RPM at 70, hmmmmm - The 1947 - Present Chevrolet & GMC Truck Message Board Network

*Fast68Chevy* · 06-24-2002, 04:47 PM

http://www.thirdgen.org/techbb2/show...hreadid=114139

check out the 9th reply by Don_89Iroc

interesting,

Longhorn Man · 06-24-2002, 05:00 PM

That MIGHT work with a certian size tire...but you'd do so much math to figure out the tire size you need, you could have already pulled the cover, counted the teeth, and filled it back up.

blue_69 · 06-24-2002, 08:01 PM

i have used this site it seems pretty close there are others i have seen too. http://home.off-road.com/~wsjc/calculators.html NO MATH required

Longhorn Man · 06-24-2002, 08:09 PM

personally...I just count the teeth. You know the oil has been there since the '80's anyways.

ckhd · 06-24-2002, 08:50 PM

Someone is smoking some SERIOUS crack!

Take my '68 for example. At 70mph, my RPMs are about 3300. Does that mean that I have 3.07s or something? Wow, someone should tell my 4.57s that they are really 3.07s! Oh wait, it's a stick, so it doesn't have a "Drive".

How about my 'burb? It's got "Drive". At 70 my rpms are about 2800. Does that mean that in "Drive" my rear gears magically transform from the 3.73 ratio to some magical ratio that's less than a 3.07?

For that silly little trick to work, you would have to have a tire size of 23.5" -24". Unless your tire height is close to that, it just aint gonna happen.

bigjimzlll · 06-25-2002, 12:49 AM

well on my 67 long bed at 70 mph im at 3700 rpm, and thats with 27 inch tires...oh yea i have 3:73 gears...hmmmm works in my case

Project1970 · 06-25-2002, 04:05 AM

that trick only works on a certain number of tire sizes and tranny combos - autos without lockup converters will run RPMs similar to a stick (or probably a lockup auto) with the next ratio numerically higher ratio. without factoring that in, my RPMs are more like a 4.11's (and I've done the real math, it's not hard at all! lol) but i've got 3.73s.

Trust me, for me, I can do the math a LOT faster than I could pull a cover and count teeth. Lucky for me I know my truck's exact history.

Huck · 06-25-2002, 07:47 AM

If you're running HEI and stock original Tach's, your RPMs will probably be off several hundred rpm at 70mph so don't get hung up on it.

Longhorn Man · 06-25-2002, 05:23 PM

Then it is time for a new tach.
HEI shouldn't have any effect on the tach at all.

Suicide-D · 06-25-2002, 06:05 PM

Autometer says that their tach might be + or - 100 rpms... My speedo doesn't read correctly so I guess I can't find out I have a 4.11 rear end...for the record... I'm turning 3300 to 3400 with 29" tires...

hanssolo · 06-25-2002, 09:31 PM

dobster- {rpm x tire diameter} divided by {mph x 336} = gear ratio

Suicide-D · 06-25-2002, 10:43 PM

here's a site I have found interesting....

http://www.dirtroad.com/gears.htm

06-24-2002, 04:47 PM	#1
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	RPM at 70, hmmmmm http://www.thirdgen.org/techbb2/show...hreadid=114139 check out the 9th reply by Don_89Iroc interesting,

06-24-2002, 08:01 PM	#3
blue_69 someday Join Date: Dec 2001 Location: canastota,new york Posts: 629	i have used this site it seems pretty close there are others i have seen too. http://home.off-road.com/~wsjc/calculators.html NO MATH required __________________ Click Here to search ebay for all 67-72 parts My 72 CST blazer build 72 CST Blazer 4x4 67 2wd swb ( project onion crate ) 86 4x4 lwb plow truck 40 x 60 foot barn to work on all this stuff

06-24-2002, 08:50 PM	#5
ckhd Registered User Join Date: Oct 2000 Location: St. Johns, Arizona Posts: 2,660	Someone is smoking some SERIOUS crack! Take my '68 for example. At 70mph, my RPMs are about 3300. Does that mean that I have 3.07s or something? Wow, someone should tell my 4.57s that they are really 3.07s! Oh wait, it's a stick, so it doesn't have a "Drive". How about my 'burb? It's got "Drive". At 70 my rpms are about 2800. Does that mean that in "Drive" my rear gears magically transform from the 3.73 ratio to some magical ratio that's less than a 3.07? For that silly little trick to work, you would have to have a tire size of 23.5" -24". Unless your tire height is close to that, it just aint gonna happen. __________________ my 2¢ - t.i.o.l.i. Bowen 1968 K20 fleet 1969 K10 swb fleet 1972 K10 Suburban 1972 C10 lwb step 1992 K1500 'burb 1995 K2500 'burb 1997 C1500 'burb 1999 K1500 2000 K1500 'burb Why do I own so many Suburbans?

06-25-2002, 12:49 AM	#6
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	well on my 67 long bed at 70 mph im at 3700 rpm, and thats with 27 inch tires...oh yea i have 3:73 gears...hmmmm works in my case __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

06-25-2002, 04:05 AM	#7
Project1970 14.1 @ 96MPH Join Date: Jul 2001 Location: Mesa, AZ Posts: 2,811	that trick only works on a certain number of tire sizes and tranny combos - autos without lockup converters will run RPMs similar to a stick (or probably a lockup auto) with the next ratio numerically higher ratio. without factoring that in, my RPMs are more like a 4.11's (and I've done the real math, it's not hard at all! lol) but i've got 3.73s. Trust me, for me, I can do the math a LOT faster than I could pull a cover and count teeth. Lucky for me I know my truck's exact history. __________________ Project1970 - LS1 Swap Complete!

06-24-2002, 05:00 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	That MIGHT work with a certian size tire...but you'd do so much math to figure out the tire size you need, you could have already pulled the cover, counted the teeth, and filled it back up.

06-24-2002, 08:09 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	personally...I just count the teeth. You know the oil has been there since the '80's anyways.

06-25-2002, 07:47 AM	#8
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	If you're running HEI and stock original Tach's, your RPMs will probably be off several hundred rpm at 70mph so don't get hung up on it.

06-25-2002, 05:23 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Then it is time for a new tach. HEI shouldn't have any effect on the tach at all.

06-25-2002, 06:05 PM	#10
Suicide-D Registered User Join Date: Jan 2001 Location: Katy, TX...but the love of my life resides in Dublin,OH. Posts: 1,408	Autometer says that their tach might be + or - 100 rpms... My speedo doesn't read correctly so I guess I can't find out I have a 4.11 rear end...for the record... I'm turning 3300 to 3400 with 29" tires... Last edited by Suicide-D; 06-25-2002 at 06:08 PM.

06-25-2002, 09:31 PM	#11
hanssolo Registered User Join Date: Sep 2001 Location: central Ill. USA Posts: 447	dobster- {rpm x tire diameter} divided by {mph x 336} = gear ratio __________________ 69 C10 longbed: 357c.i. /TH400/3.73s/AC/ beige primer/saving $ for paint ______________________________

06-25-2002, 10:43 PM	#12
Suicide-D Registered User Join Date: Jan 2001 Location: Katy, TX...but the love of my life resides in Dublin,OH. Posts: 1,408	here's a site I have found interesting.... http://www.dirtroad.com/gears.htm



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