Help selecting an "ideal" street rear end ratio

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slowmobile
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Help selecting an "ideal" street rear end ratio

Post by slowmobile » Tue Mar 01, 2016 12:51 pm

I am looking for something a little more highway friendly than my current 3.90s and 26" tires. This is a full-dress 1966 Olds Cutlass (maybe 3650 # with no driver) with a TH350 and a 470" Olds. I can provide the dyno sheet if that would help. I don't know the stall specs but the converter flashes pretty consistently to 3800 rpm.

I plan to go to a 28" tire. I want to minimize the 3.90 gear "fun-factor" loss as much as possible.

Thanks!

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Re: Help selecting an "ideal" street rear end ratio

Post by tjs44 » Tue Mar 01, 2016 1:24 pm

I have played with so many combos it's insane.Right now I have a 62 GP with a ST-10 with a 3.42 first with a 2.68 in the back.My 69 bird with a 3.27 first Tremic has a 3.31 in the back.My 421 63 lemans has a 2.67(?) first TKO 600 with a 3.90 in the back.The thing that is very close in all is a crusing highway RPM of about 2000.So you can see 3 diff combos that got to that end.If to do over I like the 2.67 ish first with the 3.90 and the .67 OD to get me there.In fact I really think with my cam intake I could go to a 3.55 in the back without having to pull down into 4th too often.Tom

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Re: Help selecting an "ideal" street rear end ratio

Post by Brian P » Tue Mar 01, 2016 6:49 pm

I ran the numbers and I get 2800 rpm at actual 60 mph with the 28" tall tires and your 3.90 ratio with no overdrive and that doesn't include the torque converter slippage. My tiny econobox isn't even geared that short!

Taller axle ratio will help up to a point but eventually your high-slippage torque converter will slip too much if you gear it tall. And as you note, it won't help with the fun factor. I'd like 2000-ish rpm at 60 mph also.

The real fix for this is the same fix that the auto manufacturers have implemented: wider ratio spread and more ratio choices and lock up that torque converter whenever you can.

Any possibility at all of going to a 4 speed automatic with a lock-up torque converter? I'm not familiar enough with the old stuff to know what will fit. 4L80E with a standalone controller?

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Re: Help selecting an "ideal" street rear end ratio

Post by peejay » Tue Mar 01, 2016 7:08 pm

I did a 700R4 in a '68 with a 455 and 4.11 gears, worked out real nice. You can buy new shifter parts for 4 speed action with the stock shifter, even get a new gear indicator panel too.

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Re: Help selecting an "ideal" street rear end ratio

Post by PackardV8 » Tue Mar 01, 2016 8:40 pm

I was there back in the bad old days and caught between slow shifting Borg-Warner 3-speed overdrives and slick-shifting but direct top 4-speeds. The best automatics were only 3-speed direct top. There was then and is today no po'boy way to get ideal dig off the line and ideal cruising RPMs. Fortunately, today, all it takes is money to get there. Today's multi-speed multi-overdrive manual and automatic transmissions can deliver the best of both worlds. Six speeds is as near as your checkbook.
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Re: Help selecting an "ideal" street rear end ratio

Post by F-BIRD'88 » Wed Mar 02, 2016 11:57 am

Going from a 26" tire to a 28" tire only drops the cruise rpm by 200 rpm.

If you change to a 30" rear tire the rpm drops by 400 rpm.

30" tire P275 70R 15 70 series tire
A P225 70R 15 will probabily fit the front.
Then when you go drag racing use a P235 60R15 drag radial 26-8.5-15 (true height 25.5")

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Re: Help selecting an "ideal" street rear end ratio

Post by Speedbump » Wed Mar 02, 2016 12:29 pm

I've fought the same battle with the 55 Chevy PU, 505 CID BBC. I have a 4.11 rear and a 31" Hoosier tire. Started with a switch pitch TH-400 and a DNE overdrive on the rear. Worked with that combo/converter for a year and just couldn't get happy. I finally threw money at a 4L80/ HD 3K stall lock up converter done by Joel on Joy. It's only been around the block a couple of times so far but the difference is crazy good. the launch gear ratio is the same as the old in direct and the OD is the same as the old high in OD and you can feel the converter lock up for cruise. I also got the TCI controller which gives tons of options including full manual shift at the flip of a switch. Haven't got it all sorted out yet but my advice for a sweet ride like yours seems, go for the newer E OD trans. With your CID I would think four gears would do but there are six out there if you want the wow factor.
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Re: Help selecting an "ideal" street rear end ratio

Post by slowmobile » Wed Mar 02, 2016 3:36 pm

I'd love to have an OD trans. The guys that build 2004Rs and 700R4s are reluctant to stick one of their transmissions behind this thing. It seems to be right on the threshold of what the majority of them are uncomfortable with. Peak torque was 617 ft/lb at 4400 rpm. That doesn't seem all that outrageous to me...

Thanks so far for the insight.

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Re: Help selecting an "ideal" street rear end ratio

Post by Speedbump » Wed Mar 02, 2016 4:13 pm

Exactly why I went with the 4L80E. I'm willing to pay for reliable AND power.
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Re: Help selecting an "ideal" street rear end ratio

Post by pdq67 » Thu Mar 03, 2016 2:58 pm

This stuff is why I would damned near give my left nut for one of the 1969 Cougar Eliminator 2-speed OD rear ends!! Ford only made like 6 or maybe 7 prototypes before shit-canning it!!

What a waste!

Funny, if not mistaken, the sucker was made like a 2-speed 12-bolt that doesn't exist instead of a 2-speed 9" rear..... Go figure about Ford doing things???

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Re: Help selecting an "ideal" street rear end ratio

Post by autogear » Thu Mar 03, 2016 3:12 pm

Pardon my stupidity; but why not a Gearvendors on your current trans or TH400. I dunno how well it plays in your tunnel though.

Mentioned elsewhere in the thread
TKO600s are 2.87:1 1st gear
Legend 5spds are a 2.66:1 1st

PDQ:
Dana built that axle. Theres a fair article on it in the June '69 issue of HRM titled 'two-upmanship" Its definitely the way to do it best from an engineering perspective

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Re: Help selecting an "ideal" street rear end ratio

Post by Dan Timberlake » Thu Mar 03, 2016 8:27 pm

autogear wrote:Pardon my stupidity; but why not a Gearvendors on your current trans or TH400. I dunno how well it plays in your tunnel though.

Mentioned elsewhere in the thread
TKO600s are 2.87:1 1st gear
Legend 5spds are a 2.66:1 1st

PDQ:
Dana built that axle. Theres a fair article on it in the June '69 issue of HRM titled 'two-upmanship" Its definitely the way to do it best from an engineering perspective
==============

Looks like PDQ was able to sweet talk somebody into scanning the HRM Eliminator article back in 2007
http://www.camaros.net/forums/11-bench- ... -test.html

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Re: Help selecting an "ideal" street rear end ratio

Post by slowmobile » Fri Mar 04, 2016 11:01 am

autogear wrote:Pardon my stupidity; but why not a Gearvendors on your current trans or TH400. I dunno how well it plays in your tunnel though.

Mentioned elsewhere in the thread
TKO600s are 2.87:1 1st gear
Legend 5spds are a 2.66:1 1st

PDQ:
Dana built that axle. Theres a fair article on it in the June '69 issue of HRM titled 'two-upmanship" Its definitely the way to do it best from an engineering perspective
I am actually thinking this might be the best route to take. I wish the OD ratio was a little better but it would sure knock the rpm down at highway speeds nonetheless....

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Re: Help selecting an "ideal" street rear end ratio

Post by Speedbump » Fri Mar 04, 2016 11:54 am

They make a good unit. I had the DNE unit which is only a manual shift. That part was just OK. The down side is, depending on your engine's manners, you still have the converter to screw up your cruise RPM. With the "E" overdrive trans, you can get a serious HD lock up converter + 3500 stall and, IMO and recent experience, that's the key to good cruise AND idling comfortably in gear with a 243 @ .050 cam.
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Re: Help selecting an "ideal" street rear end ratio

Post by Rizzle » Sun Mar 06, 2016 11:20 am

There is the possibility of using a universal bell housing TH350C if converter slippage is playing a large part in cruising rpm. Might also be possible to do a high stall/lockup to mimic a 6 speed (I've driven some older equipment which did the high stall/lockup, shift into converter, and lockup again to effectively double the trans gears) , with a slightly taller rear gear to noticeably drop the cruise rpm.
That is, if you can find a th350C converter with a strong enough lockup to take your high torque numbers...however, it should let you use the majority of your current go-fast trans parts.

Or, and probably the better idea, is the 4l85e.

What type of shifter/shift pattern do you currently have?

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