gear ratios? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tex truckie · 08-19-2006, 11:16 AM

hello all ,
i have a 72 super its all original.
402 cid,400 turbo,holly 650,hugger orange/white(sweet looking truck)
i get around 11.5 mpg.
i was worndering if any one might know a couple of things or have suggestions about:
1.when cruising 70 mph my tach is at 3000 rpm,of course this is not helping my mpg,is there any way to get this lowered without affecting performance greatly?
2.i am unsure of my factory gear ratios,but when people say a higher gear would help this out does that mean a lower number?
3.also i hear overdrive tranny would help,i was thinking like a 3 speed overdrive turbo 350 from a 80's model chevy,would a 350 turbo hold up behind this BB 402 or would it being to much torqe and blow it out?
all info and or suggestions are apricciated .

dougs69c10 · 08-19-2006, 11:25 AM

put a 3.07 gear in the truck, with the torque of the bb you can easily walk all over the gears

Longhorn Man · 08-19-2006, 11:28 AM

Agreed.
The overdrive 350 never existed. You may be thinking of the 700R4.

Gary · 08-19-2006, 12:57 PM

I have much the same truck as you except I have the 3.08 (3.07?) gears and the 700r4 I still get about 12mpg. GRANTED my foot when I drive that truck is made of lead so this could be mostly due to ME not the truck

And yep it'll romp all over those lower (er higher?) gears with no problem at all.

81 rustruck · 08-19-2006, 01:55 PM

I converted my 81 to 700 r4.The rpm goes from 2800 on the highway to 1800.I get about 700 km out of two tanks now instead of 500.

The swap was easy for me it is a short box with a long tailshaft th350.
I only had to move the cross member.
My only compaint is the 700 r 4 runs a really tight converter stock.

Longhorn Man · 08-19-2006, 03:19 PM

you guys would probably get a little more MPG if you went to a 3.73 or a 3.42 with the 700 R4.

tex truckie · 08-20-2006, 06:44 PM

thanks for all the great input!!!!!
at this point i believe getiing the 700-r4 will be the way to go and then work the gears if nessacary.
i wonder will my existing torque convertor work well with this tranny?
as well what might be the best way to set up a tranny for long term storage.
thanks again.

Gary · 08-20-2006, 07:51 PM

Quote:

Originally Posted by Longhorn Man

you guys would probably get a little more MPG if you went to a 3.73 or a 3.42 with the 700 R4.

I've got (what's supposed to be) a 3.73 posi in the shed, only it's 6 lugs so trying to decide the best plan of attack, swap guts or swap rearends and axles.

Longhorn Man · 08-20-2006, 08:19 PM

Any idea what year the axle is out of?

**kwmech** · 08-20-2006, 08:33 PM

You didn't say whether the truck was a 1/2 or 3/4 ton. If building a 700,you will have to have one custom built to handle the torque of the big block and there is a heavy duty convertor available-- HDT--. All the beefy stuff inside plus torrington bearings instead of washers. A large cooler is a big plus.

08-19-2006, 11:16 AM	#1
tex truckie Registered User Join Date: May 2006 Location: dallas Posts: 8	gear ratios? hello all , i have a 72 super its all original. 402 cid,400 turbo,holly 650,hugger orange/white(sweet looking truck) i get around 11.5 mpg. i was worndering if any one might know a couple of things or have suggestions about: 1.when cruising 70 mph my tach is at 3000 rpm,of course this is not helping my mpg,is there any way to get this lowered without affecting performance greatly? 2.i am unsure of my factory gear ratios,but when people say a higher gear would help this out does that mean a lower number? 3.also i hear overdrive tranny would help,i was thinking like a 3 speed overdrive turbo 350 from a 80's model chevy,would a 350 turbo hold up behind this BB 402 or would it being to much torqe and blow it out? all info and or suggestions are apricciated .

08-19-2006, 11:25 AM	#2
dougs69c10 Registered User Join Date: Feb 2006 Location: Birmingham,Alabama Posts: 260	Re: gear ratios? put a 3.07 gear in the truck, with the torque of the bb you can easily walk all over the gears __________________ 1969 c10 stepside 250 inline six, 4speed granny tranny, 4.11 posi rear end

08-19-2006, 11:28 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: gear ratios? Agreed. The overdrive 350 never existed. You may be thinking of the 700R4.

08-19-2006, 12:57 PM	#4
Gary Senior Member Join Date: Jan 2004 Location: Harvest, AL Posts: 545	Re: gear ratios? I have much the same truck as you except I have the 3.08 (3.07?) gears and the 700r4 I still get about 12mpg. GRANTED my foot when I drive that truck is made of lead so this could be mostly due to ME not the truck And yep it'll romp all over those lower (er higher?) gears with no problem at all. __________________ Gary Harris 85 Silverado - 87 Grand National 2007 Ardmore Car Show

08-19-2006, 01:55 PM	#5
81 rustruck Registered User Join Date: Apr 2006 Location: Canada Posts: 12	Re: gear ratios? I converted my 81 to 700 r4.The rpm goes from 2800 on the highway to 1800.I get about 700 km out of two tanks now instead of 500. The swap was easy for me it is a short box with a long tailshaft th350. I only had to move the cross member. My only compaint is the 700 r 4 runs a really tight converter stock.

08-19-2006, 03:19 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: gear ratios? you guys would probably get a little more MPG if you went to a 3.73 or a 3.42 with the 700 R4.

08-20-2006, 06:44 PM	#7
tex truckie Registered User Join Date: May 2006 Location: dallas Posts: 8	Re: gear ratios? thanks for all the great input!!!!! at this point i believe getiing the 700-r4 will be the way to go and then work the gears if nessacary. i wonder will my existing torque convertor work well with this tranny? as well what might be the best way to set up a tranny for long term storage. thanks again.

08-20-2006, 08:19 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: gear ratios? Any idea what year the axle is out of?

08-20-2006, 08:33 PM	#10
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,571	Re: gear ratios? You didn't say whether the truck was a 1/2 or 3/4 ton. If building a 700,you will have to have one custom built to handle the torque of the big block and there is a heavy duty convertor available-- HDT--. All the beefy stuff inside plus torrington bearings instead of washers. A large cooler is a big plus.



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