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Username Post: Rear end ratio (gear ratio)
widall2
Poster
Posts 43
02-08-19 07:25 AM - Post#2759377    

What rear end ratio is recommended for a 1955 car with a stock 6 cylinder with power glide, P215 75 R15 tires that is mostly driven on the highway on 100 mile + trips? Also what is a safe RPM for highway driving that provides long engine life?
TR56210
Poster
Posts 63
02-08-19 08:45 AM - Post#2759385    

3.55 should be the stock gear ratio with power glide.

My opinion, engine longevity, is more about over maintenance than in RPM. Have a compression test done to assess overall health. Then check and change oil, cap, rotor, plugs etc per schedule and you are doing everything right. You could temporarily install a tachometer, wire to coil, power and ground, to confirm rpm. If it is the increased rpm when under load, then that’s more a limitation of the transmission not having an overdrive.

Can’t tell from your post if your concern is increasing mpg, highway performance or the power glide being a power glide.
widall2
Poster
Posts 43
02-08-19 11:00 AM - Post#2759397    

Thanks for the response. I am interested in improving gas mileage and not beating up the engine. At 65 MPH my engine is turning about 3000 RPM. My engine is in very good condition and I want to keep it that way. We take several 1000 mile trips per year in this car.

TR56210
Poster
Posts 63
02-08-19 12:28 PM - Post#2759403    

I opted for a 700r4 transmission over a 400, to get the overdrive feature and I hope save gas and more highway friendly.

Other than manual transmission, 3 or 4 speed, most would say the straight 6 is the most economical to drive. I am a fan of the 700r, uncertain as to mounting to a 6 cylinder motor. Maybe some company makes an add on overdrive for the powerglide. The best I think you could get was to drop the rpm from 3000 to maybe 2200.

4.11 gears might help.

Changing rear gear ratio, might get you to around 2000, maybe.

Until that power glide gives up the ghost, I would keep driving it. Once it does, you might have some options that might get you more Mpg.




craig32
Frequent Contributor
Posts 1300
02-09-19 02:06 PM - Post#2759492    

4.11 gears will hurt, 3.36 gears will help.


Original57
Poster
Posts 43
02-09-19 06:35 PM - Post#2759512    

I had the same concern with my '57 but a sml block v8 and 3 speed on the column and 3.55 rear end. I was about to install a 3.08 rear end ($1k)simply to reduce rpm when a friend set me up with the old technology manual trans. Borg Warner R-10. It reduces RPM 29%. Basically adding a 4th gear. Has idiosyncrasies,
but worth it. I did not have to alter drive shaft

I DO NOT know if the old R-10 will fit on a 6 cyl without research.
a cheap option is to get tall tires on the rear but your speedometer reading & riding height would be off but you can check mph via GPS.

Some guys on chevy talk reeaallyy helped me get mine going. The were so knowledgeable, I could look up the old posts and you could ask them questions.
Gene_Schneider
Ultra Senior Member
Posts 12204
02-10-19 02:41 PM - Post#2759571    

If the engine is in good condition 65 to 70 MPH is a safe speed. A 3.36 will reduce your engine speed by only 5%. A 3.08 will be like have a 1000 pound trailer on the back power wise. 3000 is a safe RPM for that engine.
ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible

widall2
Poster
Posts 43
02-11-19 08:48 AM - Post#2759620    

Thanks to all of you for your input. From this and some from friends I think I will go with changing to a 3.36 gear. My ’55 presently has a 3.70 gear, the car was originally purchased by a lady school teacher and may have been ordered special. I used a cheap tach to check RPM so my RPM numbers may be incorrect. I like the ides of an automatic with overdrive however adaptors would have to be used and then the final ratio I believe would be too fast for the horsepower of the 6 cylinder. I have several late 50’s 3 speed plus overdrive transmissions and I would go that way if I was 20 years younger. I even have a 4 speed light duty transmission (I think a Saginaw) and a heavy duty curved side three speed plus overdrive unit that, based on articles that I have read, can be adapted to the 4 speed with minor alterations. Some day I will dig out the article and put this stuff up for sale.

One more question, will 3.36 gears fit in the ‘55/’56 third member? I have read that they will not because the pinion gear is so big that the ring gear can not be moved over far enough. That article says the third member must be ’57 to ’64. Have any of you had any experience with this?

Gene_Schneider
Ultra Senior Member
Posts 12204
02-11-19 11:19 AM - Post#2759641    

Chevrolet had no option for a 3.70 ratio in a Power Glide.
With a Power Glide you will have about a 4% gain in engine speed due to the torque converter. That is why lock-up converters were used. I am sure you have a 3.55 - the diff. number stanped into the front of the housing on the right side will ID the ratio.
ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible

radioheaterwsw tires
Poster
Posts 26
02-16-19 02:43 PM - Post#2760060    

  • Gene_Schneider Said:
Chevrolet had no option for a 3.70 ratio in a Power Glide.
With a Power Glide you will have about a 4% gain in engine speed due to the torque converter. That is why lock-up converters were used. I am sure you have a 3.55 - the diff. number stanped into the front of the housing on the right side will ID the ratio.


Hi Gene:
From many years ago, being a high school'r having a used Powerglide equipped tri-five car, we kids always thought that there was about a 10% slip in these transmissions. I've never seen any printed reference to the percentage of slip. Is it so stated in any literature?
Also, is the 4% slip constant or fixed, regardless the vehicle speed or whether in low gear or drive? I always figured the least amount would be driving down a level highway at cruising speeds of say 50-70 mph. And the most going up a grade in low gear.
Would so appreciate your sharing your knowledge about this.
I always enjoy the info share on this site about old Chevies.

Rick_L
Honored Member
Posts 27564
Rick_L
02-16-19 05:21 PM - Post#2760070    

  • Quote:
One more question, will 3.36 gears fit in the ‘55/’56 third member? I have read that they will not because the pinion gear is so big that the ring gear can not be moved over far enough. That article says the third member must be ’57 to ’64. Have any of you had any experience with this?



Your information is correct. If you use a 3.36 (or 3.08) gear ratio you must use a 57-64 3rd member case. Same would be true of any gear set that's made for a 57-64 3rd member. You must use 55-56 gears if you have a 55-56 3rd member case!

Good news is that any complete 57-64 3rd member can be used in a 55-56 housing. If it is a posi you must shorten each axle shaft 1/8".

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