250 3ott 3:08 combo - The 1947 - Present Chevrolet & GMC Truck Message Board Network

68panelman · 01-02-2021, 10:43 PM

Like title says. Anyone running this drivetrain? My has 67 has all but with a 4:11 rear, 28" tires, stays happy at 60, looking for more and better gas mileage
Found a 3ott overdrive somewhat local, but then a shortened driveshaft comes into play, but I like the idea. A 5 spd will definitely be the most expensive route but I enjoy column shifting too much and don't want to cut my floor. Anyone have one of the combos for info?

Thanks, Mike

**pjmoreland** · 01-02-2021, 11:14 PM

I had a 67 that came from the factory with 4:11 gears and a 3ott overdrive transmission. Engaging the overdrive was a two-step process that involved pushing in a handle that was connected to a cable that put the transmission in a freewheeling mode where the engine could go to idle while coasting down the road. The second step was to pull an electric switch that engaged the overdrive gear as the engine RPM was allowed to drop by lifting my foot off the accelerator. One time I got 21 MPG on a long road trip. Sometimes I'd forget to disengage OD before stopping, and then first gear would be extra tall when I'd start going again and I'd have to disengage the OD quickly.

**special-K** · 01-02-2021, 11:51 PM

I think a 3spd with 3.08s would be a miserable thing. 1st is so tall as it is. When I helped my neighbor build his '68 SWB with LT1/car 4spd the original rear was 3.73s because it was a factory 3spd truck. He wanted better hwy gears so we put a 3.08 in there. He hated it from the start. Way to tall for getting going, slip the clutch city. Within a couple weeks (and no driving it) we were putting 3.42s in there and that was just right

**jocko** · 01-03-2021, 01:31 AM

28" tires / 3OTT / 4.11 = 3206 rpm @ 65 mph / 3699 rpm @ 75 mph (i.e. what you have now)
28" tires / 3OTT / 3.08 = 2402 rpm @ 65 mph / 2772 rpm @ 75 mph
28" tires / 3OTT + BW .70 OD / 4.11 = 2244 rpm @ 65 mph / 2589 rpm @ 75 mph

Your choice should depend on what you actually want to do with the truck. A change to 3.08 will give you a more reasonable highway rpm, however, the abysmal acceleration you would see might make you regret it... you'd somewhat decrease your ability to occasionally use it like a truck. The BW OD unit keeps your current deep 1st gear intact, a good thing with a 250, while giving you even slightly lower highway rpm - right in the sweet spot. I'd find a reputable driveline shop for the d/s and grab that OD before it's gone...

Also, when you say "at 60" - are you talking about 60 on your speedo or a GPS? If you're only referencing your speedo, keep in mind that it will be incorrect with 28" tires - recommend cross-ref a GPS to see how far off your speedo is if that's the only thing you're looking at for speed.

Overdriven · 01-03-2021, 03:45 AM

I think it was LocDoc who posted about an option I really liked and am considering for my truck. Using a 4spd trans and keeping the column shifter. I think he said he used a choke cable or similar to actuate the reverse lever on the trans and the other rods went to the column shifter. I’d look for a Saginaw 4 speed, they’re inexpensive compared to other 4 speeds and stout enough for the engine.

The internet says your trans should have a 2.84 first gear. Saginaw 4 speeds came with a variety of ratios identified by the number of rings or groves in the input shaft. No rings is 2.84 first gear, 1 ring is 2.52, 2 rings 3.11 and 3 rings is 3.50 first gear. If you want 3.08 rear gears I’d look for a Saginaw with 2 or 3 rings on the input shaft.

68panelman · 01-03-2021, 09:09 AM

Quote:

Originally Posted by jocko

28" tires / 3OTT / 4.11 = 3206 rpm @ 65 mph / 3699 rpm @ 75 mph (i.e. what you have now)
28" tires / 3OTT / 3.08 = 2402 rpm @ 65 mph / 2772 rpm @ 75 mph
28" tires / 3OTT + BW .70 OD / 4.11 = 2244 rpm @ 65 mph / 2589 rpm @ 75 mph

Your choice should depend on what you actually want to do with the truck. A change to 3.08 will give you a more reasonable highway rpm, however, the abysmal acceleration you would see might make you regret it... you'd somewhat decrease your ability to occasionally use it like a truck. The BW OD unit keeps your current deep 1st gear intact, a good thing with a 250, while giving you even slightly lower highway rpm - right in the sweet spot. I'd find a reputable driveline shop for the d/s and grab that OD before it's gone...

Also, when you say "at 60" - are you talking about 60 on your speedo or a GPS? If you're only referencing your speedo, keep in mind that it will be incorrect with 28" tires - recommend cross-ref a GPS to see how far off your speedo is if that's the only thing you're looking at for speed.

Mostly Hwy driving, my area is mostly flat unless I'm traveling to a truck show in PA. GPS shows 64, speedo at 60, was dead on when i had 225's but recently went with 235-75-15

68panelman · 01-03-2021, 09:11 AM

Quote:

Originally Posted by pjmoreland

I had a 67 that came from the factory with 4:11 gears and a 3ott overdrive transmission. Engaging the overdrive was a two-step process that involved pushing in a handle that was connected to a cable that put the transmission in a freewheeling mode where the engine could go to idle while coasting down the road. The second step was to pull an electric switch that engaged the overdrive gear as the engine RPM was allowed to drop by lifting my foot off the accelerator. One time I got 21 MPG on a long road trip. Sometimes I'd forget to disengage OD before stopping, and then first gear would be extra tall when I'd start going again and I'd have to disengage the OD quickly.

Seems like more driving fun

68panelman · 01-03-2021, 09:17 AM

Quote:

Originally Posted by special-K

I think a 3spd with 3.08s would be a miserable thing. 1st is so tall as it is. When I helped my neighbor build his '68 SWB with LT1/car 4spd the original rear was 3.73s because it was a factory 3spd truck. He wanted better hwy gears so we put a 3.08 in there. He hated it from the start. Way to tall for getting going, slip the clutch city. Within a couple weeks (and no driving it) we were putting 3.42s in there and that was just right

My blazer has 3:08's and 33's but the V8 helps it take off easily in second. Maybe the lil sixer will struggle in first with 3:08's.

weim55 · 01-03-2021, 08:30 PM

I did this very swap myself some years back. Zero option 71 w/ 250 3 on the tree / 3.73s. Short stepside.Had a complete drum to drum 3.07 axle left over from a parts truck. Thought I would give it a try for better gas mileage as I was doing a lot of highway at the time. Wound up being a fail. Unless it was on flats with zero wind that little six just couldn’t keep up. It would fall off at every incline and I would have to downshift into second. Driving around town it really struggled. With so little horsepower the 250 needed those 3.73s. Bear in mind I am in Colorado at about 5200 feet altitude so the engines about 20% hp down because of the altitude. Maybe a sea level truck would do better.

Steve weim55 Colorado

**LockDoc** · 01-03-2021, 08:59 PM

Quote:

Originally Posted by Overdriven

I think it was LocDoc who posted about an option I really liked and am considering for my truck. Using a 4spd trans and keeping the column shifter. I think he said he used a choke cable or similar to actuate the reverse lever on the trans and the other rods went to the column shifter. I’d look for a Saginaw 4 speed, they’re inexpensive compared to other 4 speeds and stout enough for the engine.

The internet says your trans should have a 2.84 first gear. Saginaw 4 speeds came with a variety of ratios identified by the number of rings or groves in the input shaft. No rings is 2.84 first gear, 1 ring is 2.52, 2 rings 3.11 and 3 rings is 3.50 first gear. If you want 3.08 rear gears I’d look for a Saginaw with 2 or 3 rings on the input shaft.

You are correct, I used a Muncie M21. The original shift lever was used for the 4 forward gears, and for reverse I just put it in neutral and pulled the choke knob out. I made a bracket that bolted on the side of the trans to fasten the end of the choke cable housing to. Worked good.

LockDoc

68panelman · 01-03-2021, 09:53 PM

Sounding like I should go grab that 3spd overdrive trans, then I can keep the 4:11 rear which is great for get up and go. It's out of a tri five, has same SM318 that my 67 does.

68panelman · 01-03-2021, 09:54 PM

Quote:

Originally Posted by LockDoc

You are correct, I used a Muncie M21. The original shift lever was used for the 4 forward gears and for reverse I just put it in neutral and pulled the choke knob out. I made a bracket that bolted on the side of the trans to fasten the end of the choke cable housing to. Worked good.

LockDoc

Thanks both of you for your information

**Steeveedee** · 01-03-2021, 10:44 PM

Quote:

Originally Posted by LockDoc

You are correct, I used a Muncie M21. The original shift lever was used for the 4 forward gears and for reverse I just put it in neutral and pulled the choke knob out. I made a bracket that bolted on the side of the trans to fasten the end of the choke cable housing to. Worked good.

LockDoc

That's also a seriously cool anti-theft device! Leave it in any forward gear and pull out the choke knob!

**NewBayColony** · 01-04-2021, 11:43 AM

I have a 72 C20 with a 292 and 4.56 gear and 3 OTT. I couldn't drive it too far on the highway due to excess RPMs so I got a Borg Warner Overdrive. I love it. I did not have to change the length of the driveshaft but sometimes you have to change the front yolk to match the spline of the output shaft. I hooked mine up so it works like a factory install. Easy to do. The kickdown switch mounts easy on the gas pedal bracket under the dash. The rest is just wiring. Just know that you need to use mineral gear oil in it. NAPA sells it. Here's a link about it plus there are plenty of posts on this forum about the OD units. https://www.jalopyjournal.com/forum/...ar-oil.797663/ NAPA gear oil link: https://www.napaonline.com/en/p/FNJ65201

TwoFiftyShifter · 01-05-2021, 07:54 AM

3.42 would work. Big difference from 4.11. With the 3.08s, you would just have a slow time taking off. Uphill for instance would be an annoyance. But 3.08 axles can be had cheap, and swapped in an afternoon. I would try it.

68panelman · 01-05-2021, 08:23 AM

Quote:

Originally Posted by NewBayColony

I have a 72 C20 with a 292 and 4.56 gear and 3 OTT. I couldn't drive it too far on the highway due to excess RPMs so I got a Borg Warner Overdrive. I love it. I did not have to change the length of the driveshaft but sometimes you have to change the front yolk to match the spline of the output shaft. I hooked mine up so it works like a factory install. Easy to do. The kickdown switch mounts easy on the gas pedal bracket under the dash. The rest is just wiring. Just know that you need to use mineral gear oil in it. NAPA sells it. Here's a link about it plus there are plenty of posts on this forum about the OD units. https://www.jalopyjournal.com/forum/...ar-oil.797663/ NAPA gear oil link: https://www.napaonline.com/en/p/FNJ65201

Good info here, thank you

68panelman · 01-05-2021, 08:26 AM

Quote:

Originally Posted by TwoFiftyShifter

3.42 would work. Big difference from 4.11. With the 3.08s, you would just have a slow time taking off. Uphill for instance would be an annoyance. But 3.08 axles can be had cheap, and swapped in an afternoon. I would try it.

I can see it now, having to downshift to second and the column gets hung up

01-02-2021, 10:43 PM	#1
68panelman Registered User Join Date: Jan 2011 Location: Delaware and Long Island, New York Posts: 3,450	250 3ott 3:08 combo Like title says. Anyone running this drivetrain? My has 67 has all but with a 4:11 rear, 28" tires, stays happy at 60, looking for more and better gas mileage Found a 3ott overdrive somewhat local, but then a shortened driveshaft comes into play, but I like the idea. A 5 spd will definitely be the most expensive route but I enjoy column shifting too much and don't want to cut my floor. Anyone have one of the combos for info? Thanks, Mike

01-02-2021, 11:14 PM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,309	Re: 250 3ott 3:08 combo I had a 67 that came from the factory with 4:11 gears and a 3ott overdrive transmission. Engaging the overdrive was a two-step process that involved pushing in a handle that was connected to a cable that put the transmission in a freewheeling mode where the engine could go to idle while coasting down the road. The second step was to pull an electric switch that engaged the overdrive gear as the engine RPM was allowed to drop by lifting my foot off the accelerator. One time I got 21 MPG on a long road trip. Sometimes I'd forget to disengage OD before stopping, and then first gear would be extra tall when I'd start going again and I'd have to disengage the OD quickly.

01-02-2021, 11:51 PM	#3
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,863	Re: 250 3ott 3:08 combo I think a 3spd with 3.08s would be a miserable thing. 1st is so tall as it is. When I helped my neighbor build his '68 SWB with LT1/car 4spd the original rear was 3.73s because it was a factory 3spd truck. He wanted better hwy gears so we put a 3.08 in there. He hated it from the start. Way to tall for getting going, slip the clutch city. Within a couple weeks (and no driving it) we were putting 3.42s in there and that was just right __________________ "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 ~

01-03-2021, 01:31 AM	#4
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,933	Re: 250 3ott 3:08 combo 28" tires / 3OTT / 4.11 = 3206 rpm @ 65 mph / 3699 rpm @ 75 mph (i.e. what you have now) 28" tires / 3OTT / 3.08 = 2402 rpm @ 65 mph / 2772 rpm @ 75 mph 28" tires / 3OTT + BW .70 OD / 4.11 = 2244 rpm @ 65 mph / 2589 rpm @ 75 mph Your choice should depend on what you actually want to do with the truck. A change to 3.08 will give you a more reasonable highway rpm, however, the abysmal acceleration you would see might make you regret it... you'd somewhat decrease your ability to occasionally use it like a truck. The BW OD unit keeps your current deep 1st gear intact, a good thing with a 250, while giving you even slightly lower highway rpm - right in the sweet spot. I'd find a reputable driveline shop for the d/s and grab that OD before it's gone... Also, when you say "at 60" - are you talking about 60 on your speedo or a GPS? If you're only referencing your speedo, keep in mind that it will be incorrect with 28" tires - recommend cross-ref a GPS to see how far off your speedo is if that's the only thing you're looking at for speed.

01-03-2021, 03:45 AM	#5
Overdriven Registered User Join Date: Mar 2016 Location: Long Island NY Posts: 598	Re: 250 3ott 3:08 combo I think it was LocDoc who posted about an option I really liked and am considering for my truck. Using a 4spd trans and keeping the column shifter. I think he said he used a choke cable or similar to actuate the reverse lever on the trans and the other rods went to the column shifter. I’d look for a Saginaw 4 speed, they’re inexpensive compared to other 4 speeds and stout enough for the engine. The internet says your trans should have a 2.84 first gear. Saginaw 4 speeds came with a variety of ratios identified by the number of rings or groves in the input shaft. No rings is 2.84 first gear, 1 ring is 2.52, 2 rings 3.11 and 3 rings is 3.50 first gear. If you want 3.08 rear gears I’d look for a Saginaw with 2 or 3 rings on the input shaft. __________________ 1967 C10 LWB 250 6cyl 3ott Panaramic cab “Layla” 1967 Camaro 327 ‘glide cruiser “Bailey” Last edited by Overdriven; 01-03-2021 at 04:06 AM.

01-03-2021, 08:30 PM	#9
weim55 Registered User Join Date: Jan 2017 Location: Larkspur, Colorado Posts: 904	Re: 250 3ott 3:08 combo I did this very swap myself some years back. Zero option 71 w/ 250 3 on the tree / 3.73s. Short stepside.Had a complete drum to drum 3.07 axle left over from a parts truck. Thought I would give it a try for better gas mileage as I was doing a lot of highway at the time. Wound up being a fail. Unless it was on flats with zero wind that little six just couldn’t keep up. It would fall off at every incline and I would have to downshift into second. Driving around town it really struggled. With so little horsepower the 250 needed those 3.73s. Bear in mind I am in Colorado at about 5200 feet altitude so the engines about 20% hp down because of the altitude. Maybe a sea level truck would do better. Steve weim55 Colorado

01-03-2021, 09:53 PM	#11
68panelman Registered User Join Date: Jan 2011 Location: Delaware and Long Island, New York Posts: 3,450	Re: 250 3ott 3:08 combo Sounding like I should go grab that 3spd overdrive trans, then I can keep the 4:11 rear which is great for get up and go. It's out of a tri five, has same SM318 that my 67 does.

01-04-2021, 11:43 AM	#14
NewBayColony Senior Member Join Date: Mar 2017 Location: Northbridge,MA Posts: 493	Re: 250 3ott 3:08 combo I have a 72 C20 with a 292 and 4.56 gear and 3 OTT. I couldn't drive it too far on the highway due to excess RPMs so I got a Borg Warner Overdrive. I love it. I did not have to change the length of the driveshaft but sometimes you have to change the front yolk to match the spline of the output shaft. I hooked mine up so it works like a factory install. Easy to do. The kickdown switch mounts easy on the gas pedal bracket under the dash. The rest is just wiring. Just know that you need to use mineral gear oil in it. NAPA sells it. Here's a link about it plus there are plenty of posts on this forum about the OD units. https://www.jalopyjournal.com/forum/...ar-oil.797663/ NAPA gear oil link: https://www.napaonline.com/en/p/FNJ65201 __________________ 72 C20 292 3OTT 77 GMC 2500 4x4 Longstepper 350 4sp 50 3100 ex Airforce/Town of Mendon fire truck 72 GMC 1500 2wd short stepside 250 4sp 78 GMC 2500 4x Longstepper 292 4sp 54 6400 Chevy 5 window 4 sp 2sp vac rear end Shell oil truck/dump truck (Norman) Last edited by NewBayColony; 01-04-2021 at 12:02 PM.

01-05-2021, 07:54 AM	#15
TwoFiftyShifter Registered User Join Date: Nov 2008 Location: Gwinnett GA Posts: 1,803	Re: 250 3ott 3:08 combo 3.42 would work. Big difference from 4.11. With the 3.08s, you would just have a slow time taking off. Uphill for instance would be an annoyance. But 3.08 axles can be had cheap, and swapped in an afternoon. I would try it. __________________ 1982 K10 SWB 1987 V20 - Sold - Doh!



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