I have 3.70 in my 66. 70 mph @ 2,100 rpm
Car will feel alot quicker with 3.70

Here is what I have;
 331 ci FSB with Holly 670 Street Avenger carb run on 93 octane.
Baumann upgraded AOD.
8 " rear with 3.25 gears and Auburn gear traction control.
 25.5 " dia. rear tires.
1950 rpm @ 70 mph
 22-24 mpg highway,  on the open road I tend to hypermile.
MPG around town depends on how hard the skinny pedal is applied, but a guestimate would be about 10-15.
 I am pleased with the car's performance off the line, plenty quick for an old guy like me. 
 I thought about trying 3.55's but right now it seems to get out of first real quick, for me anything higher that that might render 1s't just about useless, for me anyway. 

My engine was cammed for torque and has plenty to pass on WV mountains without kicking down to direct drive. One of the criteria when building my car was for long distance multi day trips. 
 YMMV!

Bullet Bob wrote:

Just curious, Steve...what's the OD ratio in that TKO?

BB

The TKO 600 has a nice first gear ratio of 2.87 that will work fine on a street car with between 3:55 and 4:56 gear ratios. It has a second gear ratio of 1.89, a third gear ration of 1.28, a direct drive ratio of 1.0 in fourth and a 0.64:1 overdrive, which would give you an rpm reduction of 36% when compared to a 4 speed transmission.
 

Here is something else to consider.  Your camshaft profile will influence your choice if it is anywhere near aggressive.  For instance, a Comp 280 hr camshaft has a profile of 2,500-6,300 rpm.  You want your rpm to be above the 2,500 rpm range at highway speeds in overdrive.  Otherwise, the motor will be thrashing on itself.  That may be why one poster indicated that they went to a taller gear and improved performance without loss of economy.  Gotta know your cam profile, overdrive ratio, and tire diameter to calculate gear ratio at, say, 70 mph.
Best,
Al

TKOPerformance wrote:

When in doubt, just drive faster!

 Best advice yet

The 8" has 3.55s and 3.80s available new.  The difference between a 3.73 and a 3.80 ratio is almost meaningless in application, so if you want the 3.73 performance just get the 3.80s. 

Likewise, with the 9" It's really a 3.70 ratio, which in practice is going to be the same as a 3.73, especially when the next step up is a 3.89, and down is a 3.64. 

Its tough to keep track of all these different ratios specific to the specific rears, especially the old stuff.  The smartest thing Ford did when they built the 8.8 was to use gear ratios that were the same as commonly available bowtie ones.  I know that's sacrilege, but bowtie terminology is how most guys in the general car hobby reference rear end stuff, exhibit A is everyone calling a limited slip a "posi" regardless of make.  And, I know this is tough for most Ford guys to hear, but the 8.8 is in most ways a Ford copy of a GM 12-bolt (12-bolt has an 8.7625" ring gear, its a c-clip Salisbury type rear).  The drop out center section rears were too expensive to produce, though in keeping with Ford fashion the 8.8 is now the popular swap axle for guys looking for strength on a budget. 

I cruise all day at 70 mph with a 3.25 traclock behind a world class 5 speed and a 351 windsor.  Had 3.70's which were way too low even for a stock 351.  Of course there is the occasional 80-85 mph running with Mustang Steve and trying to keep up.

mxjeffb wrote:

Hey Bob just a quick Question what is your tire size that you are running with the 3.55 gear?

​I'm running 225/45-17 which have a diameter of right near 25".  Before the GT brakes I had 15" wheels with 60 series tires of 205 or 215 (can't recall which) that had almost exactly the same diameter and keeping up with Glens 65 5.0 running across Wyoming at nearly 85 didn't seem to make things buzz too much.

​I don't have a tach but my homebrew cacalater says that 70mph on my setup with the AOD .67 OD will spin the engine at just under 2250.  85 mph = just over 2700.

BB
 

Last edited by Bullet Bob (2/24/2017 7:16 PM)

What about some 3.73s I rode in a 66 fast back today that had a 408 AOD and the fitech efi conversion. It's running a 235/45r/17 and a 245/50r/17 in the rear. It went way nice in the city and on the freeway.

 Here is where I'm at,
3.55 at 70mph 25" tire 2238 RPM's
3.80 at 70mph 25" tire 2395 RPM's

But I'm running a 235/60/16 tire at 27" in the rear.
3.55 at 70mph 27" tire 2072 RPM's
3.80 at 70mph 27" tire 2218 RPM's

So as Bob is saying that he likes his 3.55 and I also think the RPM range looks good with the 3.55 and 25" tire.
I think I'm going 3.80 because of the fact I like to run a bigger 27" tire in the rear and that brings me to about the same RPM range as Bob and a good blend of off the line and highway driving.


I'm running the car flat on springs with 25" tires in front and 27" rear to give the car 1" down slope rear to front.
I know this is not the stock stance but I like a little more tire in the big rear fender well.

 

Last edited by mxjeffb (2/25/2017 11:05 AM)

mxjeffb wrote:

 Here is where I'm at,
3.55 at 70mph 25" tire 2238 RPM's
3.80 at 70mph 25" tire 2395 RPM's

But I'm running a 235/60/16 tire at 27" in the rear.
3.55 at 70mph 27" tire 2072 RPM's
3.80 at 70mph 27" tire 2218 RPM's

So as Bob is saying that he likes his 3.55 and I also think the RPM range looks good with the 3.55 and 25" tire.
I think I'm going 3.80 because of the fact I like to run a bigger 27" tire in the rear and that brings me to about the same RPM range as Bob and a good blend of off the line and highway driving.


I'm running the car flat on springs with 25" tires in front and 27" rear to give the car 1" down slope rear to front.
I know this is not the stock stance but I like a little more tire in the big rear fender well.

 

Man, that is a good lookin' HT, Jeff.  Sits perfect for my old eyes.  Are you going to be able to join the Texas bunch for the cruise to the bash?  It's not really much of a cruise, only 600 miles or so but you'd probably still have a good time.  Hope to meet you there.

​You do realize that my RPM numbers are with an OD.

​John:  I felt a little guilty about that after I found out you were running the C4. 

BB

Last edited by Bullet Bob (2/25/2017 11:23 AM)

50vert wrote:

Ron68 wrote:

Not wanting to steal a man's thread, but I have a couple of questions along the same lines.
351W with 98 Explorer GT40 heads, Comp Cams XE256H-10 cam, 4R70W trans with a 9 inch 3.25 Trac Lok rear end, 245/45/17 rear tires (235/45/17 in front) 60 mph gives me about 1900 rpm. Great on the freeway, but in town doing about 35 or so, in OD, it tends to lug a bit. Also, the off the line performance isn't quite what I thought it would be. I'm wondering about going to shorter gears, maybe the 3.55 or 3.70. Oh FYI gurus - what should I do? (remember, I used to run a C4 and 60 was about 2900 rpm) Mostly local driving with some occasional freeway trips.

3.55 rear with the wide ratio 4R70W is the equivalent of a 4.11 rear with a C4 in first gear so of the line, ya oughtta be able to light them up at will.
I believe that 35 is way too slow to be engaging overdrive with a 3.25 rear. How are you tuning the 4R70W? Baumann box? I don't use one, but I'm sure there is a way to adjust shift points under a lot of different conditions.
 

I am using the (Baumann) US Shift Quick 1 control box and I am using two of the pre-programmed shift programs. I guess I could go in and try to set up my own programming, but I admit it seems a bit complicated to do that. It looks like you need two people to do that process, one to drive, and one to run the laptop, and they need to be savvy on what the programming requires. One of those" just gotta bite the bullet and do it" deals. Even so, I am still pondering a change in the rear gear ratio.
I see on the US Shift website that I can update my Quick 1 box to a newer Quick 4 box without too much cost or trouble. I may look into that. I'm sure the software has been updated, as the Quick 1 software seems really clunky and dated.