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gearheadslife 01-30-2013 04:11 PM

rpm to low?
Wasn't sure if I should post this here or in the engine area.. you'll see why..

my c-10 if I install a 2004r and 29 tall tires..
with my 3:08 gears the rpm is = 1514rpm???@65

3:73's would be =1514@65

and 4:11 would be 2018@65mph

as you can see the wallace racing cac. comes up with same rpm for 3:08&3:73 so that can't be correct..

anyways is 1500rpm to low to cruise at 65mph..

the engine I'm looking at is the 96-02 vortec350 with adding the ramjet cam
thinking 1500 might be to low for a 350 to push a brick through the air and not lug the engine..
figure'n 375 ft lb at 3500rpm (guess as I dn't have the power curves of the gm 350 vortec stock or with the ramjet cam)
so at 1500 if it was 200ft lb.. not sure if it be enough..
ideas/insight welcome

vinniekq2 01-30-2013 06:51 PM

1500 RPM is a little low but not unreasonable.
.7 X 3.08 and 29 tall tire = 60 mph @ 1500
.7X3.73 and 29 tall tire= 50 mph@1800

rick 427 01-31-2013 11:13 AM

IMO...If adjusted properly,it won't lug the engine. However, with the 3:08 gear,I would expect performance[or lack of] similier to a ,lets say,1987 caprice,which rolled out of the factory with a 350-700R4-2:56-27"tire.Not sure you would be happy with that.And thats assuming nothing heavy gets put into the truck bed. A 3:73 would greatly improve throttle response and overall performance,and would be a must if things get hauled in the truck bed.

dwwl 02-01-2013 10:47 AM

I must be figureing wrong. I had the 200 4r as .67 overdrive, or is that .70 without overdrive locked up. is there a chart somewere that gives you the correct numbers based on RPM,s, gear ratio, tire size. Like to print that out. haven't corrected the speedometer on my project yet eather.

vinniekq2 02-01-2013 10:58 AM

Gear Ratio Speed Calculator

painted jester 02-01-2013 11:50 AM

I'll add this info to Vinnies post some of the readers may like to have these!~

= = equals

x = multiply by

/ = divided by

overall tire diameter = mph x axle ratio x 336 / rpm

rpm = mph x axle ratio x 336 / tire diameter

mph = rpm x tire diameter / axle ratio x 336

axle ratio = rpm x tire diameter / mph x 336

These track formulas are so easy I've used them for years at the track they are designed for calculating with a final trans ratio of 1 to 1!

If your getting mph a little low on a pass calculate the tire diameter you need to do the mph your looking for! If your running out of gear calculate, if your pass is bellow rpm and your not at the top of your final gear calculate!

A lot of the tables on the web have rounded off factors and errors its so easy these days with calculators to do math and algebra why look at the internet! copy these and laminate them keep em safe! Ill bet all the old guys on here have these and more and calculate with them and then someone looks on the web gets a graph or quotes a table and insults fly! Like dcr and scr tables!!!

At least use the formulas to check the internet tables to see if the table is correct and then you know its right when you quote it!!

Some one is going to ask " what about overdrive " :confused: These are track formulas! Or road formulas for 1To1 overdrive is to save fuel your rpm will drop a little in final drive ( expected)


dwwl 02-01-2013 12:29 PM

thank alot. Got tons of information and now one more bit. Now i'll locate tire dilmentions by tire size and i'll have it all.:mwink:

painted jester 02-01-2013 12:51 PM


Originally Posted by dwwl (Post 1641346)
thank alot. Got tons of information and now one more bit. Now i'll locate tire dilmentions by tire size and i'll have it all.:mwink:

Measure tires with a tape ! dont go by the code on the tire @ 5 different manufacturers can all be coded 2.75 x 15 and one can be 1/2 ,3/4,1, ETC inch taller then the other so dont think all tires are the same size because they are tagged with the same code!!!!


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