06-24-2002, 04:47 PM | #1 |
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RPM at 70, hmmmmm
http://www.thirdgen.org/techbb2/show...hreadid=114139
check out the 9th reply by Don_89Iroc interesting, |
06-24-2002, 05:00 PM | #2 |
its all about the +6 inches
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Location: Hilliard Ohio
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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.
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06-24-2002, 08:01 PM | #3 |
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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
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06-24-2002, 08:09 PM | #4 |
its all about the +6 inches
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personally...I just count the teeth. You know the oil has been there since the '80's anyways.
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06-24-2002, 08:50 PM | #5 |
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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.
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06-25-2002, 12:49 AM | #6 |
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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
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06-25-2002, 04:05 AM | #7 |
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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.
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06-25-2002, 07:47 AM | #8 |
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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.
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06-25-2002, 05:23 PM | #9 |
its all about the +6 inches
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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 |
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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 |
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dobster- {rpm x tire diameter} divided by {mph x 336} = gear ratio
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06-25-2002, 10:43 PM | #12 |
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