327-3.08 diff
Thread Starter
Heel & Toe
Joined: Sep 2008
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From: Belmont Ca
327-3.08 diff
I'm trying to avoid going to a 5 speed, is it possible to gear a m-20 and 3.08 with slightly bigger tire for hiway driving? What the biggest tire size you've used?? I've got 205x75x15 stock size.
Gary
Gary
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My dad has that combo on his 67 convertible and a 308 with a M21 on his coupe. you wont gain any real fuel millage. doing 55 or 60 you are still doing 2,500 3,000 rpm. getting around 15 or so.
Thread Starter
Heel & Toe
Joined: Sep 2008
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From: Belmont Ca
At present I'm at 70mph at 2,800rpm. If I go to a bigger tire, I can go down to 2500-2600 rpm. Not bad without changing the trans. But- can I start off in first gear with a 28.5-29 " tire ?? Tell me who's got this setup????
Tech Contributor
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From: Connecticut, USA
That might not be the best setup although a M-20 and 308 was a setup from the factory. I doubt you'll see a lot of improvement in MPG. The start up may be a dog as well. A BB with more torque works ok but a 327? On a similar note I built a 273 diff for my buddy with a base 180hp 76 4 spd. The 76 uses a ST-10 w/ a 2.64 first gear. I didn't think he'd like it but he loves it for cruising.
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From: columbus oh
Drifting
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From: nj
you talking an early vette?
horsepower and year?
How far and often do you drive 70mph and where is that limit legal?
Can't beat a 327-300 with a 3:08 for being livable under varying driving conditions especially top end.
Mathematically you can get some theoretical advantage messing around with tire size but you would do well to make sure your distributor and carb are set up correctly and you drive smoothly.
No problem getting rubber in 3 gears with my old 62 327-300 and 3:08s, if that's important to you.
How far and often do you drive 70mph and where is that limit legal?
Can't beat a 327-300 with a 3:08 for being livable under varying driving conditions especially top end.
Mathematically you can get some theoretical advantage messing around with tire size but you would do well to make sure your distributor and carb are set up correctly and you drive smoothly.
No problem getting rubber in 3 gears with my old 62 327-300 and 3:08s, if that's important to you.
Instructor
Joined: Oct 2000
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From: Picton Ontario
I guess it depends on the year of the car. I use 225/75 R15 tires on all 4 corners with 3.08s in the 57. Some cars can't stop 225/75 R15s from scrubbing the body or inner panels on the front.
Bob
Bob
Last edited by Bob W; Mar 14, 2009 at 05:13 PM.
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Burning Brakes
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From: Ventura Calif
For a non OD trans 3.08's and a M21 are about as good as it gets! Some people go to a new Richmond T-10 that has a 2.8(?) low gear for a little less slouch when starting. 205x75x15 are about a medium size tire. I use 215x70 on my Midyear (same OD) and 225x70's on the rear of my C1.
Thread Starter
Heel & Toe
Joined: Sep 2008
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From: Belmont Ca
I have a 66 coupe 327-300 with a m-20. Bought it in 2007 and I had to convince my wife this is my New old car. Anyway I love it except trying to make it more freeway friendly. I drive 30m to work and want to be a daily driver 2-3 times a week.
Last edited by bagman12; Mar 15, 2009 at 03:55 AM.
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From: eastern NC
When I first bought my '63 (with 3:70 gears and 4-speed) back in 69 I seemed to be going through rear tires a lot (4 pair in 9 months...
). It must have been an alignment issue... 
Anyway, a friend gave me a pair of 9.15-15's from his Dad's Cadillac (pretty tall!!!) that had quite a bit of tread left. He said they would last me for a while... They sure did, I could burn the clutch after that, but not those tires...
Don't remember how it affected the mileage though, but hey, at 30 cents a gallon for Sunoco 260, who cared...
GUSTO
). It must have been an alignment issue... Anyway, a friend gave me a pair of 9.15-15's from his Dad's Cadillac (pretty tall!!!) that had quite a bit of tread left. He said they would last me for a while... They sure did, I could burn the clutch after that, but not those tires...
Don't remember how it affected the mileage though, but hey, at 30 cents a gallon for Sunoco 260, who cared...
GUSTO
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From: Southeast Missouri
Unless this chart is out of date, several states have a speed limit less than 70. I think NJ still had 65 as a limit a few weeks ago when I was there, but don't remember positively.
http://www.motorists.org/speedlimits...d-limit-chart/
(After a quick check, I think here's a better link to US speed limits around the country . . . .http://en.wikipedia.org/wiki/Speed_l..._United_States)
http://www.motorists.org/speedlimits...d-limit-chart/
(After a quick check, I think here's a better link to US speed limits around the country . . . .http://en.wikipedia.org/wiki/Speed_l..._United_States)
Last edited by Ron Miller; Mar 15, 2009 at 08:17 PM.
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From: Bridgewater New Jersey
I have a 3.08 in my 65 L79 coupe. Did bog on launch with the M21. went to a 7/8 shaft M20 for the 2.56:1 first gear - still not a great launch but much improved over the old 2.20:1 gears.
That said - the 3.08 rear will give you 25.6 mph per 1000rpm (no allowances for slippage or tire expansion.)
So 70 mph is 2735 rpm This is calculated using the original 65 sales brochure. My Dunlop 215 75 15's hit this speed exactly (cross checking with a stopwatch and the roadside mile markers.) I get some rubbing in the front now as the springs are a little tired - with fresh springs or a lift kit you could probably add 3-5% with a taller tire - nothing compared to a 5 or 6 speed.
Mitch
That said - the 3.08 rear will give you 25.6 mph per 1000rpm (no allowances for slippage or tire expansion.)
So 70 mph is 2735 rpm This is calculated using the original 65 sales brochure. My Dunlop 215 75 15's hit this speed exactly (cross checking with a stopwatch and the roadside mile markers.) I get some rubbing in the front now as the springs are a little tired - with fresh springs or a lift kit you could probably add 3-5% with a taller tire - nothing compared to a 5 or 6 speed.
Mitch
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From: San Antonio Texas
It is 70 MPH on all freeways here in Texas.. 80 MPH out in West Texas. 65 at night..
I have 3.36 gears with close ratio 4 speed.. 427/390.
I can run ok at 65 - 70, but steadily get passed down here... Can also cruise ok at 80, but don't like to..
My tires are 215/70/15...
I will double check, but I think I am doing 3000 rpm's at 65..
Cheers, Zale
I have 3.36 gears with close ratio 4 speed.. 427/390.
I can run ok at 65 - 70, but steadily get passed down here... Can also cruise ok at 80, but don't like to..
My tires are 215/70/15...
I will double check, but I think I am doing 3000 rpm's at 65..
Cheers, Zale
Instructor
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From: Sonora CA
I have the same set up as you on my 67/327/300-4 spd with 205/75/15 radial tires, and I love it. I have had my car 40 years.
Presently on a long trip averaging 65 mph (60 to 70+ mph most times) I'll do 17.5 mpg. When it was new, I did 20 mpg coming cross country (NY to CA). If I averaged 55, I'd probably do 20 mpg again.
Off the line you will suck, but the high speed accelleration is great once you get past 60. A 3:08 with the standard 4 speed is a great highway cruising car. You can take on those Honda's but start at 60 mph.
Presently on a long trip averaging 65 mph (60 to 70+ mph most times) I'll do 17.5 mpg. When it was new, I did 20 mpg coming cross country (NY to CA). If I averaged 55, I'd probably do 20 mpg again.
Off the line you will suck, but the high speed accelleration is great once you get past 60. A 3:08 with the standard 4 speed is a great highway cruising car. You can take on those Honda's but start at 60 mph.
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With the exception of my Cutlass with a 4.55 and Richmond 5sp (2.41 posi), EVERYTHING I own now has a 3.08 posi and M20. Yes, leaving from a dead stop is not like a rocket, but from 10-15mph, they're gone! And they will wind forever in each gear! With the 56, it's just beginning to really come alive in 2nd gear at 60mph.
From my viewpoint, a 3.08 and M20 is an excellent combo.
You should see people clear a path when I get on the Interstate with the rusty POS 51. I love it!
Tom Parsons
From my viewpoint, a 3.08 and M20 is an excellent combo.
You should see people clear a path when I get on the Interstate with the rusty POS 51. I love it!
Tom Parsons
Last edited by DZAUTO; Mar 16, 2009 at 12:29 AM.
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From: Beach & High Desert Southern California
If your goal is a lower engine rpm cruising down the highway, the 3.08's and the M20 will accomplish the goal.
I don't know if it will gain any fuel mileage. I say this because a friend and I traded ourselves into a 70' Super Bee back when gas lines formed at every station (traded for a 59' Apache Truck, I don't know which would be worth more today). The Super Bee had the 383 with the pistol grip 833 (4-speed) and the 4-Barrel and air grabber hood. It was crude with a bench seat, and that shifter sticking up from the floor. It originally had 4.10 gears, that we swapped for a 2.73 pumpkin from a wagon (the Chrysler 8.75 made this easy). The ET's dropped from 15.45 to 16.35 (at some wild trap speed like 86 mph). The Super Bee would no longer wind out in 4th gear, with the 2.73's it would actually go faster in 3rd and lose speed after a shift to 4th (still well over the then 55 mph legal limit). The mileage, the expected increase we had hoped for, never improved. The good thing about the 2.73's was the car became much more civil at 65' or 70' (with a girlfriend snuggled next to me
on the big bench seat).
Good Luck with the gear choices (the modern 5-speeds do have an advantage).
I don't know if it will gain any fuel mileage. I say this because a friend and I traded ourselves into a 70' Super Bee back when gas lines formed at every station (traded for a 59' Apache Truck, I don't know which would be worth more today). The Super Bee had the 383 with the pistol grip 833 (4-speed) and the 4-Barrel and air grabber hood. It was crude with a bench seat, and that shifter sticking up from the floor. It originally had 4.10 gears, that we swapped for a 2.73 pumpkin from a wagon (the Chrysler 8.75 made this easy). The ET's dropped from 15.45 to 16.35 (at some wild trap speed like 86 mph). The Super Bee would no longer wind out in 4th gear, with the 2.73's it would actually go faster in 3rd and lose speed after a shift to 4th (still well over the then 55 mph legal limit). The mileage, the expected increase we had hoped for, never improved. The good thing about the 2.73's was the car became much more civil at 65' or 70' (with a girlfriend snuggled next to me
on the big bench seat).Good Luck with the gear choices (the modern 5-speeds do have an advantage).