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

[FstCarsNGuitars]

Hey Big Block guys, what diff gears are you running? I currently have a 68 swb with a 454/TH400 setup and I have 3.73 diff gears...can you see where this is headed yet? I understand 3 speed trans is the real issue here and will eventually be installing an A/T with a 4th gears, but for now this is what I have and I will make it work.. being that this is not my daily driver and I would like to cruise the highway. From what I understand a 3.08 gear option would most likely help with the higher rpms at 70+ mph, according to most rpm calculators I found online.

I'm not too sure how I will feel about "losing" some bottom end, so I'm curious what others running a 3 speed trans have "remedied" for this.
Thanks!

[KEWLKATDADY]

I drove my '67 for years with a 468/th400 with 3:73.

I would have loved having a 4th gear, but I wouldn't sacrifice the low end for the lower RPM on the high end.

I'd definitely recommend a 4spd tranny.

[jocko]

don't mess with the rear gearing - 3.73 is perfect for your setup and the addition of an o/d trans. 200R4 or 700R4 will give you highway cruising in o/d AND increase your low end torque multiplication. 700R4 and 2004R have deeper 1st gears than the TH400. If you are eventually going to put in a 4 spd auto - don't waste $ on the 3.08 as an interim measure now, because it won't work well with the eventual o/d trans, unless you're gonna cruise at 110 mph.

[Fitz]

Originally Posted by FstCarsNGuitars

Hey Big Block guys, what diff gears are you running? I currently have a 68 swb with a 454/TH400 setup and I have 3.73 diff gears...can you see where this is headed yet? I understand 3 speed trans is the real issue here and will eventually be installing an A/T with a 4th gears, but for now this is what I have and I will make it work.. being that this is not my daily driver and I would like to cruise the highway. From what I understand a 3.08 gear option would most likely help with the higher rpms at 70+ mph, according to most rpm calculators I found online.

I'm not too sure how I will feel about "losing" some bottom end, so I'm curious what others running a 3 speed trans have "remedied" for this.
Thanks!

572, TH400, 4:10 rear.
My 'remedy'? A Gear Vendors 0.7:1 overdrive.
6 speeds forward, I don't lose any bottom end and at 70 on the freeway it turns 2300 RPM!

[Tom Vogel]

Originally Posted by Fitz

572, TH400, 4:10 rear.
My 'remedy'? A Gear Vendors 0.7:1 overdrive.
6 speeds forward, I don't lose any bottom end and at 70 on the freeway it turns 2300 RPM!

^^^^^^ This

[bs46488]

I picked up a previous project that he used to run a SBC with the 3.73 gears.

Well I dropped a big block with a th400 in it. While trying to get a driveshaft made, I got a lecture from the guy about the 3.73 being too much gear. He recommended something down in the low 3's.

I haven't driven the truck yet, but I'm sticking with the 3.73 for now (and I'm shopping for a new driveshaft shop) I don't foresee myself cruising the highway at 70 for long long periods of time that I would worry about the RPM issue.

[FstCarsNGuitars]

Originally Posted by bs46488

I picked up a previous project that he used to run a SBC with the 3.73 gears.

Well I dropped a big block with a th400 in it. While trying to get a driveshaft made, I got a lecture from the guy about the 3.73 being too much gear. He recommended something down in the low 3's.

I haven't driven the truck yet, but I'm sticking with the 3.73 for now (and I'm shopping for a new driveshaft shop) I don't foresee myself cruising the highway at 70 for long long periods of time that I would worry about the RPM issue.

I will say that I do love the throttle response provided from the 3.73 gearing and its a little too much fun! As for the gearing I was recommended the same thing and that's what sparked my question about diff gears. I had my driveshaft made locally here in AZ by "Dick's Driveshafts" (funny name I know) and he did an awesome job. He's a great guy, he's been making them for 34+ years..I only paid $120 for mine because I provided the yoke ends and It was done in 2 days!

[FstCarsNGuitars]

Originally Posted by Tom Vogel

^^^^^^ This

x2 I would have to agree that solution would work best, however I would have to save up my pennies for that! lol I think I saw them for about $2600?
I wonder if the cost of building a 700r4 would be worth the trouble or possibly using a 4l80E which I believe would require the use of a controller.

[68350]

I have 3.08, 3.42, and 3.54 gears in all of my big block trucks. 3.08 is really not a low end take off gear but for cruising its perfect. Big blocks make low end torque so a higher rear gea won't be terrible.

[ho70]

Originally Posted by 68350

I have 3.08, 3.42, and 3.54 gears in all of my big block trucks. 3.08 is really not a low end take off gear but for cruising its perfect. Big blocks make low end torque so a higher rear gea won't be terrible.

I have 3.08's. Still chirp the tires and good for treks to the lake

[chevy2tom]

Rat Infestation

[72chevyk10]

4.56's are awful with a 454. even with 35's i dont like going over 60. i get better mpg in town than i do on the freeway. for my setup im thinking 4.10's would be the money gear

[Mick Je]

I'm running 3.73s with 402/400 turbo trans. Fine around town, but screams going 70 at 3,000 rpms on the freeway. I'm thinking about adding a gear vendor overdrive.

[67ctener]

I also have a 402/400 with 3.73 gears. Never driven the truck so can't speak on drive ability. I know the truck will be a weekend cruiser and rarely see the freeway so stop light to stoplight torque is what I'm looking for

[67 chevelle]

A built 2004r is a great option for your 373 gears , a posi is a good idea also , 342 gears are another option

[Steeveedee]

3.55s in mine. It's a bit steep for freeway cruising, but it is a Custom Camper, so that's a good gear when I'm towing. I've thought about an O/D, but that would just be a needless expense at this time. I'd never recover the monetary savings, given how little I drive it.

[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 ?

[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]

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

[CST10]

3.08 in my truck with a 462 ci/turbo 350 combo. The gearing doesn't hurt the low end too much as traction in the truck is the issue here. I can smoke the tires a good 50-70 feet and still cruise at 70 mph with 2500 RPM's. If I was racing it and no pleasure cruising it I would change to 3.73's.

My 6 cylinder 68 has 3.73's. Can't go over 60-65 mph without rolling over 3000 RPM's. No good for hey cruising.

[davischevy]

I'm running 3.73's' but I'm having an overdrive trans built as we post.

[chevy2tom]

468 th400 with 355 gears not pleasant sounding @70 mph on the highway lol, having a 200 4r installed in the next couple weeks. Read an article on the interweb that my 355 will turn into a 273 with the overdrive, we'll see.

[FstCarsNGuitars]

Originally Posted by chevy2tom

468 th400 with 355 gears not pleasant sounding @70 mph on the highway lol, having a 200 4r installed in the next couple weeks. Read an article on the interweb that my 355 will turn into a 273 with the overdrive, we'll see.

yeah I hear you on that, at 65 mph I'm pushing about 3500+ rpm, not too pleasant but the throttle response is awesome

From this point I'm thinking I will stay with the 3.73 gears and look for an old used 200r4 or 700r4, build it up and change out my yoke on the driveline. If I have to get a new driveline, then so be it. It would be worth it to me to upgrade the trans for an additional gear on the highway, best of both worlds in my opinion. I appreciate everyone's input!

oh and for anyone wondering, here is a diagram of the different transmissions and lengths...