Gear choice for street
 

Currently have 2.56 gears and Saf-T-Trak rear un my 68 FB with 400HO and TH 400. I'm looking at replacing gears and carrier to either 3.36 or 3.55 and the correct posi unit. Plan to eventually put in an OD. Which would be the better street gears for the 400HO engine? Thanks

I would just wait until you decide WHAT OD and what final ratio you decide on in that trans.Tom

A buddy used 3.42 gears with his 2004R behind his 455.
Nice combination.

once upon a time i had an awful lot of fun with an M20, 3.90’s, and 26 inch tall tires ...

What are your goals with the car? Is it just for around town or do you plan to take it on the Interstate? I guess we need to know what speeds you want to cruise around at...

I like to end up with a final gear between 2.70 and 3.00.Tom

A move from a 2.56 to a 3.36 or 3.55 will be very welcome for sure. I have run both and a 3.73 as well without an overdrive transmission. Up to around 70 MPH, any of those gear choices are OK IMO. I did run the 3.73 in a 70 TA with 15" wheels. The other gears with 14" wheels. Obviously, if you want lazy, low RPM cruising at high road speeds, overdrive will be your next move.

OD is a real highway opener. The 4L80 is well ratioed for 3.54:1

I ran 4.11 and 3.73 back in the 70's with small block Ford's and Chevy's with one being a 69 Z with DZ engine. Just sold my 2010 Camaro SS that had 3.9l gears. Right now I have 26.5" 15inch tires on the Firebird. Plan to keep the Poncho engine. I'm looking for some spirited driving, cruising on the back roads with occasional hiway driving. The 2.56 are great on the hiway but lacking on the street and back roads. I know there's no "one size fits all" but looking for something to take advantage of the 400 engine without going too radical.

Very happy with my 2.75 rear gear in the 9" BUT I have a Nash 5 speed with 5th being 1 to 1.
As Tom S posted,try to get to a 2.7 to 3.0 as the final drive when on the highway.

Tom V.

3.23 is as high as I would recommend with 1:1 but if you go overdrive 3.73 is steep but very livable and will give you a lot of grunt off the line with a 26" tire

I have 3.89 rear gear, 28 inch tires and overdrive behind a 428 and it’s a great overall combination. If you really plan on using an OD in the future I would stay between 3.60 and 3.90 or you will be going in and out of od around town and missing a lot of fun.

An interesting tidbit is the 28" tire compared to the stock tire is almost exactly one gearset. I ordered my GTO with the 3.36 rear, but when I computed engine RPM the move to a 28" tall tire worked out nearly the same as if I had 3.23 gears and stock tires. Just something to keep in mind, and also why people with taller tires are also happier with a little lower gear choice.

Next also take into consideration what the overdrive ratio is with the trans you choose because the trans will also influence final drive ratio.
200-4R is .64
700-4R and 4L60e are .70
4L80e is .75
TKO-600 is .68

Mike,I think TKO 600 can be had in two diff OD ratios.Tom

Yep. .68 and .81 now. Used to be .64 and .82

The .81 isn't all that useful for an overdrive and not commonly chosen.

I'm with Tom on his original post. You might want to explore which overdrive you want before deciding on the final gear ratio.

From my experiences you'll probably want that 3.55 in there as I've found that's a great all around gear that works well with either scenario, and works nice with a .68 or .70 or even .75 overdrive ratio. Until the overdrive goes in, running a little taller tire on it makes it very livable on the highway as well.

I am running a 4L80E with a 3.55 rear end. I feel it is a perfect street/strip combination. My motor has plenty of torque, so 3.55 is more than enough to get the car moving and it's relaxing on the highway. 70 MPH at 2400 RPM, with converter locked up.

So that's a short tire combo. Probably around 26 inches.

Just to show what a taller tire will do.

My 4L80E setup runs 4.10's out back with a 28" tire and I can cruise 70 at 2600 rpm with converter locked. 2400 rpm is 65 mph.

Hell,I try to keep all mine about 2000 at highway speeds.Tom

That's usually what I like to have as well Tom and what I would recommend to others. This particular car I like the 4.10s' (been in the car for 35 years) and the 4L80E doesn't really offer enough overdrive to suit me but I needed the beef of that trans for this particular car so it's a little out of the norm.

All my other overdrive setups here cruise nice at 2000 rpm on the highway with either 3.55's or 3.73's and .63 to .64 overdrives.

I totally agree with your post Tom S. 2000 rpm at 75 mph for me, if I had a choice..
Lots of OEM Trucks cruise BELOW 2000 with their 10 speed trans at 75 mph. Smaller engines and big diameter tires

Tom V.