Is 3.08 to low to use 4th gear around Town? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Arl37 · 05-04-2013, 10:45 PM

I have a 69 C10 with a new crate 350 and a 700r4. It has a 3.08 rear end and 255/60R 15s. When i am in 4th around town and trans is not locked up it seems to work pretty well shifting in and out of 4th. When I engage the lock up for 4th it shifts into 4th earlier and stays in it longer plus it drops the RPM about 200. dosent really lug in the classic sense but just dosent feel right to be driving around at 40 MPH at 1600 RPM. On the highway at 70 it turns 1800 locked up.
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**LockDoc** · 05-04-2013, 10:56 PM

Quote:

Originally Posted by Arl37

I have a 69 C10 with a new crate 350 and a 700r4. It has a 3.08 rear end and 255/60R 15s. When i am in 4th around town and trans is not locked up it seems to work pretty well shifting in and out of 4th. When I engage the lock up for 4th it shifts into 4th earlier and stays in it longer plus it drops the RPM about 200. dosent really lug in the classic sense but just dosent feel right to be driving around at 40 MPH at 1600 RPM. On the highway at 70 it turns 1800 locked up.

Here are some threads that might help with some info... I think you will find that the 3.08 will be too high for the 700r4.

http://67-72chevytrucks.com/vboard/s...d.php?t=542624

http://67-72chevytrucks.com/vboard/s...d.php?t=540257

http://67-72chevytrucks.com/vboard/s...d.php?t=560277

Just my 2¢ worth.

LockDoc

**jocko** · 05-04-2013, 11:59 PM

yep, your effective final drive is too high (too low numerically) and resultant rpm is way too low with that combo (for around town driving - might be ok on the highway if you cruise at 80+..) 35 and 45 mph rpm shown in the chart below. Will either lug the motor or experience the problems you're seeing. At 35, you're even below idle rpm. You could just drive it in Drive, not Overdrive, in town, yes?

**special-K** · 05-05-2013, 12:12 AM

You don't want to be in overdrive with any gear ratio at 35 mph. Personally,my mark is 50+ mph for using o/d. The 3.08s will work with the 700r4 but you have to be selective on using o/d.

Arl37 · 05-05-2013, 08:42 AM

Thanks for theguidance. Toying with the idea of just putting a 3.73 in it.
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Tx Firefighter · 05-05-2013, 09:24 AM

GM built a zillion trucks with 2.73 and 3.08 gears and the 700R4 trans. They can work just fine.
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Leftlane · 05-05-2013, 09:57 AM

I can't recall my past 350/700r4 trucks ever engaging lockup prior to around the 40-45 mph range. Highway rpm in the 1600-1800 range are acceptable.
Beyond that speed/ rpm you should be fine.
When you go to accelerate, it should disengage and drop gears anyway.
(If its set up for full automatic function)
Is your lockup on a manual switch?
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1972RedNeck · 05-05-2013, 10:06 AM

A lot depends on the weight of the truck.

bugmasta · 05-05-2013, 11:07 AM

I have 3.08 gears with a 700r4 and drive around town in drive and only use overdrive when I am on the freeway. Will eventually change to 3.73 once I can find a rearend local I can swap.
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**mr48chev** · 05-05-2013, 11:21 AM

Quote:

Originally Posted by 1972RedNeck

A lot depends on the weight of the truck.

And what you are hauling/towing. Personally I'd just drop it down into 3rd around town and call it good. That keeps it from "wanting" to go into od in town.

**Boog** · 05-05-2013, 11:28 AM

I had an 88 with 305/R4 with 2.73 gear and 29" tall tires. Yes the 305 was a real noodle for power but with the low cruise rpms it had a terrible time staying in od at 70 mph. The slightest incline or headwind forced it to downshift into 3rd. Consequently it didn't get great mpg either.
The small blocks sweet spot as I've come to call it is normally between 1800 and 2200 rpm. They are more efficient in that range.
As said you have the option of using 3rd gear instead of od. It's the same as these trucks came with back then. 3.08 is a good all around gear for a non od application.
A 3.42 or better yet a 3.73 gear works great with an od trans and 28 or 29" tall tires. You get better pulling power and od to bring the rpms back down at cruise speeds.
Figure out how you intend to mainly use the truck and tune it for that. Low speed around town use or mainly highway.

05-04-2013, 10:45 PM	#1
Arl37 Registered User Join Date: Jul 2011 Location: Valley Center, Ca Posts: 277	Is 3.08 to low to use 4th gear around Town? I have a 69 C10 with a new crate 350 and a 700r4. It has a 3.08 rear end and 255/60R 15s. When i am in 4th around town and trans is not locked up it seems to work pretty well shifting in and out of 4th. When I engage the lock up for 4th it shifts into 4th earlier and stays in it longer plus it drops the RPM about 200. dosent really lug in the classic sense but just dosent feel right to be driving around at 40 MPH at 1600 RPM. On the highway at 70 it turns 1800 locked up. Posted via Mobile Device Posted via Mobile Device Posted via Mobile Device

05-04-2013, 11:59 PM	#3
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,985	Re: Is 3.08 to low to use 4th gear around Town? yep, your effective final drive is too high (too low numerically) and resultant rpm is way too low with that combo (for around town driving - might be ok on the highway if you cruise at 80+..) 35 and 45 mph rpm shown in the chart below. Will either lug the motor or experience the problems you're seeing. At 35, you're even below idle rpm. You could just drive it in Drive, not Overdrive, in town, yes? Attached Images

05-05-2013, 12:12 AM	#4
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: Is 3.08 to low to use 4th gear around Town? You don't want to be in overdrive with any gear ratio at 35 mph. Personally,my mark is 50+ mph for using o/d. The 3.08s will work with the 700r4 but you have to be selective on using o/d. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

05-05-2013, 08:42 AM	#5
Arl37 Registered User Join Date: Jul 2011 Location: Valley Center, Ca Posts: 277	Re: Is 3.08 to low to use 4th gear around Town? Thanks for theguidance. Toying with the idea of just putting a 3.73 in it. Posted via Mobile Device

05-05-2013, 09:24 AM	#6
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	Re: Is 3.08 to low to use 4th gear around Town? GM built a zillion trucks with 2.73 and 3.08 gears and the 700R4 trans. They can work just fine. Posted via Mobile Device

05-05-2013, 09:57 AM	#7
Leftlane Registered User Join Date: Dec 2002 Location: St. Louis, MO Posts: 118	Re: Is 3.08 to low to use 4th gear around Town? I can't recall my past 350/700r4 trucks ever engaging lockup prior to around the 40-45 mph range. Highway rpm in the 1600-1800 range are acceptable. Beyond that speed/ rpm you should be fine. When you go to accelerate, it should disengage and drop gears anyway. (If its set up for full automatic function) Is your lockup on a manual switch? Posted via Mobile Device __________________ 1971 C10 SWB 350/350 A/C PS PDB

05-05-2013, 11:07 AM	#9
bugmasta Registered User Join Date: Aug 2009 Location: North Bend, WA Posts: 272	Re: Is 3.08 to low to use 4th gear around Town? I have 3.08 gears with a 700r4 and drive around town in drive and only use overdrive when I am on the freeway. Will eventually change to 3.73 once I can find a rearend local I can swap. Posted via Mobile Device __________________ 68 c20 build: http://67-72chevytrucks.com/vboard/s...d.php?t=358130 Club longbed ...cause girls like'em long!!!!!

05-05-2013, 11:28 AM	#11
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 13,584	Re: Is 3.08 to low to use 4th gear around Town? I had an 88 with 305/R4 with 2.73 gear and 29" tall tires. Yes the 305 was a real noodle for power but with the low cruise rpms it had a terrible time staying in od at 70 mph. The slightest incline or headwind forced it to downshift into 3rd. Consequently it didn't get great mpg either. The small blocks sweet spot as I've come to call it is normally between 1800 and 2200 rpm. They are more efficient in that range. As said you have the option of using 3rd gear instead of od. It's the same as these trucks came with back then. 3.08 is a good all around gear for a non od application. A 3.42 or better yet a 3.73 gear works great with an od trans and 28 or 29" tall tires. You get better pulling power and od to bring the rpms back down at cruise speeds. Figure out how you intend to mainly use the truck and tune it for that. Low speed around town use or mainly highway. __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross



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