4000 rpm @ 55 mph - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bellemare_486 · 02-21-2008, 07:25 PM

I have a '67 gmc 3/4 ton "camper cruiser". Rebuilt the whole truck, 350 4-speed. Put a tach in it and 4000 rpm @ 55 mph. I went to the gmc dealership in the city I live in and their answer was "get a new truck". I just need to change my rear gears. I think the term is "full floating " axle, 8-stud. Any advise would be appreciated.

67shortie · 02-21-2008, 07:29 PM

2 options,

Taller gears/tires or install an overdrive trans.

I run 3.42's and a 31 inch tall tire and cruise about 2700 at 65 with a th350 auto.

71swb4x4 · 02-21-2008, 07:39 PM

I agree, change gears and/or tires. The first thing you may want to do is make sure the tach is right.

mrein3 · 02-21-2008, 08:25 PM

Quote:

Originally Posted by 71swb4x4

I agree, change gears and/or tires. The first thing you may want to do is make sure the tach is right.

I agree. I typed in 4.56 gears in a rpm calculator and couldn't get to 4000 r's at any combo of tires that you'd find on one of these rigs. With a 4-speed there is no torque converter slip so it just doesn't make sense that you are seeing those r's at 55mph.

69halfton · 02-21-2008, 09:19 PM

yeah I'd say that your tach is way out of whack.

that truck better have some whicked BALLS on the streets if its geared that low.

whats the 60 foot times?

67ChevyRedneck · 02-21-2008, 09:33 PM

If your truck is like mine, your speedometer may be off. Different sized tires, different tranny, etc could make it off. Mine is off by about 9%

Ex: 55 is really 60 mph, 65 is really 71 mph, 70 is really about 76 mph

I did some quick rpm calcs as well... with a 27 inch tire (small tire) and 4.56 gears you'd have to be going 70 mph to get 4000 rpm.... I think you need a new tach

Even if I drop my truck from OD to third I'm only spinning 24/2500 rpm's at 55 mph with a 3.73 rear.

http://www.csgnetwork.com/multirpmcalc.html

jorgensensc · 02-21-2008, 10:22 PM

Are you sure you don't have a 6 cylinder tach? That could possibly be the culprit also!
Shawn

jgrh64 · 02-21-2008, 10:27 PM

Does it sound like the motor is racing that high ? My old 64 had 4:10 gears and it was hell on the freeway,always thought it would blow up.

ERASER5 · 02-21-2008, 10:38 PM

My old tach has a switch for 4, 6,and 8. Is your on 8? As said before, 4k at 55 does not compute.

dznucks · 02-22-2008, 01:10 AM

i agree check the # of cylinders setting on the back of the tac.

67chevemall · 02-22-2008, 01:31 AM

Quote:

Originally Posted by ERASER5

My old tach has a switch for 4, 6,and 8. Is your on 8? As said before, 4k at 55 does not compute.

What they said.
Mine is high rpm's on the highway but not that high...

Get a timing light that has the RPM's built in. They are great to test your tach too...

MrBeast · 02-22-2008, 02:04 AM

Quote:

Originally Posted by bellemare_486

I have a '67 gmc 3/4 ton "camper cruiser". Rebuilt the whole truck, 350 4-speed. Put a tach in it and 4000 rpm @ 55 mph. I went to the gmc dealership in the city I live in and their answer was "get a new truck". I just need to change my rear gears. I think the term is "full floating " axle, 8-stud. Any advise would be appreciated.

Get me a picture of your rear end, and I will see if I can tell you what axle it is. As well your tire size, I will be able to tell you if your tach, or speedo is off, and what R's your engine should pull at highway speed, I do have a calculator.

Longhorn Man · 02-22-2008, 04:11 AM

Does your SPID show an optional gear ratio on it?
I don't have any charts handy, but can anyone plug in 31 inch tire (typical size) and 5.13 gear.
If you have the eaton/corp rear, the 4.10 ratio (most all v-8/auto 3/4 ton would have it) is as high as you can realisticly locate. That would put you at about 3200 @ 70 MPH if memory serves.
Otherwise, you'll want to locate another rear axle, or an overdrive unit.

crazy longhorn · 02-22-2008, 04:27 AM

http://www.slowgt.com/Calc1.htm#RPMInGear You can play with tire size, but from what I see, even with a 29" tire, you wont be past 3300 @ 55 on a 5.13.....crazyL

Blue'72 · 02-22-2008, 04:32 AM

If all else fails, get a new truck..

Just kidding!!

**WorkinLonghorn** · 02-22-2008, 04:36 AM

His gear ratio would have to be 6.71/1. Even with 26" tires he'd have to have a 5.63 ratio (I have no idea what exists beyond 4.57) So it's pretty safe to say the tach,speedo or both are way off (or his clutch is slipping!)

haysonj · 02-22-2008, 05:33 AM

Quote:

Originally Posted by Blue'72

If all else fails, get a new truck..

Just kidding!!

Or shift out of second gear

1969 GMC · 02-22-2008, 05:51 AM

Quote:

Originally Posted by WorkinLonghorn

His gear ratio would have to be 6.71/1. Even with 26" tires he'd have to have a 5.63 ratio (I have no idea what exists beyond 4.57) So it's pretty safe to say the tach,speedo or both are way off (or his clutch is slipping!)

The eaton's common gear ratios are 4.11, 4.57, and 5.14. There is a 5.83, but I think they are too uncommon, so we can probably rule that out.

If it's a D60, I believe the tallest oem gear would be 4.56. Aftermarket, though, you can do 4.30, 4.88, 5.13, 5.38, 5.86, 6.17, and 7.17, plus higher gears which I think we ruled out

stllookn · 02-22-2008, 08:13 AM

Quote:

Or shift out of second gear

That's what I was thinking!

Trent_McAllister · 02-22-2008, 10:14 AM

Regardless any problem with your tach, you still probably want to swap out rears. It's really not that painful, I was able to locate a DANA 3.56 to replace my Eaton 4.11 on a 1968 custom camper and I am happy with the RPM drop. I never had a tach before, but the road noise is now louder that engine noise at interstate speeds. That's what you want to get the speed up and the RPM down, that way you don't have to monkey making bigger tires fit in the wells. Make sure to get some new U-bolts, the old ones will most likely break when you try to impact them off. Also I had to have my driveshaft length slightly changed. I can't remember why though.

**special-K** · 02-22-2008, 11:31 AM

Tires will never make up all that difference anyway.Gears is the way to go.

bellemare_486 · 02-23-2008, 05:00 PM

Quote:

Originally Posted by Trent_McAllister

Regardless any problem with your tach, you still probably want to swap out rears. It's really not that painful, I was able to locate a DANA 3.56 to replace my Eaton 4.11 on a 1968 custom camper and I am happy with the RPM drop. I never had a tach before, but the road noise is now louder that engine noise at interstate speeds. That's what you want to get the speed up and the RPM down, that way you don't have to monkey making bigger tires fit in the wells. Make sure to get some new U-bolts, the old ones will most likely break when you try to impact them off. Also I had to have my driveshaft length slightly changed. I can't remember why though.

Years ago I swapped rear gears out of a '66 plymouth fury into a '67 dodge pickup, it was easy and what a difference. Would you know where I could get more info on possible gears for my '67? thanx

bellemare_486 · 02-23-2008, 05:04 PM

Quote:

Originally Posted by 1969 GMC

The eaton's common gear ratios are 4.11, 4.57, and 5.14. There is a 5.83, but I think they are too uncommon, so we can probably rule that out.

If it's a D60, I believe the tallest oem gear would be 4.56. Aftermarket, though, you can do 4.30, 4.88, 5.13, 5.38, 5.86, 6.17, and 7.17, plus higher gears which I think we ruled out

I should have counted the teeth when I had the cap off. I'll do this and could you please tell me where I could source these aftermarket gears ? thanx

bellemare_486 · 02-23-2008, 05:10 PM

Quote:

Originally Posted by 71swb4x4

I agree, change gears and/or tires. The first thing you may want to do is make sure the tach is right.

I got alot of replies, most pointing at my tach. I'll pull the tach out of my other truck and try that tomorrow, thanx

olblue60@aol.com · 02-23-2008, 05:14 PM

If you go to an interstate with the mile markers, you can check it against your truck odometer. It is a direct corolation to your speedometer to check if it is correct or not.

02-21-2008, 07:25 PM	#1
bellemare_486 Registered User Join Date: Feb 2008 Location: Vernon, B.C. Posts: 17	4000 rpm @ 55 mph I have a '67 gmc 3/4 ton "camper cruiser". Rebuilt the whole truck, 350 4-speed. Put a tach in it and 4000 rpm @ 55 mph. I went to the gmc dealership in the city I live in and their answer was "get a new truck". I just need to change my rear gears. I think the term is "full floating " axle, 8-stud. Any advise would be appreciated.

02-21-2008, 07:29 PM	#2
67shortie Green club Join Date: May 2002 Location: Central CA Posts: 1,059	Re: 4000 rpm @ 55 mph 2 options, Taller gears/tires or install an overdrive trans. I run 3.42's and a 31 inch tall tire and cruise about 2700 at 65 with a th350 auto. __________________ 71 short/fleet ECE 4/6 drop, LQ4/700r4/3.42 gears 70 SS396 Chevelle 427/200-4R/3.73posi 61 Lincoln Continental, black on black Son you are going to drive me to drinkin if you don't stop drivin that hot rod lincoln Last edited by 67shortie; 02-21-2008 at 07:30 PM. Reason: Shlepped

02-21-2008, 07:39 PM	#3
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: 4000 rpm @ 55 mph I agree, change gears and/or tires. The first thing you may want to do is make sure the tach is right. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

02-21-2008, 09:19 PM	#5
69halfton Registered User Join Date: Apr 2007 Location: King City, Ontario, Canada Posts: 974	Re: 4000 rpm @ 55 mph yeah I'd say that your tach is way out of whack. that truck better have some whicked BALLS on the streets if its geared that low. whats the 60 foot times? __________________ - Jason If you can turn, Youre not going fast enough. Favorite caption: first car to 200 mph in the 1/4 mile, no parachute, drum brakes all the way around.. ..back when men were men.

02-21-2008, 09:33 PM	#6
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Re: 4000 rpm @ 55 mph If your truck is like mine, your speedometer may be off. Different sized tires, different tranny, etc could make it off. Mine is off by about 9% Ex: 55 is really 60 mph, 65 is really 71 mph, 70 is really about 76 mph I did some quick rpm calcs as well... with a 27 inch tire (small tire) and 4.56 gears you'd have to be going 70 mph to get 4000 rpm.... I think you need a new tach Even if I drop my truck from OD to third I'm only spinning 24/2500 rpm's at 55 mph with a 3.73 rear. http://www.csgnetwork.com/multirpmcalc.html __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications! Last edited by 67ChevyRedneck; 02-21-2008 at 09:34 PM.

02-21-2008, 10:22 PM	#7
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	Re: 4000 rpm @ 55 mph Are you sure you don't have a 6 cylinder tach? That could possibly be the culprit also! Shawn __________________ 1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE

02-21-2008, 10:27 PM	#8
jgrh64 Prestige member Old Farts Club Join Date: Nov 2005 Location: Cleveland Posts: 502	Re: 4000 rpm @ 55 mph Does it sound like the motor is racing that high ? My old 64 had 4:10 gears and it was hell on the freeway,always thought it would blow up. __________________ Hey sombodys gotta do it!

02-21-2008, 10:38 PM	#9
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: 4000 rpm @ 55 mph My old tach has a switch for 4, 6,and 8. Is your on 8? As said before, 4k at 55 does not compute. __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60

02-22-2008, 01:10 AM	#10
dznucks Registered User Join Date: Feb 2006 Location: Smyrna, Ga Posts: 1,647	Re: 4000 rpm @ 55 mph i agree check the # of cylinders setting on the back of the tac.

02-22-2008, 04:11 AM	#13
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 4000 rpm @ 55 mph Does your SPID show an optional gear ratio on it? I don't have any charts handy, but can anyone plug in 31 inch tire (typical size) and 5.13 gear. If you have the eaton/corp rear, the 4.10 ratio (most all v-8/auto 3/4 ton would have it) is as high as you can realisticly locate. That would put you at about 3200 @ 70 MPH if memory serves. Otherwise, you'll want to locate another rear axle, or an overdrive unit.

02-22-2008, 04:27 AM	#14
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	Re: 4000 rpm @ 55 mph http://www.slowgt.com/Calc1.htm#RPMInGear You can play with tire size, but from what I see, even with a 29" tire, you wont be past 3300 @ 55 on a 5.13.....crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

02-22-2008, 04:32 AM	#15
Blue'72 Skinned knuckles Join Date: Oct 2006 Location: Longview, Washington Posts: 1,126	Re: 4000 rpm @ 55 mph If all else fails, get a new truck.. Just kidding!! __________________ '72 C-10 longbox soon to be short step! '72 Chevelle (sold in January) '07 Sebring '01 Ranger (I've gotta long commute..) If you try to fail, and succeed, which have you done? My build thread - http://67-72chevytrucks.com/vboard/s...d.php?t=276904

02-22-2008, 04:36 AM	#16
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,886	Re: 4000 rpm @ 55 mph His gear ratio would have to be 6.71/1. Even with 26" tires he'd have to have a 5.63 ratio (I have no idea what exists beyond 4.57) So it's pretty safe to say the tach,speedo or both are way off (or his clutch is slipping!) __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

02-22-2008, 10:14 AM	#20
Trent_McAllister Registered User Join Date: Jan 2002 Location: Elizabethtown, KY, USA Posts: 136	Re: 4000 rpm @ 55 mph Regardless any problem with your tach, you still probably want to swap out rears. It's really not that painful, I was able to locate a DANA 3.56 to replace my Eaton 4.11 on a 1968 custom camper and I am happy with the RPM drop. I never had a tach before, but the road noise is now louder that engine noise at interstate speeds. That's what you want to get the speed up and the RPM down, that way you don't have to monkey making bigger tires fit in the wells. Make sure to get some new U-bolts, the old ones will most likely break when you try to impact them off. Also I had to have my driveshaft length slightly changed. I can't remember why though. __________________ My Truck: 1968 C-20 Custom Camper

02-22-2008, 11:31 AM	#21
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: 4000 rpm @ 55 mph Tires will never make up all that difference anyway.Gears is the way to go. __________________ "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 ~



Register or Log In To remove these advertisements.

02-23-2008, 05:14 PM	#25
olblue60@aol.com member Join Date: Dec 2003 Location: Silver Spring Md.20901 Posts: 2,031	Re: 4000 rpm @ 55 mph If you go to an interstate with the mile markers, you can check it against your truck odometer. It is a direct corolation to your speedometer to check if it is correct or not. __________________ 1967 C/K10 Chevy Panel 1968 C/K20 Chevy Pickup 1968 C/K10 Chevy Suburban 1941 Chevy Sedan Delivery 1960 Chevy Sedan Delivery 1972 Camaro SS 396 #s matching 1971 Harley FX Superglide

Register or Log In To remove these advertisements.