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Old 01-09-2009, 09:25 AM
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Best rear end ratio?

I have looked at some other threads and am still not sure what is best for me.
I am a Painter/Pinstriper not an Engine Man.
I have a 65 Chevy C10 Fleetside
1962 283 60 over, with Roch 4bbl, solid aluminum fan, with elec fan add on.
Trans is 1989 700R4. with the 3/4 throttle valve.
Rear end is 3.73
tires are regular stock tires...I guess I should know the numbers huh?
It turns 3000 at 65 mph..maybe 3100-3200 at 70 and I am getting maybe 12 mpg with it.
I have never touched the mixture on it, because I am afraid I will lean it out too far and screw something up.
This is my daily driver/shop truck..not a racer.
I travel to rod runs, and car shows so I need to get the hwy mileage up.
The biggest load I pull is a very small trailer with my motorcycle.
What is the best rear end gear to get the rpms down lower?
How low RPMS can I go before engine lugging becomes and issue. I can recognize it when it's lugging but is there damage I could be doing before I notice the lugging?
So I guess I don't even know the "operating rpm range". I am never taching it out. I doubt I ever rev it over 4 grand.
I am planning on switching out rear ends so I can have disks, and I need to know what ratios to look for in the new rear end.
Sorry for the length of the thread, and thanks for all the help you guys give on here...I love this site.

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Old 01-09-2009, 10:00 AM
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Something is WRONG with your numbers ... you should not be turning that high a RPM with a 3.73 @ 60 unless you have 14 inch rims/tires on your truck. Maybe your speedometer is OFF ? or you have more gear gear than you realize. Maybe the TACH is reading wrong.

Please measure the height of your rear tires, report it here ... and then we can address this a little more accurately.

The speedometer not reading right is my first guess.

Deuce
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Old 01-09-2009, 10:53 AM
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Quote:
Originally Posted by CustomConspiracy
Rear end is 3.73
tires are regular stock tires...I guess I should know the numbers huh?
It turns 3000 at 65 mph..maybe 3100-3200 at 70 and I am getting maybe 12 mpg with it.
With a standard 15 inch tire ( 235/75/15R ) the average height is around 28 and 1/2 inches tall. That means the tire would rotate 707 times in a measured mile. With a 3.73 gear in the rear end ... ( 3.73 X 707 ) your driveshaft should rotate 2640 times in one mile. With the 700R4 in high gear ( a 30 per cent reduction ) your engine should rotate 1850 times to travel a mile. So ... at 60 MPH ... your engine should be turning 1850 RPMS



Hypothetically ... In the ballpark numbers for your combination ( with a 15 inch tire of around 28 inches tall ) would a 17 tooth gear ( red ) on the transmission shaft ( drive gear ) and a 44 tooth on the speedometer cable side ( driven ).
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Old 01-09-2009, 11:46 AM
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Deuce is correct,,,

something is way wrong!!!
(the convertor, tranny, tach and speedo gear are all wrong/shot???)

with just 24" tall tires and taller 3.55 gears and "NO" overdrive my 70mph is "3200" =next to no slip in the tranny=20mph additional for each additional 1,000rpms in 1:1 third gear

here's a handy calculator (along with others on the link) to help you sort it out:

http://www.tciauto.com/Products/Tech...culators.asp#6
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Old 01-09-2009, 02:04 PM
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Sounds like He's running in 3rd??
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Old 01-10-2009, 11:23 PM
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I vote for 3.08 or thereabouts, but I'd stay out of OD in the hilly parts.
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Old 01-11-2009, 07:00 AM
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Tire size

The tires are 215-70R 15
I got all these numbers on I 100 mile drive I took the other night.
Usually I can hear feel, and see when the trans goes into OD.
The truck is pretty loud...it has those vette style side pipes, and I was also jammin the stereo pretty loud, and it was raining.
It COULD BE it was NOT getting OD and I was just missing it altogether.
I'll try to do another test today and confirm my numbers. I had checked the speedo a while back against the sat nav thingy, and it was pretty close but I'll do it again.
I'll get back to ya maybe tonight.
Thanks for getting back to me.
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Old 01-11-2009, 08:28 AM
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Custom,
the 3.73 gears are not that far off from a stock 4.X liter=roughly 283 cubes SUV OD automatic set up....

mpg is a balancing act of available flywheel TQ (cubes and rpms dependent) multipied by the gears ratios versus weight and air resistance...
gear it too tall, and hwy motor Hg vacuum to maintain speed will be lower which means the carb is running richer....

fair chance, based on where you live, just taller tires will put you in the overall "sweet spot" 18-20Hg at 70mph for best power per unit of gas
(once the tranny is sorted out and maybe some tuning for cruise timing)

the true goal is to use the least amount of gas pedal to maintain the speed....

my $.02:
for best mpg,,,$40? for a vacuum guage is money well spent!!!
high Hg=lean, low Hg=rich on the carb fuel metering....
mount it on the dash hooked to a manifold vacuum source,,,don't let it fall below 12Hg accelerating...
tune the motor to show highest Hg while driving at cruise speed on a level road...
keep the Hg as high as possible,,,if 72mph is 16Hg and 65mph is 20Hg that is very very roughly 3+mpg difference...

"back in the day" vacuum Hg guages were a oem car option,,,to help drivers improve mpg....
here's a pic' of the one in my car,,,note the green=economy, <12Hg=poor

second reason it is very smart to have one is it will tell you immediately if almost anything is not right in the motor...
ex:
just a loose plug wire which is causing a weaker spark (due to gap resistance) will make the needle bounce,,,you won't hear or feel that but mpg will suffer

third reason is it does directly tell you "real world" results for any changes (tires diameter, plug gaps, etc) you want to try....
I recently gained 2Hg at 60mph by just changing the voltage sensing location for .4V additional to the coil and dist....
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Old 01-15-2009, 04:17 PM
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OK so I am an idiot

Ichecked it two days in a row now and I must have been really tired or something the other night.
It was late, raining, in some hills, and I had some stuff in the back, plus there was a bit of a head wind now that I think about it.
There is a very good chance it was running in 3rd a lot due to all that.
So it turns out that on flat ground it turns 2600 rpms at 60....and about 3000-3100 at 70 in OD.
I guess eventually I will be going to some larger tires when I can swing the bucks or find a real deal. The probelem is.....I am real fussy about wheels and tires and most of what I see out there.....well it's just not my style.
I am still wondering what the lowest rpms are that are safe?
Does anyone know where the torque just dies at the low end on a 283?
Thanks for all the good help as usual.
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Old 01-15-2009, 05:03 PM
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According to this speed/RPM calculator: http://wahiduddin.net/calc/calc_speed_rpm.htm
You have a 5.13:1 rear end ratio.....
I figured a 215/70 15 at close to 27" diameter (13.5" radius)
If you are running in Drive rather than Overdrive it figures out to about a 3.55:1 ratio.
JA
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Old 01-16-2009, 09:08 AM
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from your original post: "I can feel it lugging"
(that should not be happening!!!!)

fair chance the TVR cable,,,which tells the tranny valve body when to shift,,,based on pressure,,, is not adjusted correct...
that will destroy a tranny,,more sooner than not enough rpms....
here's a thread with more info:
what is this on my 700r4

if the tranny is fine then wrong ign timing and maybe wrong carb jetting is the problem,,,,pull all the spark plugs and look for a uniform "chocolate milk" color on the insulator

the cam duration determines the practical minimum motor operating rpms...
(numbers just to illustrate)
typical stock 194/200 duration cam 283 is 200ft/lbs+ TQ at only about 1500rpms...
a "race cam" 240/250? would be about <100ft/lbs at 1500...
(that's why a stock motor can fully "engage" the convertor at 1500 from a stop sign and a race cam motor has to let it slip to 3000 before engaging,,,where there is enough TQ with that cam)

ask a "car savvy" friend to test drive it and look it over carefully to help sort it out....
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