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Discussion Starter · #1 ·
I am in the prcess of rebuilding the 8" rear in my 66 fastback. I have a set of 3:50 gears in now but am considering going to a 3:80 or 4:11 ratio. The engine is a 289 with 10:1 comp 220 cam edlelbrock heads and rpm intake with 1 5/8 headers and a t5 tranny. I put the 3:50 gears in when I had a 4 speed in the car but now that I have overdrive it seems like I should go with more gear. What do you think is the best gear to run with this combo?
 

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Go with 4.11. A t5 overdrive fifth is a .68 ratio. Multiply .68x4.11 is 2.79, new final ratio, much higher than 3.50.
 

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Discussion Starter · #3 ·
4:11 was my first choice, but I am concerned that with the 3:35 first gear ratio in the t5 it may be too much for the car.
 

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Depending on the size of the rear tire. A 26 inch tire with 4.11 at 6000 rpms will top out and shift gears at at 34 mph. A 3.80 ratio will go 36.75 mph according to the calculator. Check out Project 67 in the Project Cars link.
 

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Discussion Starter · #6 ·
The tires I have are 25" tall, the other thing is that most likely I will be switching to the 2.95 first gear cluster t5 next year. That would make more sense for the 4:11 I think.
 

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On 2006-04-16 14:38, Matte wrote:
When more power for passing on freway is needed isn´t it possible to use the possibility to change gear rather than changing axleratio..?
Sounds easyier to me...
Yeah, I agree. But it seems to me he is after more rpms at a certain speed to be able to floor it and pass without shifting down.
Also, the engine isn't working as hard to increase speed.
 

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On 2006-04-16 14:52, mikes82GT wrote:
On 2006-04-16 14:38, Matte wrote:
When more power for passing on freway is needed isn´t it possible to use the possibility to change gear rather than changing axleratio..?
Sounds easyier to me...
Yeah, I agree. But it seems to me he is after more rpms at a certain speed to be able to floor it and pass without shifting down.
Also, the engine isn't working as hard to increase speed.
Yes... but.. aren´t we installing 5-speed to keep rpms down at highwayspeeds? and then changing gearratios untill revs are closer and closer to non OD revs seems nonlogical to me... OK 4:11 and od makes 2:79 or so that isn´t to bad. I have been running 3.0 and fourspeed aond that you can live with but one wishes for lover revs at highways..
 

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OD is used for cruising, not aceleration. As mentioned, it allows your engine to drop revs, after you get up to speed.

There are kits and builders that can put together a T-5 with a .80, .81 .83 or so OD instead of the .67 value that is normally used. That gets you closer to a working 5-speed transmission. To be really strict the T-5 is more a 4 speed with Overdrive 5 gear. A true 5 speed would have 5th as 1:1.

No one reads the owners manual, but in there is advice to the same effect.

With a 3.35 1st gear, going over 3.50 or 3.55, with street tires (commonly 26" dia.)is a waste of 1st gear. If you are using 28" dia. tires/slicks it might be worth it.
 

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Have 2.95 first t5 with 4.11's right now. Love it. Will be going with 4.30's in the soon to be 9 inch.
 

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If your street driving with street tires the 4.10 are to much with a T-5 3.35 first gear. (1 gear become useless) I would stick with the 3.50's you have it gives you an excellent 11.72 ratio in first and a great ratio in OD for cruisin.....
 

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Promotion built my T5z with a 0.59 OD, I run 4.30's and Love it, 5th gear is 2.55:1. Almost like a 6 speed minus 5th gear.
 

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I"m running a 3.35 first gear and a 3.89 rear gear in a 9 inch. First gear is almost non existant in my car. It makes for a good hole shot but you might as well start off in second. But I built with plans on installing a 2.95 first gear and 1.1 5th gear. So if you are planning on changing to a 4.11 in the near future go ahead and get the higher first gear cause you won't even use it if you don't.
 

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I used to run a 3.35 first gear T5 with 4.33:1, shift to 2nd comes very quickly. Last year changed to a 2.95:1 first gear (G-Force). This year I will try swapping the 4.33:1 to a 3.89:1. Now this is with a low reving (6500rpm) stroker. If I was still running a 289c.i.... I think 4.11 or 4.33:1 , a short tire and the 2.95 first gear would be a good match. On a side note, I don't think your 8" will survive very long!
 

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I ran the same setup...3.35 low T5 in a '84 GT..mild 302 (ran 7.90 in the 1/8 on street tires). It came with 3.73's in the 7.5 POS rear end. I put 4.10's in. ET dropped, MPG dropped, first gear was now totally useless with street tires (could induce tire spin EASILY and 1st gear was VERY short winded....hit the throttle, be ready to shift). I normally took off in 2nd gear with the 4.10's and had no worries about it. Went back to the 3.73s and never looked back.

Why not try to find a set of gears somewhere in the middle between 3.55 and 4.11? Maybe you can find a set of 3.80s? That'd be pretty dern close to ideal with the 3.35 low and .68 OD. I loved the .68 OD with the 3.73's. Car had enough power to pull to about 5400 in OD. Dunno how fast that is, nor do I care....one of the stupidest things I've ever done next to topping out my old crotch rocket at 170+.
 
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