I pulled this from another board that TEX posted it on.

"I frequently do math to try to make predictions about which gear ratios will work. For example,

FD*MPH*TD/336 = RPM

FD = Final Drive (which equals tranny gear*t-case gear*axle ratio)

MPH = Miles Per Hour

TD = Tire Diameter

RPM = engine Revolutions Per Minute

You can also turn the ratio around to say that:

MPH = (RPM*TD)/(336*FD)

Using that ratio, you can calculate the effect of different tires & gear ratios from a baseline.

Let's say that most Auto trannies have a 2.5:1 first gear, 1.5:1 second, & 1:1 3rd (not taking into account the effect of converters). And most transfer cases are 1:1 in high range & 2:1 in low.

And let's say you have a truck that works well in low-range, 2nd gear, with 4.10 gears & 35" tires. And you want to know what gears you'd need to run 1st gear high range with everything else on the truck staying the same. Well, you'd plug all of the numbers into the formula using your max RPM to determine tire speed. Then change the t-case ratio & tranny ratio & recalculate with the same RPM & MPH to find your "new" gear ratio."

what kind of track are you running? deep mud or shallow fast track? Tex will be on here before long with more info for you but this should get you started.

http://www.prestage.com

http://www.4lo.com/

 

mongoose,
THANKS FOR THE INFO. I RUN A 4750 LB SUPERSTOCKER OR CLASS 3 TRUCK.(JUST DEPENDS ON WHAT PART OF THE COUNTRY YOU ARE IN) NOT SURE OF MPH; SPEEDO IS BROKE. I JUST DIDN,T WANT TO TURN THE BOGGERS SO FAST AS TO MAKE THEM UNAFFECTIVE. I PROBABLY SHOULD GIVE LOW A TRY AND SEE IF IT SEEMS TO QUICKEN THINGS UP USING ALL THREE GEARS INSTEAD OF THE TWO. I PERSONALLY STAY FROM THOSE DEEP PITS. THEY KIND OF SCARE ME

 

Buddy of mine has a similar setup. ~800HP 488", TH400, divorced Rockwell t-case (2:1 low), 3.90 front/4.10 rear gears, 38.5 Boggers - he's about 1k lighter though (CJ7). He's launching in 1st low & immediately going to 2nd. I don't think he gets into 3rd at all on 125' tracks. He's pulling the left front off the ground on launch.

Another friend has ~800HP 523", C6, 205, and I THINK 4.10's with 38.5" Boggers. This truck is heavier (full-bodied Ranger on full-size frame) & he too is having trouble blowing away the tires. I think it might be that he needs to play with air pressure & maybe tune the suspension a bit.

Was just talking to another friend last night who's blowing away his 38.5" Boggers on launch with only ~450-500HP in his CJ7. He runs 4.88's in low range. Turns out he's running 18lbs of air in his tires - EEK! Needless to say, I suggested he drop the pressure a WHOLE bunch /wwwthreads_images/icons/smile.gif.

TEX

http://sites.netscape.net/gumboracing

 

good point TEX, i think 18 is way to much for racing!!! more like 4 or 5 would be in order.

 

What kind of suspension? What rpm are you launching from?
What is the rpm range of your engine? Are the axles locked?
I see you run fast tracks, but what are the lengths of the tracks you run?

 

He's running 125' tracks - at least this one he mentioned was 125'.

TEX

http://sites.netscape.net/gumboracing

 

UNFORTUNATELY JUST ABOUT ANY PLACE WE RUN DOWN HERE REQUIRES STOCK SUSPENSION WITH MODIFICATIONS. WE RUN LADDER TYPE SLAP BARS TO KEEP AXLE WRAP UNDER CONTROL. THE REAR IS LINCOLN LOCKED. WE MIGHT LEAVE THE LINE AT ANYWHERE BETWEEN AND INCLUDING 3300 AND 4000 DEPENDING ON TRACK CONDITIONS. MOTOR MAKES ALL ITS JUICE FROM THE NEIGHBORHOOD OF 4000 RPM UP TO 7200/7300 RPM. THIS WEEKEND AT THE RACES WE TRIED LOW LOCK AND JUST MASHED THE GAS FROM ABOUT 2300 RPM. TRUCK SEEMED TO LAUNCH QUITE A BIT BETTER. JUST HAD TO SHIFT RIGHT OFF THE LINE INTO 2ND. BATTERY DIDN'T HOLD OUT LONG ENOUGH TO LET US PLACE ANY HIGHER THAN THIRD. THOSE MSD IGNITIONS SURE DO LIKE A LOT OF JUICE. KEEP ALL THE HELPFUL IDEAS AND SUGGESTIONS COMING. THANKS!!!
MUD DUCK