|
02-21-2008, 07:25 PM
|
#1 |
|
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 |
|
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 |
|
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, 08:25 PM
|
#4 | |
|
Registered User
Join Date: Feb 2000
Location: Center City, MN, USA
Posts: 3,253
|
Re: 4000 rpm @ 55 mph
Quote:
__________________
'70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN |
|
|
|
|
|
02-21-2008, 09:19 PM
|
#5 |
|
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 |
|
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 |
|
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 |
|
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 |
|
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 |
|
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, 01:31 AM
|
#11 | |
|
Registered User
Join Date: Apr 2003
Location: NB
Posts: 3,367
|
Re: 4000 rpm @ 55 mph
Quote:
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...
__________________
1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
|
|
|
|
|
02-22-2008, 02:04 AM
|
#12 | |
|
high-Tech Red Neck
Join Date: Feb 2008
Location: Livingston, MT/On my boat WA/BC/AK
Posts: 2,294
|
Re: 4000 rpm @ 55 mph
Quote:
__________________
"He used to be a pretty nice feller, now he's a welder!" |
|
|
|
|
|
02-22-2008, 04:11 AM
|
#13 |
|
its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
|
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 |
|
Fabricate till you "puke"
Join Date: Jun 2002
Location: Ill
Posts: 9,403
|
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 |
|
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 |
|
Senior Member
Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,858
|
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, 2nd owner, Work-Truck supreme. Best $300 I ever spent. |
|
|
|
|
02-22-2008, 05:33 AM
|
#17 | |
|
Registered User
Join Date: Dec 2007
Location: Santa Maria, Ca.
Posts: 1,423
|
Re: 4000 rpm @ 55 mph
Quote:
__________________
69 Chevy 4x4 383 Power 
|
|
|
|
|
|
02-22-2008, 05:51 AM
|
#18 | |
|
Registered User
Join Date: Feb 2003
Location: The Rubber City
Posts: 5,498
|
Re: 4000 rpm @ 55 mph
Quote:
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 
__________________
1969 GMC K2500  1996 Honda Accord 2007 Kawasaki KLR 650 |
|
|
|
|
|
02-22-2008, 08:13 AM
|
#19 | |
|
Saving 1 truck at a time!
Join Date: Aug 2001
Location: Kent, WA
Posts: 6,463
|
Re: 4000 rpm @ 55 mph
Quote:
 That's what I was thinking!
__________________
'68 C20 Longhorn 50th Anniversary 400/TH400 '68 C20 Longhorn 50th Anniversary 468/TH400w/buckets '72 C20 Halfhorn (Longhorn w/o cab and front clip) '69 Flxible Cruiser (look up ugly in the dictionary) |
|
|
|
|
|
02-22-2008, 10:14 AM
|
#20 |
|
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 Order
  
Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,863
|
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 ~ |
|
|
|
|
02-23-2008, 05:00 PM
|
#22 | |
|
Registered User
Join Date: Feb 2008
Location: Vernon, B.C.
Posts: 17
|
Re: 4000 rpm @ 55 mph
Quote:
|
|
|
|
|
|
02-23-2008, 05:04 PM
|
#23 | |
|
Registered User
Join Date: Feb 2008
Location: Vernon, B.C.
Posts: 17
|
Re: 4000 rpm @ 55 mph
Quote:
|
|
|
|
|
|
02-23-2008, 05:10 PM
|
#24 | |
|
Registered User
Join Date: Feb 2008
Location: Vernon, B.C.
Posts: 17
|
Re: 4000 rpm @ 55 mph
Quote:
|
|
|
|
|
|
02-23-2008, 05:14 PM
|
#25 |
|
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 |
|
|
|
 |
| Bookmarks |
|
|
Linear Mode
