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Discussion Starter #1
I'm debating on changing my final OD from .68 to .80, because my engine doesn't like to be lower than 2000 rpm with the cam I have. The majority of my driving is on roads that are posted at 50mph, which puts me around 1800 rpm and 60mph is just over 2000rpm. It's only a matter of time before my luck runs out and I get caught speeding while trying to keep the engine happy. I want to change the final ratio,but I'm trying to find what will work at 50-60 and 60-70mph without getting too high in rpm's. I am aware my combination was never meant to save fuel, but if I can at least get it optimum I'd be happy. My other option is to go with a 411 rear gear rather then the 373, but I really didn't want to change where the other 4 gears are and the 295 1st, is low enough.

Anyone else run .72 or .80 final ratio with their OD, what are your thoughts?
 

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Discussion Starter #3
I'm running a 255 60 15 BFG and wanted to stay with this size, because I can also get a drag radial in the same size. 235 60 15 is the next size for the MT SS tire.

15 inch tires are starting to slim down now that they aren't the norm like they were a few years back.
 

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Discussion Starter #5
Gotcha. Wasn't sure if you were, say, a 275/60 and could step down to a 275/50 or something like that.
Thanks for mentioning it, because that could have been the easiest fix.:)
 

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Ever think of just driving in 3rd gear? That's a lot cheaper than gears and tires...
 

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1969 Nova . . 2dr . . Chino Valley,Az USA
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Paul ..

my 5th gear is 0.72 using my stock 3.08 rear ...

I like it -- you told me to "Drive it first .. then think about gears"

for my 250 / 6 .. I think I'm good-to-go.

FYI ;

I did buy a new 5th gear & 5th gear 'slider' ... when I rebuilt my T 5 ... from 'Paul Cangialosi - and the correct gears the 1st time' .

(that was just the Big Gear ... not the small one -- I was just replacing a worn gear . If your changing "ratio's -- you need both the big gear & small gear" : may be the slider also.

.... jim
 

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Discussion Starter #8
Ever think of just driving in 3rd gear? That's a lot cheaper than gears and tires...
I can drive in 4th and get about 12mpg at 50mph or at 60mph in 5th at get 16-18mpg, so if I can improve my mileage at where I do most of my driving, it's justified. The .80 transmission gears and 12 bolt 411's I already have, the only cost will be my time to change them out and with the cost of fuel up here (almost $6/gallon) makes it worth while.
 

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1969 Nova . . 2dr . . Chino Valley,Az USA
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looking at O.D. -- small & large 5th gears (the slider may be the same for all 'ratios' ... I don't know that).

#1



#2 -- re-used the small gear -- only replaced the "larger 5th gear / with 61 tooth (large section) count teeth.



#3



#4 -- inside teeth on 5th gear 'slider'



#5 -- New Slider comes Complete ( "springs, keys, & new BRASS synchro ring ).


old slider & gear :



#6 -- new 5th gear & "complete" new slider




just gives an idea of the gears ... FYI

.... jim
 

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I’ve got a 4l80 in the 66 with a lock up converter on 26-27” tire and it drives great. I think at 70mph it’s at 2700-2800 rpm with 4.10 gear.
Love it.

In my 72 powerglide with gear vendors od and 31” tall street tire. Haven’t put it on the street but at 60mph with a 3.70 gear we calculate 2114rpm.

I had a 700r4 in the 66- not my favorite transmission. 1st to second gear drop is undesireable. Plus I wouldn’t advise one if running over450hp. I had what I believe may have been valve body problems but after upgrading all that we knew to in that trans I gave up and it’s currently in storage.

4l80e is great. Just heavy and had to modify trans tunnel in the 66 to fit.
 

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Discussion Starter #11
Paul ..

my 5th gear is 0.72 using my stock 3.08 rear ...

I like it -- you told me to "Drive it first .. then think about gears"

for my 250 / 6 .. I think I'm good-to-go.

FYI ;

I did buy a new 5th gear & 5th gear 'slider' ... when I rebuilt my T 5 ... from 'Paul Cangialosi - and the correct gears the 1st time' .

(that was just the Big Gear ... not the small one -- I was just replacing a worn gear . If your changing "ratio's -- you need both the big gear & small gear" : may be the slider also.

.... jim

I got lucky when I had my T5 built, he gave me both gears for the .80 OD made by G-Force, they were ordered and paid for but the guy changed his mind. Unfortunately there is no .72 for my build, I would have to change my first gear to the 335 and I don't really want a lower first gear. I'll have to calculate a 390 and 411 ratio and compare it to the .80 OD to see what will work best for my set up. The beauty with the G-Force OD, I'll be able to stand on the gas in OD compared to the stock gear.
 

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1969 Nova . . 2dr . . Chino Valley,Az USA
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" cost of fuel up here (almost $6/gallon) makes it worth while".

wow ......... I look @ Safeway (here in Chino valley , today) = $ 3.09

and in Prescott @ Frys Food Store == $ 2.99 ......... , sorry ....


I SEE U'R Point with a Gear Change ..... :yes:
 

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Discussion Starter #13
I’ve got a 4l80 in the 66 with a lock up converter on 26-27” tire and it drives great. I think at 70mph it’s at 2700-2800 rpm with 4.10 gear.
Love it.

In my 72 powerglide with gear vendors od and 31” tall street tire. Haven’t put it on the street but at 60mph with a 3.70 gear we calculate 2114rpm.

I had a 700r4 in the 66- not my favorite transmission. 1st to second gear drop is undesireable. Plus I wouldn’t advise one if running over450hp. I had what I believe may have been valve body problems but after upgrading all that we knew to in that trans I gave up and it’s currently in storage.

4l80e is great. Just heavy and had to modify trans tunnel in the 66 to fit.
Your 4l80e should be .75 OD so very close to what I'm looking at for final ratio, so with my 373's this might be ok.:yes:
 

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Rear Gear

My 66 has LS3 with 4L65E trans. Coan 3000 lock up converter. 12 bolt 3.73. Tru Trac. 245-45-17 tire (25.68 ") 2150 RPM at 70 MPH. 22 MPG.
 

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You don't want to drive around in 4th because your fuel mileage goes down, but you want to change the OD ratio to raise the RPM you cruise at. Don't that will make your fuel milage go down just like driving around in 4th now does. I would just drive in 4th.
 

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Discussion Starter #17
You don't want to drive around in 4th because your fuel mileage goes down, but you want to change the OD ratio to raise the RPM you cruise at. Don't that will make your fuel milage go down just like driving around in 4th now does. I would just drive in 4th.
With .68 OD 55 mph = 1736 rpm
65 mph = 2052 rpm
1.00 4th 55 mph = 2553 rpm


With .80 OD 55 mph = 2042 rpm
65 mph = 2414 rpm

This is from an online calculator and the first example is about where I am now and as you can see, I'm at 2553 rpm in 4th. Most of my distance driving is on secondary roads between 50-60mph, so I figure if I could maximize gearing at this speed and I still benefit at 65mph it would be better then what I have.
 

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I run 255/60-15's with 3.73 rear and a .62 OD... I run about 1750 at 60 MPH. When I chose my cam one of my objectives was to maximize low end torque (to help with the overdrive at lower speeds)... It is pretty happy running at 1750 or so. But I'm sure I sacrificed some top end performance to make it more comfortable at 1700 RPM.
 
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