Crazy Longhorn - rearend question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

busterwivell · 05-15-2004, 09:41 AM

Saw your post about changing rearends. I've got to do so, and just used the formula to determine that I have 411's now. I have a 700R4 and I run 3400 RPM at 75 MPH. Changing to a 307/308 would put me at 2500 at 75 MPH, which is where I would like to be. Someone told me this would make my truck a real dog off the line. I'm hoping it will not. I've been told that the 14 bolt 8 lug rear I have only has 3 gear choices, 391, 411 or 456. None of these will make me happy. I have a later model rear with 8 lugs, I'll open it up and see what gears it has. Do you think 307 gears will be ok with the 402/700R4?

**kwmech** · 05-15-2004, 11:30 AM

The options in ratio for the 14 bolt are 3.21[hard to find] 3.42--3.73--4.10--4.56--4.88 and finally 5.13s. Assuming this is a full float. Also there may be something wrong with your calcs. The final drive in a 700 is .72 so your 4.10s gears with your 700 when in 4th gear come out to 2.95 ratio. If you go to a taller gear such as a 3.08------3.08x.72=2.21---unless of course you are running a small dia. tire.

busterwivell · 05-15-2004, 02:05 PM

My tire diameter is 31 inches. When I'm in Drive, not overdrive, 55 MPH is 3400. 75 MPH in overdrive is 3400. I'll check the math again (I have the formulas posted on the garage wall) and see if I made a mistake.

crazy longhorn · 05-15-2004, 03:25 PM

I am running a 1/2 ton 12 bolt in the longhorn(4.10 gears). overdrive is .76, & on a 28" tire i cruise right at 80 MPH @ 3000 rpms. If you are still running 8 lug wheels,i agree with kwmech on the 14bolt rears. You will find 3.73 to be very common, & the 3.42 should be available. i wouldnt go any higher than 3.42, to keep that good "giddyup" on takeoff. Remember that the 700r4 has a 3.06 1st gear, which is quite a bit lower than the old 400T. i would think a mid 3s gear would give the best of both worlds.....i still wonder if you dont have a 4.57 gear ? that is a lot of revs in OD with a 31" tall tire! good luck,crazyL

crazy longhorn · 05-15-2004, 03:53 PM

Going over calculations @ Smokemup .com, my guess is your speedo could be off.....your truck is probably running faster than the speedo reads?

walker · 05-15-2004, 07:32 PM

Busterwivell, sent you a PM.

05-15-2004, 09:41 AM	#1
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	Crazy Longhorn - rearend question Saw your post about changing rearends. I've got to do so, and just used the formula to determine that I have 411's now. I have a 700R4 and I run 3400 RPM at 75 MPH. Changing to a 307/308 would put me at 2500 at 75 MPH, which is where I would like to be. Someone told me this would make my truck a real dog off the line. I'm hoping it will not. I've been told that the 14 bolt 8 lug rear I have only has 3 gear choices, 391, 411 or 456. None of these will make me happy. I have a later model rear with 8 lugs, I'll open it up and see what gears it has. Do you think 307 gears will be ok with the 402/700R4? __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

05-15-2004, 11:30 AM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,599	The options in ratio for the 14 bolt are 3.21[hard to find] 3.42--3.73--4.10--4.56--4.88 and finally 5.13s. Assuming this is a full float. Also there may be something wrong with your calcs. The final drive in a 700 is .72 so your 4.10s gears with your 700 when in 4th gear come out to 2.95 ratio. If you go to a taller gear such as a 3.08------3.08x.72=2.21---unless of course you are running a small dia. tire. Last edited by kwmech; 05-15-2004 at 11:39 AM.

05-15-2004, 02:05 PM	#3
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	My tire diameter is 31 inches. When I'm in Drive, not overdrive, 55 MPH is 3400. 75 MPH in overdrive is 3400. I'll check the math again (I have the formulas posted on the garage wall) and see if I made a mistake. __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

05-15-2004, 03:25 PM	#4
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	I am running a 1/2 ton 12 bolt in the longhorn(4.10 gears). overdrive is .76, & on a 28" tire i cruise right at 80 MPH @ 3000 rpms. If you are still running 8 lug wheels,i agree with kwmech on the 14bolt rears. You will find 3.73 to be very common, & the 3.42 should be available. i wouldnt go any higher than 3.42, to keep that good "giddyup" on takeoff. Remember that the 700r4 has a 3.06 1st gear, which is quite a bit lower than the old 400T. i would think a mid 3s gear would give the best of both worlds.....i still wonder if you dont have a 4.57 gear ? that is a lot of revs in OD with a 31" tall tire! good luck,crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears.... Last edited by crazy longhorn; 05-15-2004 at 03:28 PM.

05-15-2004, 03:53 PM	#5
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	Going over calculations @ Smokemup .com, my guess is your speedo could be off.....your truck is probably running faster than the speedo reads? __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

05-15-2004, 07:32 PM	#6
walker Registered User Join Date: Jul 2003 Location: north of Phx AZ about 30 miles Posts: 698	Busterwivell, sent you a PM. __________________ Andy,Phx AZ '67 C-10 (Ahhh, done at last. Well there is that disk front end I want to put in and...) "23 C-Cab-sold '48 Ford 8N tractor(still working) '67 Scout(Now on the road) '70 MG B.-sold

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