ATTN Crewcab owners - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Poonman86 · 08-27-2003, 07:34 PM

Hey guys
I've got my new project crewcab longbed. It has a 454 and t400 trans in it, and as you all know the gas prices are on the rise. Thinking about putting a complete 350 tbi and 700r4 out of an 89 silverado in my truck. Whatcha think. any gas mileage increase or decrease? Will it even move this big beast? Would the trans hold up? Should I just leave the big boy in there? I won't do any kind of heavy hauling just my 21FT boat. Advice?

**old Rusty C10** · 08-27-2003, 08:04 PM

i dunno about gas mileage but i haul a 20 foot boat with a four cylinder ranger all the time all over the east coast

**N2TRUX** · 08-27-2003, 08:21 PM

If you are going to covert it to SBC, don't go with the 89 motor. I can't remember the exact hp/tq numbers on it, but they aren't that great. I would find a 96-99 Vortec motor instead. BIG difference in power and torque.

What gears do you have in the rear? That will make a difference too. If it has 4.10 gears, and a T400, you might save some hassle by going to 3.73's and an OD trans.

What kinda mpg are you getting now, about 10?

**Captkaos** · 08-27-2003, 08:23 PM

Quote:

Originally posted by Poonman86
Hey guys
I've got my new project crewcab longbed. It has a 454 and t400 trans in it, and as you all know the gas prices are on the rise. Thinking about putting a complete 350 tbi and 700r4 out of an 89 silverado in my truck. Whatcha think. any gas mileage increase or decrease? Will it even move this big beast? Would the trans hold up? Should I just leave the big boy in there? I won't do any kind of heavy hauling just my 21FT boat. Advice?

My opinion is that if you put the smaller motor in there it will get worse gas mileage as it has to work harder than the BBC. I didn't notice much difference in mileage(other than more torque on tap) when I went with a mildly modified SBC to a stock BBC.

Poonman86 · 08-27-2003, 08:48 PM

I'm getting about 9 mpg. I've got 3.73's in it. Thought about putting a 4L80 with a controller in it, but came across this wrecked 89 truck cheap but haven't bought it yet. thanks for the replies.

**N2TRUX** · 08-28-2003, 12:14 AM

Poonman86
I have had a few of the SBC TBI trucks ( 88, 91,93,95,) and was not impressed with the power, or mileage. Stay with the BBC, and go for the OD trans. That should give you one or two better mpg.

Also, the 88-89 700R4 weren't that great. I went through a couple in my 88. Of course when I sold it, it had over 300k on it...

bigblock73 · 08-28-2003, 12:45 AM

I say stick with the BB and 400 trans...but get a Gear Vendors OD unit...you will then have 6 speeds as opposed to a 700r4 which only has 4. It just bolts on the tailshaft of your trans and installation is pretty straight forward. You will need to shorten your existing driveshaft as well. Also, depending on your current tire size and their condition (tread wise), you could get a taller diameter tire...that will reduce your freeway RPM's a little and help on gas.

-Jeff

tibbar · 08-28-2003, 02:32 PM

I have a 75 crew cab dually, with 400sbc, 400 turbo, 4:10 rear gears, and 8.75- 16.5 tires. I haul 6000lbs daily average of scrap iron in the bed, I get 12 mpg. I'm getting ready to build my 402 BBC to put in it.

Good luck Tibbar

08-27-2003, 07:34 PM	#1
Poonman86 Registered User Join Date: May 2000 Location: Houston,Texas Posts: 123	ATTN Crewcab owners Hey guys I've got my new project crewcab longbed. It has a 454 and t400 trans in it, and as you all know the gas prices are on the rise. Thinking about putting a complete 350 tbi and 700r4 out of an 89 silverado in my truck. Whatcha think. any gas mileage increase or decrease? Will it even move this big beast? Would the trans hold up? Should I just leave the big boy in there? I won't do any kind of heavy hauling just my 21FT boat. Advice?

08-27-2003, 08:04 PM	#2
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,280	i dunno about gas mileage but i haul a 20 foot boat with a four cylinder ranger all the time all over the east coast __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

08-27-2003, 08:21 PM	#3
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,023	If you are going to covert it to SBC, don't go with the 89 motor. I can't remember the exact hp/tq numbers on it, but they aren't that great. I would find a 96-99 Vortec motor instead. BIG difference in power and torque. What gears do you have in the rear? That will make a difference too. If it has 4.10 gears, and a T400, you might save some hassle by going to 3.73's and an OD trans. What kinda mpg are you getting now, about 10? __________________ Follow me on Facebook and Instagram @N2trux.com Articles- "Jake" the 84 to 74 crewcab "Elwood" the77_Remix 85 GMC Sierra "Scarlett" "Refining Sierra"

08-28-2003, 02:32 PM	#8
tibbar Registered User Join Date: Jun 2003 Location: turon ks Posts: 120	Gas miledge I have a 75 crew cab dually, with 400sbc, 400 turbo, 4:10 rear gears, and 8.75- 16.5 tires. I haul 6000lbs daily average of scrap iron in the bed, I get 12 mpg. I'm getting ready to build my 402 BBC to put in it. Good luck Tibbar

08-27-2003, 08:48 PM	#5
Poonman86 Registered User Join Date: May 2000 Location: Houston,Texas Posts: 123	I'm getting about 9 mpg. I've got 3.73's in it. Thought about putting a 4L80 with a controller in it, but came across this wrecked 89 truck cheap but haven't bought it yet. thanks for the replies.

08-28-2003, 12:14 AM	#6
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,023	Poonman86 I have had a few of the SBC TBI trucks ( 88, 91,93,95,) and was not impressed with the power, or mileage. Stay with the BBC, and go for the OD trans. That should give you one or two better mpg. Also, the 88-89 700R4 weren't that great. I went through a couple in my 88. Of course when I sold it, it had over 300k on it...

08-28-2003, 12:45 AM	#7
bigblock73 yeller Join Date: Oct 2001 Location: Twin Cities Posts: 13,826	I say stick with the BB and 400 trans...but get a Gear Vendors OD unit...you will then have 6 speeds as opposed to a 700r4 which only has 4. It just bolts on the tailshaft of your trans and installation is pretty straight forward. You will need to shorten your existing driveshaft as well. Also, depending on your current tire size and their condition (tread wise), you could get a taller diameter tire...that will reduce your freeway RPM's a little and help on gas. -Jeff



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