PDA

View Full Version : Correct rear gear ratio for 69 with 9"



Clsccmro
01-18-2006, 06:07 AM
I had posted this question in another thread but I think it got overlooked, so I'm asking it here again.

I'm trying to figure out what gear ratio I should get with the new 9" rear I am purchasing for my 69 Camaro. The car has about 350 horsepower with a TH350 trans and a 327 SBC. Right now it is running a stock 10-bolt. The car may get some small horsepower upgrades in the near future. I was thinking that I should get somewhere between 3.50 and 3.75, but I'm not really sure. Would I get more power to the ground with a 3.75, or more with the 3.50.

What happens when you put too high of a gear ratio in a mild horsepower vehicle?

Norm Peterson
01-18-2006, 10:31 AM
If you're going to use the car in any sort of competitive or performance driving, you could find yourself with either too much or too little gear for the course. This applies to drags, auto-x, open-road racing, or open-tracking.

For primarily street use, gearing that's too tall will catch you below the powerband more often (though the automatic can cover for you in such situations far better than if you were shifting for yourself). Too short mostly wastes gas, increases wear, and argues for a more careful warmup before hitting the Interstates in cold weather. I'd start with the cam mfr's recommendations, though I might make an adjustment for higher or lower typical highway speeds or mostly non-highway use.

I know that answer is a bit vague, but there are many factors. FWIW, my '79 Malibu with generally similar power from an EFI engine was OK with 3.27's on the street and highway (even pulling a heavy trailer in an OD 5th except up hills) but was definitely better on the local auto-x lots with 3.73's. For longer National courses or road course running I might well want something slightly taller.

Norm

6'9"Witha69
01-18-2006, 12:11 PM
I say 3.55s. I have friends with 3.73s and have experience with 2.73, 3.08 and 3.25. Many people I know find a nice compromise with 3.42 or 3.55 gears. Low enough to put the power down, tall enough to keep freeway driving easy (and I have an OD, 3.73 w/o OD is a pain).

CAMAROBOY69
01-18-2006, 12:17 PM
1. Are you going to keep that transmission in there? Any plans for a 4, 5, or 6 speed tranny in the near future?
2. Are you going to drive the car on the highway or long distances or what are you doing with it?
3. It also depends on your tires diameter.

Clsccmro
01-18-2006, 12:44 PM
I don't have any plans on changing transmissions anytime soon, since it is was rebuilt only a year ago. There are other areas that need to be focused on more than that.

I will be doing almost all city and highway driving, though this car is not my daily driver, luckily. I would like to drive longer distances with the car, however it is not currently reliable enough to take it very far.

I will also be upgrading to 4 18" wheels (rear 275/35/18, front 235/40/18).

CAMAROBOY69
01-18-2006, 12:46 PM
Somewhere around 3.55 otherwise you will hate that car on the highway with only having a 3 speed tranny. The rear gear of that tranny is probably 1:1
Especially since you said you are probably going to go with more hp in the near future.

Norm Peterson
01-18-2006, 12:56 PM
A TH350 is direct drive in top. There's also a little bit of slippage across the torque converter, as it isn't a lockup design (which also suggests choosing the taller of two close axle ratios).

Norm

CAMAROBOY69
01-18-2006, 01:00 PM
Its up to him. I have 4.11 gears and I am very bias towards high gears but with a 3 speed tranny i know he will hate anything above 3.55. My friend has 3.73 in his car with a 3 speed and now he hates his car. Never goes over 55 on the highway.
With 3.73 gears you will be reving around 3,100rpm at only 65mph. Not fun.
Oh mabey with taller your meant lean towards the 3.55. I always get that backwards.

Norm Peterson
01-18-2006, 01:16 PM
Oh mabey with taller your meant lean towards the 3.55. I always get that backwards.Yup. The descriptions "taller" and "shorter" are less apt to be misunderstood or confused if you think in terms of the mph that corresponds to the same engine speed for each of the two ratios.

Norm

CAMAROBOY69
01-18-2006, 01:18 PM
Yup. The descriptions "taller" and "shorter" are less apt to be misunderstood or confused if you think in terms of the mph that corresponds to the same engine speed for each of the two ratios.

Norm
Ok so think of taller more towards the engine MPH. Cool I like that and now I will never forget again. :usa:
"Shorter" gearing is lower MPH at same RPM.
"Taller" gearing is higher MPH at same RPM.

Norm Peterson
01-18-2006, 01:30 PM
I think you still have it backwards. Mentally fix the rpm (let's say at 3000 rpm) and let the mph "float". At this rpm, 3.50's might get you to 60 mph, while the 3.75's would only correspond to 56 mph.

"Taller" = more mph at the same rpm (3.50 is taller than 3.75)
"Shorter" = fewer mph at the same rpm

Norm

CAMAROBOY69
01-18-2006, 01:46 PM
Oh right taller for the MPH that makes more sense. I will correct that in the other post so everyone doesnt go insane. :throw:

Clsccmro
01-18-2006, 01:50 PM
so you guys are saying that I can pretty much expect to be hitting 3000 rpm somewhere around 65 mph, with a 3.50 gear?

Does that sound right?

CAMAROBOY69
01-18-2006, 01:52 PM
Around 2900rpm @ 65 with 3.55
Around 3100rpm @ 65 with 3.73

Clsccmro
01-18-2006, 01:54 PM
oh, ok

then the 3.50's will definitely work much better for the set-up I currently have.

Clsccmro
01-18-2006, 01:56 PM
So am I correct in saying that if I went with a 3.73 gear, that I would have tons of power off the line and very little at high speeds?

While with the 3.55 gear would have decent power in both areas?

Kenova
01-18-2006, 01:56 PM
If you plan on doing any long highway trips, I would say use the 3.55s. I'd even be tempted to use 3.30s or 3.23s. It is a nine inch, which makes it a little easier to change gearsets at a later time. It's always nice to have more than one set of gears that can be swapped in an hour or less.
Ken

Norm Peterson
01-18-2006, 02:38 PM
Keep in mind that those numbers are somewhat approximate and were for comparative purposes only. I was basing my math on a tire size good for 848 revolutions per mile.

Norm

vanzuuk1
01-18-2006, 02:52 PM
Taller is highway dreaming , shorter is highway screaming. Did I get that right?

skybu
01-20-2006, 05:33 AM
I am such a weeny any more when it comes to street gears. Over the years I have had street gears ranging from 2.29 all the way up to 4.56.
All of these with either a th350 or th400. I am settling in now at anything in the 3.00 - 3.50 good for the street. Depends on how revy the motor is.
A 327 with 3.55 should be about perfect. If you need a faster car get more motor not gears. I have my rear end down right now for bearings and I am changing from 3.42 to 3.08 in the process.

Kenova
01-20-2006, 07:24 PM
I am such a weeny any more when it comes to street gears. Over the years I have had street gears ranging from 2.29 all the way up to 4.56.
All of these with either a th350 or th400. I am settling in now at anything in the 3.00 - 3.50 good for the street. Depends on how revy the motor is. .
:lmao: I know what you mean! I had 3.73s in my first car and thought nothing of driving all day at 60 mph. That put the revs at 3000 rpm. Now I hate the thought of spinning faster than 2000 rpm at sixty.
Damn I hate this "gettin' old" stuff LOL
Build it with lots of grunt and gear it tall.
Ken

hotroddr
01-24-2006, 11:00 PM
I have 3.30s in my firebird with a smallblock and th350 with a 9" and that works great for me. no problems on the freeway. Enough gear to cruise comfortably with better off the line than my factory 2.78s. I used to hit 75 mph in first gear tached out with the th350 and 2.78 gears...ughh.
My burban has 3.73 gears and thats a little "short" or numerically high for my taste for the long haul with no overdrive. 3500 rpms to kind of keep up with traffic before I put way too big of sideway tires on the back.