2 series rear end ratio driving experience? - Pontiac GTO Forum
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post #1 of 13 (permalink) Old 09-24-2018, 06:06 AM Thread Starter
 
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2 series rear end ratio driving experience?

Can anyone tell me their impression of having a rear end ratio thatís geared for economy and low revving Vs the usual 3 or 4 series rear?

I have a 3.55 open rear in my 72, and the engine is turning 3200 at 70. Memory is vague, but 30 years ago when I had my previous GTO this didnít seem to bother me too much, but now after driving cars with over drive all these years it seems like the motor screaming.

Not looking to drag race, and want to eventually go to a 5 speed, ($$$$$) but in the mean time I was thinking of swapping rears ($).
Thanks

WS
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post #2 of 13 (permalink) Old 09-24-2018, 06:18 AM
 
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Can anyone tell me their impression of having a rear end ratio that’s geared for economy and low revving Vs the usual 3 or 4 series rear?

I have a 3.55 open rear in my 72, and the engine is turning 3200 at 70. Memory is vague, but 30 years ago when I had my previous GTO this didn’t seem to bother me too much, but now after driving cars with over drive all these years it seems like the motor screaming.

Not looking to drag race, and want to eventually go to a 5 speed, ($$$$$) but in the mean time I was thinking of swapping rears ($).
Thanks

WS

I have a 3.55 posi in my '71 GTO with a TH400 transmission. I personally like that ratio. Gives me the best of both worlds. A little red light to red light action and I can cruise with it. I occasionally will go on an interstate and I drive about 60-62 mph at about 3000 rpm's. I'm not overly fond of the engine working that hard, but this is only 1-2 times a year for a short distance. My normal driving is about 45-50 mph. No rush. Not sure what your speed limit is or how often you constantly drive at 70 mph. Here in NY, we're 55 mph on the main roads and 65 mph on the interstate. Maybe a 3.23 or 3.08 would suit you better.
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post #3 of 13 (permalink) Old 09-24-2018, 06:30 AM Thread Starter
 
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I am also in NY, but frequently drive in Jersey and upstate NY. Speed limits are 55 to 65 and traffic is usually moving along at 70 to 80. I feel like a menace to navigation going 60 to 65 in the right lane of the GSP. Wondering what a 2.78 would be like before i go for a 5 speed in the future.
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post #4 of 13 (permalink) Old 09-24-2018, 09:19 AM
 
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Originally Posted by 11th Indian View Post
Can anyone tell me their impression of having a rear end ratio thatís geared for economy and low revving Vs the usual 3 or 4 series rear?

I have a 3.55 open rear in my 72, and the engine is turning 3200 at 70. Memory is vague, but 30 years ago when I had my previous GTO this didnít seem to bother me too much, but now after driving cars with over drive all these years it seems like the motor screaming.

Not looking to drag race, and want to eventually go to a 5 speed, ($$$$$) but in the mean time I was thinking of swapping rears ($).
Thanks

WS

Ahhh, 30 years ago....... national speed limit was 55MPH, so you probably did not have a problem with the gears you had. But now, many highways are 65-70 MPH with traffic rolling at 75-80MPH, or more. I personally get uncomfortable revving past 2600RPM's on those long trips, but some don't have a problem with it.

This is all assuming you have an automatic trans.

I have a '76 Lemans with 350CI/TH350, added a Q-jet, and 2.78 gearing. When it was on the road, I did not have a tach, but I could comfortably run 70MPH and the best I could squeeze out of it was almost 20MPG at steady flat road driving. The down side was around town. With that stiff of a gear, and the weight of the car, it requires more gas to get it rolling and up to speed only to be hitting your brakes for the next stop light. Gas mileage went down the tubes and it seemed like I got about 13-14MPG - it burned gas, and most of my driving back then was local and in town.

First, you are not driving a car that is going to get stellar gas mileage - they are not today's cars with the electronics, variable timing, fuel injection, lock-up converters, and overdrives. So my suggestion is to select a gear that satisfies both local and highway driving -short of installing an OD type transmission.

Try one of the online speed/RPM/gear calculators and see what turns up as a needed gear to run 70MPH @ 2500-2600 RPM's. 3.08's are a typical gear for highway cruising, but the 3.23 or 3.36 may be a better all around gear. Still not going to be zippy like 3.55's. You will lose performance with 3.36 through 2.78's. This is where the bigger cubes of a 455 excel as they have more torque to pull these kinds of gears and get the car rolling with less sacrifice in performance.

So there is no magic gear ratio that will give you everything you want. It will have to be a compromise to find some kind of middle ground based on your driving requirements and RPM range of the engine. The OD transmission is really the way to go nowadays, but more work, modifications, and $$$.

Also, I will add this. I have a '73 Plymouth Fury 360/727 trans, 2 Bbl, 2.76 gears. Chrysler offered different stall converters based on the car and engine combo. The '73 Fury with the 360CI got a factory 2500 RPM stall converter. It is a "loose" converter in that it takes a little gas to get the converter up to speed in order to get the car moving. BUT, this really adds to the performance of the car. Even with the stiff gears, it is in my opinion pretty zippy when you nail the gas and the engine goes into its lower power band and starts to pull hard. So around town, I have a little fun with it. On the highway, around here in Charlotte, it'll run 70MPH and not be breathing hard and I have no problem running 80MPH keeping up with the flow of outside lane traffic. At 95MPH its still running great and doesn't feel like it is screaming. Now I probably don't get 15-16MPG's with the car, but I did not buy it for gas mileage, that's why I have a Hyundai. But the point here is that you could consider a compromise between performance & highway cruising by going with a more modern "tight" 2500 stall converter that does not have the slip when taking off using reasonable acceleration, but will zing right on up to 2,500 RPM's when you nail the gas and give you that acceleration performance. Then at highway speeds of 70MPH and above, your engine RPM's will be down to a reasonable cruising speed.
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post #5 of 13 (permalink) Old 09-24-2018, 04:04 PM Thread Starter
 
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Thanks for the response and information.

I have 3 speed stick in the 72 and rarely need to start off in first gear. Driving around town with a stock low compression 400 the engine never feels labored. However on the highway taching out at 3000 rpm to go 70 does make the Flowmaster 40s get a bit noisy. I was wondering whatís the all around experience was from someone who actually had a similar car with a lazy rear gear ratio.

I figure that being able to go 70 at around 2500 would be preferable to having more response from a dead stop as I know this current engine is no power house, but it does have decent enough torque to get out of its own weight. I get about 10 MPG now and just accept that.

My previous project was a 1999 Corvette that ran mid 11s in the quarter and would top out over 175mph. Been there done that a few dozen times, made some vids and got it out of my blood. Going back to my youth with an old Pontiac that may keep me out of trouble,. I plan to eventually go with a 5 speed, but until then was wondering if a 2.78 or 3.08 so I can drive on the highway would be a total disappointment just motoring around town.
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post #6 of 13 (permalink) Old 09-24-2018, 07:46 PM
 
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Just curious. Where do you drive to in upstate NY??
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post #7 of 13 (permalink) Old 09-24-2018, 09:02 PM Thread Starter
 
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Just curious. Where do you drive to in upstate NY??
I have family in Goshen and friends in Middletown. Also like to drive through Bear Mountain, Seven Lakes drive and the area.
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post #8 of 13 (permalink) Old 09-25-2018, 09:48 AM
 
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"I have 3 speed stick in the 72 and rarely need to start off in first gear."

With a 3-speed manual trans, I would not go any lower than 3.23 - which many came from the factory with. The 3.08's may be too stiff of a gear to pull with a clutch.

My '68 Lemans had the 3-speed with 3.23's. I went with a taller tire which drops the ratio and was closer to a 3.08. It required more slip of the clutch to get rolling and if on any kind of hill/incline when pulling off, takes a little more. So you may burn up the clutch a little premature if you are OK with that. It worked for me, but I had a little more power than your 400CI to pull it. Pontiacs have torque, so the lower gears work. Would not even consider 2.78 with a manual trans.
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post #9 of 13 (permalink) Old 09-26-2018, 07:18 AM Thread Starter
 
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Good point about burning up the clutch.... something to consider......
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post #10 of 13 (permalink) Old 09-26-2018, 06:46 PM
 
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I just put a posi in my 69 rear end that had 3.23 gears with an open system. At 80 or 90 she purrs like a big cat - I like to run mine as fast as I can. I get plenty of launch off the start and a little brake pressure gets the tires spinning. I do have a 3 sp auto. I considered moving up to 3.55 but I didn't want the added RPMs on the highway and I really don't race it other than some "fun runs".

If you car was stock with 3.55 and you put posi in it keeping your current gears you'll be very happy with the results. Keeping the current gears will save you money, no need to change unless you don't like the gear set you have.

*67K miles... Under Construction* :)
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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