Hi,

Currently restoring a 1967 valiant with 225L slant and 904tf.

my question is will I notice a different if I upgrade my diff gears to a 3.45?:1 ring and pinion or even a little bit higher if I can find it?

I've been told I will have quicker acceleration, but is it noticeable or is it a waste of (AUD)$300?

Thanks, Ben

Changing your rear end gears is one of the quickest ways to notice a difference in the way your car will take off, however it is a trade-off. The closer to 1:1 you get the higher your top speed, as you increase this ratio you will gain a lot of off-the-line-quickness (all other things being constant of course).

I'm sure others will chime in with solid recommendations and more info.

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'66 Dodge Dart

I base my rear end gear choice primarily on how fast I want to cruise and how fast I can stand the engine spinning. If you like to take long high-speed road trips I think you'll find 3.45:1 to be noisy and uncomfortable. The ET and acceleration gains as compared to something close to 3:1 are small. I would spend my $300 other places first. Order of upgrades I would recommend is electronic ignition, exhaust, intake, compression, oversize valves and head porting, camshaft, torque converter. This assumes you already have disc brakes and the chassis is in good shape.

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Joshua

I second Mr. Skinner.

Currently my car has 3.55:1 gearing, with 14 inch wheels, which yields about 3000 rpm at 60 mph. After an hour of traveling, one gets tired of the buzzing, and wishes he could shift out of second gear… But… oh my, it is all ready in third.

I also have high compression, over sized valves, 4V carburetor, headers, twin pipes out the back, some kind of rpm happy cam, and the engine loves spinning 3000 to 5000 rpm, something a stocker won’t come close to.

If you have a stock engine, stick with the ratio that came with the car, it won’t like a steady diet of high rpm.

Josh:
It is most important to have a safe vehicle with suspension and brakes in good working order.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

As a reference, with a 2.76 ratio and 14" wheels with P225-60s, 60 mph cruise is 2500 rpm. So about 500 rpm less than Bill's car. At 2500 rpm the engine is in it's sweet spot for torque and is not working very hard....

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Ted,

I would have thought it would lower. 2.76 / 3.55 x 3000 rpm = 2300 rpm. My tires are: P205/70R 14 or about 24.5â€￾ in diameter.

Ops, now I see you have 60 series skins, a bit smaller in diameter than my 70’s so there is that 200 rpm difference.

Ben, just wondering, do you have 13 inch or 14 inch wheels?

As you can see, just one size difference can raise or lower rpm at a given road speed. If I had Ted’s skins mounted on my Rallye Wheels, I would be turning 3200 rpm at 60 mph.

So what I’m getting at here is, you can change your gear ratio a little by going up or down in tire diameter.

Bill

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

3.55s are too much for a daily driver,since it's a '67 car,you probly have 2.94 gears (last year before most cars got 2.76s) check your gear ratio,I consider 3.23s to be the best,but if yours has 2.94s,It isn't worth changing it.

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V8?? We don't need no stink'n V8!!

I am fairly sure they are 14 inch, pretty small.

Guessing the bigger the wheel the less RPM?

I am still thinking of the 3.55 Fuel econ isn't something I'm worrying about at this stage (not at all). And the extra noise... The car I drive atm Is quite loud.

Engine isn't fully stock had abit of work:

2bbl Holley long runner intake
extractors
head ported and polished
head shaved
new elec ignition system

next step oversize valves and thinking a more radical cam but that's a different story.

If the wheel size makes a difference I guess I will look into seeing if I can put on bigger wheels? Is this possible or am I going to get clearance issues?

1967 VC Valiant.

Brussell,

This gearing thing is all about converting revolutions per minute to revolutions per mile. To match an engine’s torque & power curves for optimal economy and performance.

I apologize if you all ready under stand the following tutorial, take hart, someone else out there will be able to make use of this example.

When transmission is in top gear, third in these old cars, every turn of the crank shaft, equals the turns of the tail shaft, coming from the transmission, or 1:1; that is; no gear reduction, direct drive.

When that rotation goes through the rear end, it is reduced by whatever gearing your rearend has, be it 2.94:1 or 4:11:1. In other words, the drive shaft turning at the same speed as the crank shaft has to rotate 2.94 times to every turn of a rear wheel, 4.11 times per wheel turn, or whatever you have for a ratio. Tire diameter also contributes to the final drive ratio as well, where a small diameter tire has to rotate more times, than a large diameter tire to travel the same distance, in this example; one mile.

The reason for this mathematical manipulation is to find a rearend ratio to match the rpm of an engine’s torque sweet-spot, or if racing, peak out put at the ¼ mile point.

Once you know several variables, one can calculate number of wheel turns per mile, and determine revolutions per minute at a given speed.

Circumference of tire in inches:
D= diameter C= Circumference

C = 3.14 x D

Distance:
One mile = 5,280 ft or 12 x 5,280’ = 63,360 inches

Number of wheel rotations per mile:

63,360 / C = revolutions per mile of rear wheel

Example:

Using a tire P205/70R14 that has a 24.5 inch diameter, and a rearend with 3.24:1 ratio, find engine rpm while traveling at 60 mph.
Circumference of tire, or length of tread in inches:
C = 3.14 x D

C = 3.14 x 24.25â€￾ = 76.93â€￾

Number of tire revolutions per mile:
63,360â€￾ / 76.93â€￾ = 823.60 rpm

Rearend gear x wheel revolutions = 3.24 x 823.60 = 2,668.46 rev. per mile of drive shaft & engine.

At 60 miles an hour, one covers a mile in one minute. So; 2,668.46 rev per mile equals rev per minute.

With 3.24: 1 gear and P205/70R14 tire, the engine is turning up at 2,668 rpm

To calculate first (2.45) & second gear (1.45) rpm at 60 mph, multiply direct drive rpm by gear’s ratio.
Second gear rpm = 2668 rpm x 1.45 = 3868 rpm
First gear rpm = 2668 rpm x 2.45 = 6536 rpm

This example gives one an idea how engine speed and vehicle velocity relate. Knowing your engine’s operating parameters, one can select proper gearing, and tire size (up to a point) to optimize performance. Generally stock gearing & tire size will be best suited to a stock engine build. Any modifications and your on your own to figure out the best combination for what one wants to do with the car, be it street & strip, or full race…

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Ben,
Since your not worried about mileage, sounds like you could use some lower gears and a higher stall torque converter if you go to a more radical cam.
Will this car be used for racing?
or just a fun street car?
I was assuming that you were trying to improve torque and mileage to reduce the fuel costs........

_________________
Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

This car is just a fun street car.

If I put in a 3.55:1 and take out my stock I can just swap them back if I'm not happy correct?

It'll be a while before I get the new cam, bigger valves and stuff. Next step for me is the higher compression.

It's a chore on a stock rear, unless you have a 8 3/4 rear. They are much faster because you swap out the pumpkins in a few hours. On a stock rear you need to unhook the springs and shocks......a few more steps.....

_________________
Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

You could have the best of both worlds. Run you 3.55's with a 25.5-26 tall tire for the street and get a set of 24" slicks/drag radials for the drag strip.Much easier than a gear swap in a 7 1/4.