Big tire guys what gear ratio are you running?

Getting closer to having the back half car to the point of purchasing a center section for it. I have 4 quotes from builders but they all have different gear ratio suggestions. 3 Strange centers vs 1 Moser. Gear recommendations from 3.89.4.11,4.56 and 4.88. Rear is a Ford 9 inch with a set of offset length 35 spline axles. Moser axles and 13 inch drum brakes.. The car is a 70 Buick that will be in the 3200 lb range hopefully. TH400 transmission and a 464 with Aluminum Stage 2 heads. 3 inch exhaust fed by TA Stage 2 headers. Tires are MT 31x18.50x15 on widened Buick rally rims 15x14. I am not planning of racing this car as it's going to be a more streetable car. No huge cam or such. Want to be a dependable ride to drive on Sundays etc. From all the gear ratio pages, I keep getting 3.89 gear selection. I am just hoping that it accelerates well and not be a slug, but I don't want to run 3000 plus at 70 mph. I'd rather keep the TH400 and am not wanting a Gear Vendor unit at this time. A 200 or 700 trans isn't going to happen either. The guy that suggested the 4.11 is really a good guy and stands behind his work. I have friends that he did work for. I just don't want the higher rpms like the 3.90 in my other car. He suggests a Detroit Tru Trac posi unit, Strange center section and I forget the gears name.Am I paranoid or will the 4.11 be too much? I know I have a tall tire,but it's eating at me..

 

70 stage 1 The original owner took the 364s gears out and installed 430 gears back in the day. With a 27 -28 inch tire 40-45 mph at 3000 rpm. You Definitely need to pick the right cam and stall converter also.

 

Less math fill in the blanks, been actuate for me. Your 3.89 should be about 3000 at 70mph

https://spicerparts.com/calculators/engine-rpm-calculator

 

Yep if you look at the link under 3speed in 3rd, thats the same number, the auto figures in some converter slip, but since every converter is different that number is just a round about close estimate. I see too many numbers in my life so less math is good.......1 less thing I can mess up

 

So, I talked to the builder and went with his suggestions. But very similar to my research.
Yukon Nodular case 3.250 bore
Daytona Billet 1350 yoke
35 spline Tru Trac Posi unit
Motive Gear in 3.70 ratio
New Timken bearings,small parts, gasket, and set up to go

It will run about 2807 rpms at 70mph. I may decide later to get a Gear Vendors OD unit,but I don't think that rpms range it terrible to cruise at.The Nodular center is 20 lbs heavier than the aluminum one,which doesn't worry me..

 

I would be surprised if those tires actually measured to 31"..

 

Yup...31 inches ......

 

Larry, How much gear ratio do you lose with a Gear Vendor unit? Not worried about fuel mileage,but losing some rpms.

 

Usually the tires are no where close to their advertised diameter.

 

Thanks Larry..I know of a near new GV setup sitting on a shelf that I am going to start bugging the guy...

Joe, The advertised size is 31.0 but when I got to investigating,it was closer to 31.20 inches..They are kinda heavy with the Buick wheels too..

 

Old thread. Good question in my opinion. I'm psyched to get my car together and drive it. I'd like to skip the traiiler for trips under 2 hours.



Above are the cam specs I have. I believe I have a 3000-3400 converter. I do have 3.55 gears in my 9" rear. 27.8" rear tire ( no load). For freeway cruising about 70 mph, I believe a 3.00 gear is likely in my future?


Thoughts?

 

4.56 with the same sized tire

 

....and I'd hit about 90 in 1/8 mile at 7000 rpm?

 

31x18.5-15 tire. It was hitting 128 mph in the quarter with the Th-350 crossing the traps at 6700. For street use I swapped in a billet 2004R and it cruises the highway at 2800 RPM at 80 MPH. 2300 rpm at 65 mph. There is no converter slip with the lockup converter which is nice as it still has the 3800 stall to launch the car. The new engine is built to rev to 7500 but keeping the revs down is nice so the overdrive is good.

 

So you're having success with a built 2004r. I really do want a 5 or 6 speed manual but the cost is up there a bit. A built 200 is about half the cost of a Tremec.

I don't know a lot about the engine I have in the car. It's a strong runner and a huge cam. My guess is to get the car where I want it to be (civilized with mongo torque), I'll need to swap the cam/converter and change my gear ratio from 3.55 to 3.00.

Sometimes it's better to just live with what you have and enjoy it for what it is. Use the trailer when needed?

 

If I ever do it again I will use a gm 6 speed auto, but yes so far the 200 has been great. It was about $4000. There is nothing better than being able to shift into overdrive on the highway, highly recommended. Art car has been building 1000 Hp 200s for a long time. Lots of GN guys going fast with the 200 as well. My car is under 3000 pounds so that helps too.