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Old 10-17-2019, 01:13 PM   #26
dsraven
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Re: Rear Diff questions

check this site for the gear ratios of each trans in each gear. maybe that, along with the rpm calculator, will help you figure it out. it would be good to know the torque/hp curve of the engine so you know roughly what to expect at each shift point.

http://hotrodworks.net/hotrodmath/quarter.html
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Old 10-18-2019, 01:48 PM   #27
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Re: Rear Diff questions

.76 is the final ratio of a 700r4. with a 27 inch tire your rpm at 65 would be 1893. likely not enough torque to keep things moving without more gas pedal.
3.08 gear 1:1 final ratio with th350 puts you at 2500 rpm... maybe a tad high.
3.73 gear at final ratio of.73 27" tire puts you at 2293 rpm. enough rpm to have throttle response.
my advice would be if you are stuck on 3.08 gears go th350.
if you are stuck on 700r go 3.73 or 3.42.
3.42 is most common in 4x4 s10s which is a usable width for these trucks.. like posted above develop a ideal final result. plan accordingly so you dont spend money twice.
all info from this http://www.csgnetwork.com/multirpmcalc.html
and this https://tiresize.com/calculator/
figure what size tires you want with final stance plug that tire size in to find diameter. plug that into rpm calculator. good rule of thumb would be ideal final cruise speed of 60 mph between 2100 rpm and 2500 rpm.

Last edited by Marty68; 10-18-2019 at 01:54 PM.
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Old 10-18-2019, 02:44 PM   #28
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Re: Rear Diff questions

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Originally Posted by regan56 View Post
interesting...assume modifictions to the perches tho?
spring pads will need to be modified for any rearend you use except stock
easy peasy
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Old 10-21-2019, 12:57 PM   #29
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Re: Rear Diff questions

when we are talking about RPM and the need for more when required, wont the 'kickdown' take care of that and shift down as required? i mean i dont want to have to mash teh gas when i need to downshift, but at those RPMS, say in OD at 1900 RPM when i push the pedal, the 700r4 should downshift and push me into 3rd. not like its just gonna lug along until it gets up to speed... or am i just not getting this?


as far as an HP output from my 327..its unknown. i wont be in a position to make any dyno runs either. certainly hotrodders and gearheads in the past never (maybe some but not all) dyno'd their HP output to perfectly match their trans and gears before finishing.

i appreciate the effort guys, i really do...guess im tryna keep the 700 and the 3.08s i have, but if a TH350 is the better opinion then ill make that happen. yes it will be a high rev on teh hwy, but that wont be a very often type scenario. im looking for the best in-town duo with the 3.08s. i can also leave the 700 in 3rd round town and have the OD for hwy no?
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Old 10-21-2019, 05:51 PM   #30
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Re: Rear Diff questions

Will you also be using g the 700r4 lock up converter? If so the lock up would have to unload, then gear down. Slight lag.
Check the gear ratios of each trans and do the math for rpms. 350 is the easier due to electronics req for 700r4. If you want quick and easy then throw in the 350 and see if you like it. You can always change it later either way.
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Old 10-22-2019, 02:47 PM   #31
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Re: Rear Diff questions

The cable on a 700r4 is not a kickdown cable like on a turbo 350. The 700r4 is a throttle valve cable that tells the transmission the throttle position. If not hooked up correctly you can smoke the 700r4.
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Old 10-22-2019, 07:25 PM   #32
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Re: Rear Diff questions

Ya..the pros and cons lead me to the TH350 route. I won't be doin alot of hwy with this thing so fuel economy isnt top of mind. A good city drive is what is the most likely use.

My next thread will be about the converter selection...
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Old 12-08-2019, 10:49 PM   #33
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Re: Rear Diff questions

Was putting around in the garage this AM and for ships and giggles I checked the ratio on my rearend. Counted 3.73. I did it 3 times just to make sure. Was told it was a 3.08 set guess it’s not.

Keeping the 700R4 but I found some 16” 88-90 Iroc Rims (8” front and 9” rears) instead of the 15”.
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Old 12-09-2019, 12:27 AM   #34
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Re: Rear Diff questions

This thread made me curious. My 54 on a GMC sierra classic chassis with 350, 700R4 and 3.08 turns 1100 rpm at 60 MPH with 33" tire. It has all the low end torque i need and seems to deliver bout 15 MPG. Any of this seem right?
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Old 12-09-2019, 02:33 AM   #35
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Re: Rear Diff questions

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This thread made me curious. My 54 on a GMC sierra classic chassis with 350, 700R4 and 3.08 turns 1100 rpm at 60 MPH with 33" tire. It has all the low end torque i need and seems to deliver bout 15 MPG. Any of this seem right?

with a 33" tire and 30% overdrive, a 3.08 should turn about 1300 rpm. sounds like you have a 2.73
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Old 12-09-2019, 03:04 AM   #36
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Re: Rear Diff questions

try this out for rpm/gear calculations. easy to use.

http://www.csgnetwork.com/multirpmcalc.html
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Old 12-09-2019, 11:53 AM   #37
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Re: Rear Diff questions

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with a 33" tire and 30% overdrive, a 3.08 should turn about 1300 rpm. sounds like you have a 2.73
Thanks but 3.08 for sure. Marked on diff gears and still on factory sticker ( I saved it). May be tach is off but its a good auto meter. Ran a second tach and it read same. was a little surprised at the mileage however.
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Old 12-09-2019, 12:06 PM   #38
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Re: Rear Diff questions

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Originally Posted by vintovka View Post
Thanks but 3.08 for sure. Marked on diff gears and still on factory sticker ( I saved it). May be tach is off but its a good auto meter. Ran a second tach and it read same. was a little surprised at the mileage however.
well, then i would actually bet your speedo is off. its hard to dial in a speedo with larger than stock tires, and at 1100 rpm that is closer to 50mph, so you are reading almost 17% high, so 15mpg - 17% is 12.5mpg, which is closer to what sounds right for your setup. even a gps speedo requires you to drive a carefully measured mile.
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Old 12-09-2019, 12:17 PM   #39
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Re: Rear Diff questions

Thanks But The original speedo is for show. And yes, getting it even close to being accurate is a PITA. Its also way off at certain speeds, but fortunately always reads faster than actual, which i like. ( its also a brand new Chinese repop that was assembled by a starving, cock eyed Prisoner - have to sit way up to read odometer!).

For real speed i have a dedicated in dash 6" GPS and its pretty accurate hence my quandary. Maybe i bent the laws of physics in my favor for once. Not a big deal just an old curious engineer here.
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Old 12-09-2019, 12:36 PM   #40
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Re: Rear Diff questions

with 350 efi, 700r4, 3.27 gear and P235/70R15 28" dia, truk gets 21 mpg
efi has a lot to do with that
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Old 12-09-2019, 12:48 PM   #41
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Re: Rear Diff questions

EFI may be down the road for me. Just happy now cruising the ocean view 101 almost every day with window down to listen to the glasspacs roar. FWIW Oregon has very, very lax rules concerning exhaust noise. Many here on the coast have no mufflers at all due to either desire or rust. Usually the latter!!
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