Gas Mileage/ Top End Speed for 72 C20 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tjbindy · 06-04-2013, 09:06 AM

Just bought a beautiful 2 wheel drive 1972 C20. The truck is a light blue; almost a turquoise in color with steel flatbed. Anyway it has a 400 in it bored .030 over. Rear end is a Dana60. Tranny is automatic but not sure which. Frankly I can only feel one shift point so I am wondering if maybe it is a powerglide.

Anyway it has LOTS of power, but no acceleration. 65mph is absolute top end. It absolutely loves to drink gas. I need it to be a little more highway friendly. I may use it occasionally to tow a trailer but that would be very occasionally. Truck will mostly be used to run errands and carry my garden tractor around.

cdowns · 06-04-2013, 09:18 AM

8-12 is the norm
spend a bunch of money and maximize your maintenence an you can bump it up to 14-16 pretty easily// or spend a hole lot more for like a gearvendor box a better rear end and a new power supply system like lots of guys are goin with the newer powerplants with fuel injection

PanelDeland · 06-04-2013, 09:27 AM

WELCOME!!! First thing is "We need pics".I just wanted to get that in since it will come up.Second,it sounds like it's in good tune.Being a C20 it may have lower rear gears.Jack it up and rotate the tire one full turn while counting revolutions of the driveshaft.I'm guessing you will get between 3-4 driveshaft rotations to tire rotations.If your truck is an auto 3:07/8 would be normal factory gears though others were optional.You can swap rear ends faily inexpensively or spend more and go to an auto OD.Tire size also plays in the equation.Look in the glovebox door and see if the SPID is still there(it's a label with specs on your truck).You should have codes for trans and rear gears(if optional) and someone can decode it for you.We can better help you achieve your goal from there.

fourspeedwagon · 06-04-2013, 02:12 PM

TJ- First, you should crawl under the truck and see what trans it has. If you only detect one shift and it tops out at 65, you may not be engaging 3rd gear. Does it have a tachometer? rpm and mph, along with tire size and we can figure out what the rearend gear ratio is. If it has extraordinarily low gears, then a swap would be the way to go, if it's running around on the interstate in second gear then we can start to chase why.
Note the shape of the trans pan and number of bolts or post a pic of the pan and we can help. Greg

luvbowties · 06-04-2013, 03:15 PM

Quote:

Originally Posted by tjbindy

Just bought a beautiful 2 wheel drive 1972 C20. The truck is a light blue; almost a turquoise in color with steel flatbed. Anyway it has a 400 in it bored .030 over. Rear end is a Dana60. Tranny is automatic but not sure which. Frankly I can only feel one shift point so I am wondering if maybe it is a powerglide.

Anyway it has LOTS of power, but no acceleration. 65mph is absolute top end. It absolutely loves to drink gas. I need it to be a little more highway friendly. I may use it occasionally to tow a trailer but that would be very occasionally. Truck will mostly be used to run errands and carry my garden tractor around.

"...LOTS of power, but no acceleration. 65mph is absolute top end" do NOT sound as if they should go together.
You may like this site to determine which trans you have, before reporting back with answers to others' questions.
Sam
>>http://www.nastyz28.com/chevy-transm...tification.php <<

**LockDoc** · 06-04-2013, 06:56 PM

-
As far as the rear axle ratio there will be a code number stamped on the rear axle housing in the location shown in the pictures below. The only thing you will use for this lookup will be the first two or three letters. '67-'69 will have two letters and '70-'72 will have 3 letters. Look the letters up for your year of truck in the charts I posted in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

LockDoc

72chevyk10 · 06-04-2013, 07:21 PM

if you hold the trans in 1st and manually go to 2nd and then drive does it shift 3 times then? my old th350 i could never tell it was ever in 1st gear cause it shifted so soft.. and rpm never changed for the shift. but it would do all 3 if i used the shifter. 65 tops sounds about right if its got low gears. my rig i never take over 60 cause of 4.56's even with 35"s it'll wind up to 2600 at 60. i know it'll do more (had it to 90

) but i dont like winding the big block higher then it needs to

Mustard72 · 06-04-2013, 09:11 PM

I had a 4.57:1 Eaton rear end in mine before I tore it all apart. 3200 RPM at 60 mph with stock sized tires (29" diameter or so). Lots of low end grunt with that rear end for a stock 350. That being said, I've had the truck since I was 16, and it's seen the high side of 100 mph more than once. That comes in at about 5300 RPM, but it didn't seem to have too much trouble getting to that speed. Don't get me wrong, it was no drag race star or anything, but I'm thinking your truck should easily do more than 65. Something's wrong. Lots of power, no acceleration, and 65 top end don't add up.

I also don't think a C20 with a V8 would have come with a powerglide in 72. More than likely a TH350 or TH400. The TH350 has basically a squarish pan with one of the corners angled. The TH400 has a more irregularly shaped pan.

I could get 10-12 mpg with mine typically. Once I got about 14 on a fresh tune up, but that was pretty much a rare occasion, and subject to debate (like most mileage figures).

tjbindy · 06-04-2013, 10:35 PM

Quote:

Originally Posted by fourspeedwagon

TJ- First, you should crawl under the truck and see what trans it has. If you only detect one shift and it tops out at 65, you may not be engaging 3rd gear. Does it have a tachometer? rpm and mph, along with tire size and we can figure out what the rearend gear ratio is. If it has extraordinarily low gears, then a swap would be the way to go, if it's running around on the interstate in second gear then we can start to chase why.
Note the shape of the trans pan and number of bolts or post a pic of the pan and we can help. Greg

Based on the info from another post. The tranny is a TH350. As for the rear end. . . did not see any markings; I will check again tomorrow. However; with the rear end jacked up.. . both wheels off the ground. 1 rotation of driver side rear wheel resulted in drive shaft rotating 2 revolutions plus about 10 degrees.

**special-K** · 06-04-2013, 10:42 PM

If your truck has the factory drivetrain it will likely have 3.54s w/automatic,if you're saying you have a 402,and the trans will be a TH400. Powerglides were gone by '72. The optional ratios go down (higher ratio) from there. I've driven all day on the interstate at 75-80 mph with 4.10s on a C/20. Maybe you aren't coming out of 2nd.

**LockDoc** · 06-04-2013, 11:52 PM

Quote:

Originally Posted by tjbindy

Based on the info from another post. The tranny is a TH350. As for the rear end. . . did not see any markings; I will check again tomorrow. However; with the rear end jacked up.. . both wheels off the ground. 1 rotation of driver side rear wheel resulted in drive shaft rotating 2 revolutions plus about 10 degrees.

Unless you have a positraction rear end I think you only jack up one wheel.

LockDoc

PanelDeland · 06-05-2013, 03:49 PM

I have another ? Does it not go faster or is it just that you feel it is running too high RPM's.Many have driven modern vehicles with OD for so long they think they are overwinding the engine when it's a normal RPM for this era vehicle.

Aruba1 · 10-02-2021, 08:15 PM

Bringing this thread back from the dead. I just did 150 miles on my 1972 C30 with an LS1, NV4500, 1989 C3500 rear end and brakes. Did an average of 80 mph and topped 90 mph a few times. No problems. Pickup shakes a little bit but I think one wheel is out of round. I had it as fast 110 mph in the past.

06-04-2013, 09:06 AM	#1
tjbindy Registered User Join Date: May 2013 Location: Indianapolis, Indiana Posts: 4	Gas Mileage/ Top End Speed for 72 C20 Just bought a beautiful 2 wheel drive 1972 C20. The truck is a light blue; almost a turquoise in color with steel flatbed. Anyway it has a 400 in it bored .030 over. Rear end is a Dana60. Tranny is automatic but not sure which. Frankly I can only feel one shift point so I am wondering if maybe it is a powerglide. Anyway it has LOTS of power, but no acceleration. 65mph is absolute top end. It absolutely loves to drink gas. I need it to be a little more highway friendly. I may use it occasionally to tow a trailer but that would be very occasionally. Truck will mostly be used to run errands and carry my garden tractor around.

06-04-2013, 09:18 AM	#2
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Gas Mileage/ Top End Speed for 72 C20 8-12 is the norm spend a bunch of money and maximize your maintenence an you can bump it up to 14-16 pretty easily// or spend a hole lot more for like a gearvendor box a better rear end and a new power supply system like lots of guys are goin with the newer powerplants with fuel injection __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

06-04-2013, 09:27 AM	#3
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,992	Re: Gas Mileage/ Top End Speed for 72 C20 WELCOME!!! First thing is "We need pics".I just wanted to get that in since it will come up.Second,it sounds like it's in good tune.Being a C20 it may have lower rear gears.Jack it up and rotate the tire one full turn while counting revolutions of the driveshaft.I'm guessing you will get between 3-4 driveshaft rotations to tire rotations.If your truck is an auto 3:07/8 would be normal factory gears though others were optional.You can swap rear ends faily inexpensively or spend more and go to an auto OD.Tire size also plays in the equation.Look in the glovebox door and see if the SPID is still there(it's a label with specs on your truck).You should have codes for trans and rear gears(if optional) and someone can decode it for you.We can better help you achieve your goal from there. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil"

06-04-2013, 02:12 PM	#4
fourspeedwagon Registered User Join Date: Oct 2010 Location: Yakima Valley, Washington Posts: 580	Re: Gas Mileage/ Top End Speed for 72 C20 TJ- First, you should crawl under the truck and see what trans it has. If you only detect one shift and it tops out at 65, you may not be engaging 3rd gear. Does it have a tachometer? rpm and mph, along with tire size and we can figure out what the rearend gear ratio is. If it has extraordinarily low gears, then a swap would be the way to go, if it's running around on the interstate in second gear then we can start to chase why. Note the shape of the trans pan and number of bolts or post a pic of the pan and we can help. Greg

06-04-2013, 06:56 PM	#6
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 26,069	Re: Gas Mileage/ Top End Speed for 72 C20 - As far as the rear axle ratio there will be a code number stamped on the rear axle housing in the location shown in the pictures below. The only thing you will use for this lookup will be the first two or three letters. '67-'69 will have two letters and '70-'72 will have 3 letters. Look the letters up for your year of truck in the charts I posted in this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=578793 LockDoc Attached Images __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

06-04-2013, 09:11 PM	#8
Mustard72 Registered User Join Date: Jul 2012 Location: Ladysmith, WI Posts: 641	Re: Gas Mileage/ Top End Speed for 72 C20 I had a 4.57:1 Eaton rear end in mine before I tore it all apart. 3200 RPM at 60 mph with stock sized tires (29" diameter or so). Lots of low end grunt with that rear end for a stock 350. That being said, I've had the truck since I was 16, and it's seen the high side of 100 mph more than once. That comes in at about 5300 RPM, but it didn't seem to have too much trouble getting to that speed. Don't get me wrong, it was no drag race star or anything, but I'm thinking your truck should easily do more than 65. Something's wrong. Lots of power, no acceleration, and 65 top end don't add up. I also don't think a C20 with a V8 would have come with a powerglide in 72. More than likely a TH350 or TH400. The TH350 has basically a squarish pan with one of the corners angled. The TH400 has a more irregularly shaped pan. I could get 10-12 mpg with mine typically. Once I got about 14 on a fresh tune up, but that was pretty much a rare occasion, and subject to debate (like most mileage figures). __________________ Nick Old Mustard, 1972 Chevy C20, 350, TH350, 4.10 Dana 60 rear, owned since I was 16 in 1986 http://67-72chevytrucks.com/vboard/s...d.php?t=545356 Blue Bell, 1987 K20, 6.2L Diesel, TH400/NP208, 4.10 gears Ustahav 1976 K10, 350/350/203, Gov Loc, Yooper beater trail truck.

06-04-2013, 10:42 PM	#10
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: Gas Mileage/ Top End Speed for 72 C20 If your truck has the factory drivetrain it will likely have 3.54s w/automatic,if you're saying you have a 402,and the trans will be a TH400. Powerglides were gone by '72. The optional ratios go down (higher ratio) from there. I've driven all day on the interstate at 75-80 mph with 4.10s on a C/20. Maybe you aren't coming out of 2nd. __________________ "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 ~

06-05-2013, 03:49 PM	#12
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,992	Re: Gas Mileage/ Top End Speed for 72 C20 I have another ? Does it not go faster or is it just that you feel it is running too high RPM's.Many have driven modern vehicles with OD for so long they think they are overwinding the engine when it's a normal RPM for this era vehicle. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil"

10-02-2021, 08:15 PM	#13
Aruba1 Registered User Join Date: Apr 2014 Location: Birmingham, AL Posts: 560	Re: Gas Mileage/ Top End Speed for 72 C20 Bringing this thread back from the dead. I just did 150 miles on my 1972 C30 with an LS1, NV4500, 1989 C3500 rear end and brakes. Did an average of 80 mph and topped 90 mph a few times. No problems. Pickup shakes a little bit but I think one wheel is out of round. I had it as fast 110 mph in the past.



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