Hey Big Block guys, what diff gears are you running?

[68 50th]

I have a .100 over 454 and 4.10. Would really like to try a 3.54 and/or some kind of OD.

[Mike C]

Do the TH200 4R. The gear split is SO much better than the TH700R4 1-2 spread. And the TH200 has more overdrive. The 3.73 is ideal for the 200. Don't waste your money doing something with the rear only to go OD later. Just save for that point.

One thing you haven't mentioned is tire. You can get away with a lot more gear depending on tire height. A 26" tall tire will be screaming at 70 mph compared to a 31" tire.

[68 50th]

My truck is a C20 with 265/75x15 which is 30.65 tall.

[FstCarsNGuitars]

You are Correct Mike! I did forget to mention that I did switch to taller tires and it has helped a bit with the rpm's.

As mentioned before I'm going the route of the "built 200". I went to the swap-meet this weekend and a transmission vendor there gave me a strange look when I said I wanted to use a "built" 200 for my big block. He was trying to tell me that It would be a bad idea to use a 200 and then he continued by trying to sell me one if his "built" 700's.. I found his antics to be pretty shady...especially when I said the motor was a big block 454..he asked..is that a Chevy engine?

[FstCarsNGuitars]

Originally Posted by 68 50th

I have a .100 over 454 and 4.10. Would really like to try a 3.54 and/or some kind of OD.

According to the engine rpm calculator at this site: http://www.csgnetwork.com/multirpmcalc.html
The 3.54 gearing from your current 4.10 at your current tire height would decrease the rpm's about 450-500 (about 2590) at 65mph..that's a decent drop in rpm's. Although cruising at 70mph with about 2000 would be ideal IMO. In that case.. you're looking at $200-$300 for gears versus $1000 for a transmission

[TBone1970]

Hi Steeveedee,
I have the exact year and model truck you have. My truck has the 402 with the TH400 and 3.54 gears in the diff. When on the freeway hitting 65, my RPM's are close to 4K. Does your truck also run that high?

Is there anything I can do, keeping the original tranny and diff, to improve the RPM's so they are not running so high? I would like to use it as a daily driver and hit the freeways from time to time.
Any advice or input would be greatly appreciated.
God bless,
Tbone

[RustyBucket]

TBone, old thread, but I wonder if your tach. is in error. To turn 4000 RPM at 65 mph you'd have little short 20" diameter rear tires according to this previously posted calculator :http://www.csgnetwork.com/multirpmcalc.html
What size are your rear tires ?

[Steeveedee]

Originally Posted by TBone1970

Hi Steeveedee,
I have the exact year and model truck you have. My truck has the 402 with the TH400 and 3.54 gears in the diff. When on the freeway hitting 65, my RPM's are close to 4K. Does your truck also run that high?

Is there anything I can do, keeping the original tranny and diff, to improve the RPM's so they are not running so high? I would like to use it as a daily driver and hit the freeways from time to time.
Any advice or input would be greatly appreciated.
God bless,
Tbone

I don't think mine is wound up that tight, but I disconnected the tach when I put the HEI in it (about 8 years ago) and haven't connected it back up yet. So I don't know.

According to the calculator posted above, my truck runs about 3 grand. If your tach is set for a 6-cyl, it will show about 4 grand. 3 grand is still pretty wound up. I drive 55 all the time on the freeway for that reason, and the tall camper shell really kicked my mileage in the butt. That's about 2500 RPM, which is tolerable. I'm retired, so I'm in no hurry to get anywhere if I'm driving the truck.

[rdc72]

My 72 SWB, 454/TH350 trans and 3.73 gears is hitting about 4000-4200 at 65 MPH, I am just going to go with a 700R4, I'm not racing it just a cruiser so it should be fine.


Pops

[TBone1970]

Originally Posted by RustyBucket

TBone, old thread, but I wonder if your tach. is in error. To turn 4000 RPM at 65 mph you'd have little short 20" diameter rear tires according to this previously posted calculator :http://www.csgnetwork.com/multirpmcalc.html
What size are your rear tires ?

Hi RustyBucket,
Awesome! Thanks for the calculator! The motor that came in the truck when I bought it was a SBC 350. I am returning the original size BBC 402 that belongs in it. I am a novice with rebulding an engine. Wish I knew someone local that could assist me so I don't blow this thing up upon initial startup. I am also trying to figure out the correct pulleys that belong on this. I am running Power Steering, & AC. I think I am supposed to have 3 grove crank pulley and 2 groove water pump pulley. BUT, don't know the diameters of the pulleys that I need to purchase.
I am even thinking of rebuilding the TH400 since it is the original on the truck.
Thanks for the calculator!
God Bless!
Tbone

[Steeveedee]

Get yourself a book like this- https://www.amazon.com/Hotrod-Big-Bl.../dp/0912656042

To help with your rebuild. All kinds of good pointers, even if you aren't looking to make it into a FARMTRUCK- https://www.youtube.com/watch?v=sCuGNMOHIy4