I am about to order gears for my 66. i am running a t5 with a .68 OD and 25" tall tires.
which will suit better?
I do about 50% highway driving but i do want to have a good launch off the line.

I have heard great things about both. I welcome all opinions.

My buddy has the 3.55s set up in his 85 qith the same t5 and loves it but he also says first gear is about useless. as soon as he lets out the clutch its time to shift.

what to do

 

Need more info. What ratio 1st gear do you have? What kind of cam in the motor? How tall are your tires?

 

I'm running 3.55's with a TKO 600, and with the 2.89 1st, 1st is usable on the street, but you have to roll into the throttle. The T5's are like a 3.27 or 3.35 1st.

 

This winter I am going to put in a trac lok with 3.80 gears into the 8 inch.
It is a 289 with a T5.
Willing to give up a little bit of gas for better acceleration.

Good Luck and BE Safe
Ron

 

I have the same exact issue for mine! I'm in between the same gear ratios as you and I have the same exact setup. The only thing I need to do is see if I want to fork over the $$ for posi or open rear. it's about $400 difference between the two

 

T-5 have a 1st gear of 2.95. They have all the ratios at: ddperformance.com
(FYI: The TKO-500 is 3.27. That's what I have with a 3.00 rear, works great in 1st)

I have heard 3.55 is about perfect for a T-5.

 

I think 3.80s would be okay

in first 2500 will give you 17mph and 4000 rpm at 26 and finally 6000 rpm at 39mph

In 5th 2500 will give you 71 mph and 3500 rpm will give you 86 mph

-Gun

 

Not all T5's have the same first gear, which is why I ask. I believe the T5z has a 3.27, while the standard world class trannies have a 2.95.

 

just as Gun says thee, check out one of the ratio calculators
25" is on the smaller side of things and 3.8:1 is away from best for highway side of things. You will only know what you crunch all those numbers into a calculator
some people don't mind if engine turns 2000 or 2500 at 60mph but need every bit of accelleration, i'm the other way around.
I like my 3.5:1 with the T5. again, punch the numbers.
Needs to be a calculator that accepts rear end ratio, gearbox ratios and tire size.

 

For only 7% difference between the two, you could go either way unless there's some very specific need to be able to use a particular gear at a specific speed.

Since the '66 is one of the lighter-weight years, it's easier to justify 3.55's for that car than it would be for a similarly powered but heavier car.

One of my cars is about at 10 lbs/cubic inch with 25.5" tires, 3.73's and a TR3550 (think light-duty TKO). But before they started shedding teeth, I had 3.27's back there which was entirely acceptable for daily driving and highway (full disclosure - it's an EFI car that does make a bit more midrange torque).


Norm