9" Gear Ratio

What would be a good gear ratio for this 9"? I do alot of driving on country roads, 60-70mph. And Ill be running some tall skinnys in the rear. Probably just a 350 turbo trans.

 

A 3.00 or a 3.25 would be my choice

 

What trans? Ratios all make a big difference. Check out this page with the starting line ratio, I got mine right in the sweet spot and I love it.
http://www.5speedtransmissions.com/startlineratio.html
This one will give you your RPM at certain speeds.
http://www.5speedtransmissions.com/gear_calculator.html

Good luck

 

I agree 100%.
Are you going to run a auto,auto with o/d,4 or 5spd?Also what size tires are you going to run? Overall height of the tire will also make a big diff.
It is hard to give any info without first knowing the whole picture.

 

TH350 and tall skinnys is what he said...so 3.0 to 3.25 does sound just right

 

Tall skinny prob 28" + -3:00"s I,m thinking 60 mph around 1600rpm with yer th350. Thats doing a comparison with the same set up on my coupe.

 

3:00 will be a great set up for what you want . The 3:25's will just get you out of the hole a little better .

 

I have 3.50 in the 9" under my Model A PU. It has a Y-block cranking thru a Toploader 4spd with 15 x 8.20's. It's a little much on the highway. I'm planning on using a 3.00 in the 33 Sedan I'm building now.

 

I run a 2.50 rear gear with a turbo 350,at the races use 1st and 2nd gear only .Works great on the interstate,not ideal for racing but getting low to mid 12's in 3000 lb a coupe with lots of tire spin.

 

I appreciate all the help guys! Im just looking for something cool to cruise around town in, not taking it to the strip at all, maybe a few dirt roads, haha!

 

92* day, 3.00 diff and 30" tall tires with an accurate speedometer.



When I ran 3.70's, the tach would be running 3000 rpm or so at the same speed.

Performance in the 2400# car didn't seem to fall off to any great degree.

 

I also have experimented with different gear ratios under my 32 Roadster ... from a 2.75 to a 3.91

A 3.91 and a 3.70 create too much wheel spin in a light car with street tires. A 3.50 is OK ... and it pulls very hard ... but the engine gets kind of busy @ 70/75 MPH. A 3.00 or a 3.25 is just about the same ... cannot really tell the difference in them ( in the car ... except a few RPMS on the tack ) and a 2.75 starts to lug the car in town. OK out on the interstate @ 75/80 MPH but my car actually gets BETTER mileage with the 3.00 over the 2.75. Ford also had a 3.10 back in the late 50's. I have used one but it is like a 3.00 or 3.25 ... just harder to find.

 

To add to the data:

9" rear end with 3.25's with a 28" tire, 2800 RPM @ 55 mph. (Toploader T&C 4-spd. transmission.)

 

sbc 4speed 8inch rear end 3.00gear at 65mph 2275rpm with 235 75 R15 tire 24 miles per gallon great highway gear

 

I am running a 3.25 gear in my 33 with a set of 235/75 tires. I am very happy with this setup as far as interstate driving. No overdrive just 4 speed muncie. I was running a set of 3.70 they were a lot more fun around town but got old on the road for very long.

 

What kind of car? If you are talking about a Model A, I'd actually say 3.50. I had a 3.00 with 29 inch tires and it just never felt responsive enough off the line. I know you're not going to drag it, but the whole point of driving a hotrod is to be able to get a little wheel spin and at least *feel* like the car could take the guy next to you. I ended up with a 3.89 and I love it although it's about as high as you'd want to go without an O/D transmission...

 

I have that setup on my 31.

I came up with 2800 RPM @70.
(T-400.)


Where's the math police when you need them?

 

try this link. very helpful.

http://www.angelfire.com/fl/procrastination/rear.html

 

3.25 gear, 28" tire 70 mph = 2730
3.00 gear, 28" tire 70 mph = 2520

Simple math really, (or cheat and use the Richmond Gear Calculator)

 

Thats 3155rpm at 70.

 

There's alot of good advice here man. This is a question you'll want to sit down and do some math on, then decide for sure how you're gonna drive it, and think about it a little more. My wifes comet has a 289 with c4 and 3.50 ish gears in the back. Match that with those little tiny 14 inch cookie cutter tires and NO fenderwell room for bigger meats = one overworked screamin poor little sbf motor the way she drives! Next present she gets is a 8 inch posi with 3.00 gears. Its all that thing needs! Play with the numbers before you decide man. -GothY-

 

Math police say all you need is percent of change formula if you only want to change one thing at a time.

 

Do the math. Where do you want to be rpm-wise?

 

If the tire diameter has the same numbers as the rear gear, you will get about 2000 rpm at 60. Stated another way, that condition gives engine speed at 90 the same as the rear gear.

I ran a 3.00 gear and 30 inch tire on my roadster for a long time. Worked really well.

 

Originally Posted by T-Time
To add to the data:

9" rear end with 3.25's with a 28" tire, 2800 RPM @ 55 mph. (Toploader T&C 4-spd. transmission.)

Yah, I saw this also and cant believe it. especially with a 4 sp that is locked 1:1 in 4th. I run 3:73s with 28.5" tall tires and do almost 70 mph at 3,000 rpm.

 

Opps! My bad...yes, at 70, not 55.

 

[QUOTE
Yah, I saw this also and cant believe it. especially with a 4 sp that is locked 1:1 in 4th. I run 3:73s with 28.5" tall tires and do almost 70 mph at 3,000 rpm.[/QUOTE]

3078 by doing the math