess.

Everything is wrong, except one thing; the 3.73s are right, for a manual trans, and that 236/244/110 cam. . Anything less, with a low-cylinder pressure, small-cube engine, is gonna be sluggish; especially in a heavier car.

The first thing you gotta do is a compression test. and
the Second thing is to get your speed-o and tach sorted out; one of them is a big fat liar. and
the third thing to do is get rid of that 236/244/110 cam.

It's not a bad cam; it's just way too much for a 318, in that car. To properly use it, the 318 would need about 10.3 or better compression ratio. And since the power peak of that cam is up around 5400, the torque peak could be as late as 3900 . That cam may want to be upshifted another 300 to 500 over the top,so say 5800, and best performance will then be in the rpm range of 3900 to 5800.
Ok now; estimating a tire diameter of 26.5=83" roll-out,
and with 3.73s, and a standard 2.66 low A833;
3900 in first is 31mph when the cam wakes up.
and 5800=46 mph when it's time to catch Second.
And second will hit 5800 at 64 mph.
So, 3.73s are the right gear for this cam.
Therefore, if you have a 10.3 or better compression ratio, or 155 to 160 psi cylinder pressure, then you have a great combo, just too small for the weight of the car.
But
if you have an 8/1 Scr engine, your pressure is gonna be less than 120 psi, and take off from a stop will feel like a slanty; sluggish? Ohyeah bigtime sluggish!

BTW;
with 26.5s and 3.73s; 65 =3085 / 55=2610.
60 mph = one mile in one minute, and is 2840 rpm
Oh and Don't even think about running an od box in that combo.

BTW, great looking car!
 
Last edited:
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?

View attachment 1010159
The 323 would be best suited. You should have asked the Holley techs about proper carb, would have saved yourself a ton of money. I got a Holley Street avenger on my 383 RoadRunner its a 670 and plenty of carb. your 318 prbly could go to a 570. You have to match the carb to a formula engine size rpms etc. The Holley techs will do it for you on phone.
 
a small block 318 comes with 273 gear and a big block comes with 323 gear
 
OP hasn't been around since Monday. I think we're spinning our wheels...
 
Auto Transport Service
Back
Top