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#1 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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1971 Chevy Pickup revs too high
Before I start thanks for all of your patience for us new guys!!! I posted before my truck wasn't shifting into 3rd gear. I was wrong its working fine. Its just that it runs about 4 grand when going around 55 mph. I am assuming my rear end is geared too low. How do I confirm this and what can I do to resolve this? Truck runs and idles fine.
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#2 |
Senior Member
![]() Join Date: Dec 2013
Location: Gods country East,Tn
Posts: 8,545
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Re: 1971 Chevy Pickup revs too high
Need more details ,what engine ,what model truck ,what trans, what rear differential ,what size tires ???
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![]() 1967 Factory short bed - Old school '71 - 350 / 4bolt / 487 heads / Edelbrock C3BX Muncie M-22 4 speed / Hurst Comp plus Factory 12 bolt posi 3.73 / 255-70-15 Smoothed firewall / Factory cowl induction Power disc brakes / power steering / 3.5-5" drop |
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#3 |
Registered User
Join Date: Nov 2008
Location: Gwinnett GA
Posts: 1,803
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Re: 1971 Chevy Pickup revs too high
Your transmission is slipping? Even 5.13 gears wouldn't be 4000@ 55.
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1982 K10 SWB 1987 V20 - Sold - Doh! |
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#4 |
Registered User
Join Date: Dec 2013
Location: Durango, CO
Posts: 827
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Re: 1971 Chevy Pickup revs too high
Check the transmission fluid, Step #1.
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1972 K10 Cheyenne Super | LWB, fleetside | 350/350/205 | KEEPER 1971 K10 Cheyenne | SWB, fleetside | LS Swap 5.3/4L60 | SOLD 1976 Trans Am | 400/4-spd | SOLD 1976 Trans Am | 455/4-spd | TOTALED ![]() |
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#5 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
350 chevy 3 speed automatic. 30 inch tires. The transmission doesn't feel like its slipping.
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#6 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
checked the fluid when it was in gear per instruction. Fluid looks good and has good level
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#7 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
how do I find out what differential gearing I have?
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#8 |
Registered User
Join Date: Nov 2008
Location: Gwinnett GA
Posts: 1,803
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Re: 1971 Chevy Pickup revs too high
You need to search that one man!
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1982 K10 SWB 1987 V20 - Sold - Doh! |
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#9 |
Registered User
Join Date: Aug 2013
Location: Fort Worth, TX
Posts: 309
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Re: 1971 Chevy Pickup revs too high
Are you confident you are shifting into 3rd? With a 30" tall tire and 4.10 gear, 2nd gear at 4,000 rpm would 56 mph.
My favorite speed calculator: http://www.catherineandken.co.uk/sti/tyres.html |
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#10 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
I can down shift going slow from drive into second and first and can feel it shift twice the same as starting in drive from a start and feeling it shift twice. Sorry for the caps too lazy to change now.
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#11 |
Senior Member
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Location: Falls City, Nebraska "100 Miles From Nowhere"
Posts: 2,219
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Re: 1971 Chevy Pickup revs too high
Read this, http://www.crankshaftcoalition.com/w...ar_ratio_check and this, http://www.how-to-build-hotrods.com/gear-ratio.html The best method is to pop the cover, (have a new cover gasket ready to install as well as two quarts of new 80w-90 gear oil with a GL-5 API rating) and read the number of teeth stamped on the edge of the ring gear. Such as 14 43, divide the the large number by the smaller number to get the ratio. In this case, this ratio is a 3:07...By popping the cover you take the guess work out of the equation and you determine whether or not you have a positrack as well as changing the gear lube that God only knows the last time it was changed.~Ghostrider~
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Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" ![]() "Be American, Buy American" ![]() |
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#12 |
Registered User
Join Date: Aug 2013
Location: Fort Worth, TX
Posts: 309
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Re: 1971 Chevy Pickup revs too high
To get 4000 rpm you would need a gear ratio in the 6.5:1 range. Chances are slim that you have a gear of that nature. Are you confident the tach is accurate? Does it have adjustments for 6 cylinder vs 8 cylinder?
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#13 |
Registered User
Join Date: Dec 2013
Location: Durango, CO
Posts: 827
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Re: 1971 Chevy Pickup revs too high
You can also jack up the rear and rotate a wheel, count number of turns at wheel vs. driveshaft, and do the math to figure your ratio...
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1972 K10 Cheyenne Super | LWB, fleetside | 350/350/205 | KEEPER 1971 K10 Cheyenne | SWB, fleetside | LS Swap 5.3/4L60 | SOLD 1976 Trans Am | 400/4-spd | SOLD 1976 Trans Am | 455/4-spd | TOTALED ![]() |
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#14 | ||
"I ain't nobody, dork."
Join Date: Sep 2007
Location: Whidbey Island, Washington
Posts: 8,940
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Re: 1971 Chevy Pickup revs too high
You said "about 4 grand". Do you have a tach in the truck?
I'm betting it's cruising at around 2,500 to 2,800 RPM's and you're just used to modern vehicles with Fuel Injection and Overdrive transmissions That cruise at 70 mph at 1,800 RPM's. These old heaps can cruise at 3,000 RPM's all day long and not even give a rip. Gary
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'cuz chicks dig scars... My 1972 GMC 1500 Super Custom (Creeping Death) "long term" build thread. The Rebuild of Creeping Death after the wreck Quote:
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#15 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
Its the stock dash tac. The engine is from the 90's. Found that out when I replaced the starter! Its a 350. Ill put a strobe on the motor and see if the rpms are accurate. Thanks you guys.
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#16 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
Thanks great info looks like I have 410 gears in the rear end. Two full tire rotations were a little over 4 revolutions of the driveshaft.
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#17 |
Registered User
Join Date: Aug 2016
Location: Lake Forest, CA
Posts: 2,177
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Re: 1971 Chevy Pickup revs too high
My impala has a flex fan and no fan clutch and it sounds like its screaming 5k at 2500...
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"Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC |
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#18 |
Registered User
Join Date: Sep 2014
Location: Florida
Posts: 184
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Re: 1971 Chevy Pickup revs too high
My truck with 4.10 gearing and a TH350 screams at around 3k rpm at 60. At first I was afraid to go over 60 because I thought it would hurt the motor, now I'm afraid to go over 60 because it will hurt my wallet.
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Nick 1972 Chevrolet C10 Stepside |
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#19 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
just strobed it tacs pretty accurate. Found a pretty good tranny leak. Fixed it and will try it soon.
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#20 |
Registered User
Join Date: Aug 2016
Location: Lake Forest, CA
Posts: 2,177
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Re: 1971 Chevy Pickup revs too high
My son's first car is an 86 Scottsdale with a 350, TH400 and 4:10. Its hurting MY wallet...
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"Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC |
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#21 |
Registered User
Join Date: Sep 2015
Location: danbury, ct.
Posts: 568
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Re: 1971 Chevy Pickup revs too high
I have 410's in my 70 Longhorn with a 350 and a 4 speed. I usually keep it at about 2600 while on the highway for extended period's. I'll bump it up to 3 grand for a short burst every now and then but that is when the fuel gauge really begins too drop like a stone. I know for a fact the original owner of my truck was going to work it and he wanted the 410's. It was real easy to do the speed limit with a full load with 410's.. Back in the day 55 mph was the rule of the day and I think either most people didn't push the envelope as much or they enforced the speed limit more???? Its happens almost every day now that a freight train of cars blow by me doing at least 85... If you want to spend a considerable amount of time in the left lane you are going to need a gear change... Or... just do what I do, sit back and enjoy the slow ride....
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#22 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: 1971 Chevy Pickup revs too high
Silly question perhaps, but it's not a Pertronix III ignition is it? They can fire multiple times per piston event but don't have a tach wire so if you're hooked up to the coil for your tach, it can read double actual RPMs at times.
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#23 | |
Registered User
![]() Join Date: Nov 2007
Location: Hermiston, OR
Posts: 947
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Re: 1971 Chevy Pickup revs too high
Quote:
Clutch fan is on its way!
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Running in place gets you nowhere fast. -me 1968 GTO 4-spd covnvertible 1970 K-20 4-speed, factory AC. |
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#24 |
Registered User
Join Date: Sep 2015
Location: Anaheim CA
Posts: 42
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Re: 1971 Chevy Pickup revs too high
I thought that my 1971 K20 was reving to high also. Then I bought a aftermarket tachometer and found out that my stock in dash tach from 1971 was just wrong by about 600 RPMs.
Also I found this website very helpful, just put in your tire size, tranny type, and rear end ratio and it gives you exact numbers for what your RPMs should be. http://www.grimmjeeper.com/gears.html |
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#25 |
Registered User
Join Date: Mar 2016
Location: auburn new york
Posts: 71
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Re: 1971 Chevy Pickup revs too high
Thank you!!!!
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