Lowering RPM's at crusing speed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kecah04 · 03-25-2004, 10:43 PM

My 70 Screeeems at crusing speed. I don't have a tac since I have an automatic. I'm guessing 25 or 30 thou. What is the best way to slow the RPMs without messing with the speed?

bigvinnie · 03-25-2004, 10:44 PM

change the gearing in the differential (by lowering it numerically) or install an overdrive trans..

WRMZ71 · 03-25-2004, 10:49 PM

Yep just like he said. You probably got 3:73's thats what i had and i changed em because i drive back and forth from NC to nebraska and going down the interstate doing 80 mph with some 255 60's just works the dog piss out of the motor. It cruised along at about 3500 to 4000 rpm. I dropped down to a 3:40 here lately and now cruising at 70 is about 3,000 rpm. Either that or get you a 700R-4 thats the one with the overdrive. YOu hear all kinds of stories with them about reliability though so beware.

Longhorn Man · 03-26-2004, 01:13 AM

If you have a small block and an automatic, then you probably already have a 3.07 gear, which should get you about 60 at 2500 or so RPm...no too bad really. You'll have a hard time getting anything lower, so if the RPMs are too much for you, an over drive tranny is about your only option.
Just remember, reducing the RPMs, does not always mean better MPG. I've seen plenty of OD conversions that net 1 or 2 MPG better, and that'll take the life of the tranny to make up the difference in cost.

jorgensensc · 03-26-2004, 03:09 AM

You know I Love ya Longhorn Man, your my idol, but if you figure 2 mpg better at 1.50 a gllon for gas, and about 750.00 for a 700r4 tranny, your looking to make it up the cost in about 2 1/2 years. I figure it like this:
10 mpg x 20 gallons per tank = 200 miles per tank,
at 1.50 per gallon = 30.00 per tank
based on a 12,000 mile a year, that works out to 1800.00 per year on gas.
Now at
12 mpg x 20 gallons per tank = 240 miles per tank,
at 1.50 per gallon = 30.00 per tank
based on 12,000 miles per year that works out to 1500.00 per year on gas.
A savings of a mere 300.00 per year, but figure you can sell the tranny you have now for a little green, and you could easily pay for a 700r4 in 2- 21/2 years.
Of coarse I'm not includingthe driveshaft replacement either.

Longhorn Man · 03-26-2004, 03:22 AM

Hmmm, your numbers look good, I never bothered actually doing the math.
Good point.
However, if I am your idol...you have issues...no, you have ISSUATIONS

Thanks for pointing out my oversight.

jorgensensc · 03-26-2004, 03:26 AM

By the way Longhorn, why are we up this late?!?

Longhorn Man · 03-26-2004, 03:34 AM

I don't work on Fridays, so I can stay up as late as I want. (BTW, it is one hr later here)
I normally don't turn in untill 1 in the am or so...even on work nights.
BTW, I got your check today...your light will be going out tomorow.

jorgensensc · 03-26-2004, 03:38 AM

Cool Thhanks for the news. I'm off tomorrow too. The wifes out of town anyway, and I can't sleep when she's gone for some reason. I'm driving to Dallas tomorrow morning, so I better get some rest soon.
Back to the thread at hand, sorry kecah04.

Tx Firefighter · 03-26-2004, 09:24 AM

Quote:

Originally posted by jorgensensc
You know I Love ya Longhorn Man, your my idol

Seriously though, lower your cruising revs for your engine's health too. As RPM increases, the stress on the engine increases incrementally. An engine turning slow speeds is a much happier engine.

nate68 · 03-27-2004, 11:24 PM

gears need to match the cam also. i'm thinking about changing my 3.50s for 3.70s because i think with the overdrive the engine is bogging down.
1.50 for gas? i wish i lived there. its 2 bucks here.

Longhorn Man · 03-28-2004, 01:01 AM

Quote:

Originally posted by Longhorn Man
Just remember, reducing the RPMs, does not always mean better MPG.

This is why i get the same MPG doing 50, as I do at 80 MPH. My engine bogs down untill i hit 3000 RPM.

walker · 03-28-2004, 01:03 AM

Put on as tall a rear tire on it as you can. Get a tach, even a temporary tach, so you know what rpm you are turning. Change rear gears. Install new OD trans. Install gear vendors overdrive unit.
New tires $250
New rear gears $500(shop doing the change)
New tranny $1000
OD unit $2000

Compare these prices to a new GM crate engine @ $1200 in 100,000 miles. IMO over rev the engine, wastes a little fuel, that is about it.
BTW I have a TH400 and 4.11 rear end.

Chevyboy1962 · 03-30-2004, 02:45 AM

Hmmm, now you got me wondering what gears i'm running. lol

She does 2,500 RPM @ 60 mph, what's a good site to plug in a RPM calculator online?

I'm running a 235 inline 6 backed by a manual 3 speed.
David

skokie · 03-30-2004, 02:49 AM

Try this one.
http://www.angelfire.com/fl/procrastination/rear.html
For me 3000 gets me 70mph. Cant wait to get the 307s in.

03-25-2004, 10:43 PM	#1
kecah04 Registered User Join Date: Sep 2002 Location: Davenprort, Iowa Posts: 18	Lowering RPM's at crusing speed My 70 Screeeems at crusing speed. I don't have a tac since I have an automatic. I'm guessing 25 or 30 thou. What is the best way to slow the RPMs without messing with the speed?

03-25-2004, 10:44 PM	#2
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	change the gearing in the differential (by lowering it numerically) or install an overdrive trans.. __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project Last edited by bigvinnie; 03-25-2004 at 10:46 PM.

03-26-2004, 03:09 AM	#5
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	You know I Love ya Longhorn Man, your my idol, but if you figure 2 mpg better at 1.50 a gllon for gas, and about 750.00 for a 700r4 tranny, your looking to make it up the cost in about 2 1/2 years. I figure it like this: 10 mpg x 20 gallons per tank = 200 miles per tank, at 1.50 per gallon = 30.00 per tank based on a 12,000 mile a year, that works out to 1800.00 per year on gas. Now at 12 mpg x 20 gallons per tank = 240 miles per tank, at 1.50 per gallon = 30.00 per tank based on 12,000 miles per year that works out to 1500.00 per year on gas. A savings of a mere 300.00 per year, but figure you can sell the tranny you have now for a little green, and you could easily pay for a 700r4 in 2- 21/2 years. Of coarse I'm not includingthe driveshaft replacement either. __________________ 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

03-26-2004, 03:26 AM	#7
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	By the way Longhorn, why are we up this late?!? __________________ 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

03-26-2004, 03:38 AM	#9
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	Cool Thhanks for the news. I'm off tomorrow too. The wifes out of town anyway, and I can't sleep when she's gone for some reason. I'm driving to Dallas tomorrow morning, so I better get some rest soon. Back to the thread at hand, sorry kecah04. __________________ 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

03-25-2004, 10:49 PM	#3
WRMZ71 Cause the Chicks DIG IT!! Join Date: Nov 2003 Location: Beaufort SC Posts: 509	Yep just like he said. You probably got 3:73's thats what i had and i changed em because i drive back and forth from NC to nebraska and going down the interstate doing 80 mph with some 255 60's just works the dog piss out of the motor. It cruised along at about 3500 to 4000 rpm. I dropped down to a 3:40 here lately and now cruising at 70 is about 3,000 rpm. Either that or get you a 700R-4 thats the one with the overdrive. YOu hear all kinds of stories with them about reliability though so beware.

03-26-2004, 01:13 AM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	If you have a small block and an automatic, then you probably already have a 3.07 gear, which should get you about 60 at 2500 or so RPm...no too bad really. You'll have a hard time getting anything lower, so if the RPMs are too much for you, an over drive tranny is about your only option. Just remember, reducing the RPMs, does not always mean better MPG. I've seen plenty of OD conversions that net 1 or 2 MPG better, and that'll take the life of the tranny to make up the difference in cost.

03-26-2004, 03:22 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Hmmm, your numbers look good, I never bothered actually doing the math. Good point. However, if I am your idol...you have issues...no, you have ISSUATIONS Thanks for pointing out my oversight.

03-26-2004, 03:34 AM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	I don't work on Fridays, so I can stay up as late as I want. (BTW, it is one hr later here) I normally don't turn in untill 1 in the am or so...even on work nights. BTW, I got your check today...your light will be going out tomorow.

03-27-2004, 11:24 PM	#11
nate68 Registered User Join Date: Jan 2000 Location: ca Posts: 1,110	gears need to match the cam also. i'm thinking about changing my 3.50s for 3.70s because i think with the overdrive the engine is bogging down. 1.50 for gas? i wish i lived there. its 2 bucks here. __________________ 68 short step, 406/700r4, 9" w/discs.

03-28-2004, 01:03 AM	#13
walker Registered User Join Date: Jul 2003 Location: north of Phx AZ about 30 miles Posts: 698	Put on as tall a rear tire on it as you can. Get a tach, even a temporary tach, so you know what rpm you are turning. Change rear gears. Install new OD trans. Install gear vendors overdrive unit. New tires $250 New rear gears $500(shop doing the change) New tranny $1000 OD unit $2000 Compare these prices to a new GM crate engine @ $1200 in 100,000 miles. IMO over rev the engine, wastes a little fuel, that is about it. BTW I have a TH400 and 4.11 rear end. __________________ Andy,Phx AZ '67 C-10 (Ahhh, done at last. Well there is that disk front end I want to put in and...) "23 C-Cab-sold '48 Ford 8N tractor(still working) '67 Scout(Now on the road) '70 MG B.-sold

03-30-2004, 02:45 AM	#14
Chevyboy1962 C10 now is a GMC Join Date: Jun 2003 Location: Modesto, Ca Posts: 740	Hmmm, now you got me wondering what gears i'm running. lol She does 2,500 RPM @ 60 mph, what's a good site to plug in a RPM calculator online? I'm running a 235 inline 6 backed by a manual 3 speed. David __________________ 1962 C10 swb fleetside, big window, custom cab 350/TH350 combo...Hot Rod 1963 C10 lwb fleetside, small window 230/4 speed...Blvd Cruiser *1931 Ford 5 window coupe (soon to be Hot Rod)... Father & Son project

03-30-2004, 02:49 AM	#15
skokie Registered User Join Date: Oct 2003 Location: San Jose Ca. Posts: 3,354	Try this one. http://www.angelfire.com/fl/procrastination/rear.html For me 3000 gets me 70mph. Cant wait to get the 307s in. __________________ RIP to my son Dylan. 5/17/2002 - 8/18/2018 If you want to know more about rare forms of pediatric cancer or if you want to help fund pediatric cancer research, please check out the website for the non-profit I started. https://slothforpedcancer.org 68 build thread http://67-72chevytrucks.com/vboard/s...d.php?t=223234

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