How To Select The Right Rear End Gears

[bgstew6]

Nice tidbit from the boys next door

How To Select The Right Rear End Gears

#1

Shift Point x Tire Height
336 x Trap Speed


*Multiply your shift point by your tire height
*Multiply your trap speed by 336
*Divide top number by bottom number
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#2

5500 x 30 = 165,000
336 x 130 = 43,680

*In this example, your shift point is 5500
*Your tires are 30 inches
*Your trap speed is 130 mph
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#3

165,000
43,680

Result is 3.78

*Multipling 5500 RPM by 30 inches gives you 165,000
*Multipling 130 MPH by 336 gives you 43,680
*Dividing 165,000 by 43,680 gives you 3.78
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#4

Gears available for 9.75"/8.8" rear:
3.08
3.27
3.55
3.73
4.10
4.30
4.56

*From here, you simply select the closest gear available to 3.78 = 3.73

[Harley#356]

http://www.nloc.net/vbforum/showthread.php?t=123066
original thread by copperhead....already under the how-to index in classroom corner from before

[Black N Blown]

i came out with 4.40????

5500 x 28 = 154000
336 x 104 = 34944

154000/ 34944 = 4.40

So does what exactly does that tell me? if half the people in here went to 4.40 gears they would have to have a tire company sponsor them.

[2manytrucks]

I don't believe the 4:30's can be run on our trucks as a DD because they have no reverse cut. On the 8.8 and 9.75's for DD use it's 4:10 and then 4:56 (if you want to use reverse...LOL.). The 4:30's refenerenced are for straight line use only. I believe the only functional 4:30's are available on the Ford 10.5 Sterling rear end (F250-F350) (have a set in the garage...LOL).

There are higher ratio's as well for our carrier as well, 4:88 into the high 5's. 4:88's are pretty common when running a 38" and larger tire on the F150 4x4's.

[Harley#356]

I think its an over-optimized calculation for perfect conditions at a track and all that. mine came out to this...

5800*28 = 162,400
107*336 = 35,952

162,400/35,952 = 4.517

no way am i about to go to a 4.56 gear! haha traction would be impossible! its going to be hard enough on 4.10's.


I think a better way to calculate that out, is instead of using shift point, use what RPM you want to be crossing the traps at. Because you dont want to still be down at 4500 RPM's crossing the traps....you have power left on the line, but you dont want to be at 5500 RPM's crossing the traps, you're ready to shift into OD and not pulling as hard as you would be down lower in the RPM's a bit.

Try this calculation....you know your tire size, you know your rough trap speed, pick different gear ratios, and you'll see what RPM's you'll be putting down at the end of the 1/4

(Gear Ratio * 336 * Trap Speed) / (Tire Size) = RPM's

This actually comes pretty close to accurate. Before on my 3.55 gears and 28" tires I was crossing the traps around 4500 to 4600 RPM's...still power left on the plate. do the calculation...

(3.55 * 336 * 107) / (28) = 4558.2 RPM's

Now with my 4.10's, I'm crossing the traps closer to 5200 RPM's

(4.10 * 336 * 107) / (28) = 5264.4 RPM's

Calculation comes out nearly dead on. Now if i decide I cant get the power down and want to drop that down a little, going to 30" slicks would drop the RPM's by 350

(4.10 * 336 * 107)/(30) = 4913.44 RPM's

As opposed to dropping to 3.73 gears and keeping the 28" slicks which would drop the RPM's by 475 RPM's

(3.73 * 336 * 107)/(28) = 4789.32 RPM's


At this point, with my power level and track times, my 4.10's are perfect as long as traction doesnt become an issue. If it does, then stepping to 30" slicks would be a midway jump between going down to 3.73 gears.

[dsblk93gt]

I've always just figured a RPM/MPH to get me to the 1/4 in 4th/D so that I am just past my peak HP...or just past the shift point.

[Harley#356]

ideally you dont want to shift into 4th/OD at the end of the 1/4. You want to be at the top of 3rd. I always turn my OD off just incase so it doesnt for some reason shift right before the traps

[03hdsc]

well i know i am at the very end of3rd when i cross the line...my trap speed is very bad though...for the mods i have on it..time is not so bad ut speed is just plain retarded...still have not figured out how i am putting down consistant 13.2-13.4's and only going 99mph at the end of 1.4 mile...makes no sense...especially when i am beating cars that are going up to 108 in the 1.4

[Harley#356]

yeah 99 is a bit on the low side, my 13.3's were around 104 mph. gotta remember though you are pushing a TANK down the track, your mph is going to be lower than a car going 13.3. so you're only like 3-4 mph on the slow side. 108 is around 12.8-12.7 range

[03hdsc]

yeah 99 is a bit on the low side, my 13.3's were around 104 mph. gotta remember though you are pushing a TANK down the track, your mph is going to be lower than a car going 13.3. so you're only like 3-4 mph on the slow side. 108 is around 12.8-12.7 range

yeah i know...must be all that torque pulling me off the line>>>>????

[Harley#356]

the 5.4's are torque monsters like to get up and GO!....its just that effort to keep going is pretty hard pushing a cinderblock wall through the air