And was barley able to keep up. I have the 5.4 with the superchip, FIPK, and cat-back. He has a cat-back, FIPK, and a throtle body spacer.

I belive our engin specs are very close (don't remember what size engin he has) but one big diffrence is that he has a 4.10 rear end while I have the 3.55. (He has a V8 Chevy I belive possibly the 350, 1996, again I think. Z71 4x4, don't know how much this weighs)

Do you think this gives him the upper hand in take off ability or what???

I do have more of a top end, as he can go to 95MPH and then the limiter cuts out on him, and stock (before the chip) I was able to pin my truck at over 100 MPH. So in top end I may have him but I really don't care about that. I just wanted to know that if I changed my rear end to a 4.10 or something higher than a 3.55 would this help me in my "takeoff" ability?

I may do it anyways just becuase if I put on larger tires in the future, I would want to compensate for the larger radius of the tire with a diffrent gear ratio.

I hate losing to my friends Chevy, although people like to ride in my SCREW (nicer ride, more room) than his.

 

dcovell,
The gear ratio does make a HUGE difference in off the line performance. Also just so ya know the higher the number on the ratio the lower the gear. So if you switch from a 3.55 to a 4.10 it is switching to a lower gear ratio not a higher gear ratio . I wish like hell I had 3.73 or 4.10's in my screw. It would really wake this puppy up. Well good luck & keep working on kickin that chevy's a$$.
Later....Mike

 

anybody know if that is possible with the Screw 4x4? I currently an running 3.55 LS..... would like to lower, bu tnot all the way to 410... I think 370 woulld make just enuf difference...

curious...

Mosa

 

I have the 3.73's, and you won't notice much seat of the pants difference from the 3.55's. I have never raced vs a 3.55 truck, as that's not what trucks are for. However, if you were towing, you would notice the difference in these gears, the 3.73 would not downshift as much, staying higher in the rpm range.

If you are considering changing gears, it is possible. If you have a 4x4 you must swap front and rear. You're looking at ~$1500 to do this. Better be serious about it...

 

I thought the Z71 came with a 454 engine. That would explain a lot. Any Chevy people out there?

Tommy

 

I'm just going by my experience. My friend has virtually the same truck with 3.73's and he pull's on me big time off the line.
I think even 3.73's make a huge difference in overall performance.
Don't get me wrong it's not like like adding a 100 h .p . (Although I wish it was) because I be gettin new gear's. he he .
Later....Mike

 

What kind of rear end does the Lightning have???

When I talk to my friend again I will (discretly) ask him what kind of engine he has.

 

Also another question, I suck at math so help me out here.

If assuming the tires stayed the same, what RPMs would you turn at 70MPH with a 3.73 and a 4.10 also assuming you turn about 1900 RPMs a 70 MPH?

 

dcovell,

Multiply the final gear ratio x 549 (don't ask how I got 549!).

With 255/70-16 tires and a 3.55 rear end, you should be doing 1950 rpm at 70mph in 4th OD. If you changed to a 3.73 rear end, your rpm would be 2048. With a 4.10 rear end, the rpms go up to 2250, or about 15% higher than with the stock 3.55; you can figure a 15-20% drop in gas mileage.

Other tire sizes will give different rpms but the ratio between the different rear ends will be the same.

Hope this helps.

 

dcovell.

I will take a stab at the math. The 3.55 ratio says the driveshaft turns 3.55 times for each rev of the rear wheel (3.55:1). The 4.10 says 4.1 turns for each wheel rev etc. Both the 3.73 and 4.1 will require the ENGINE to turn FASTER to turn the wheel at the same speed. The fact that it will turn faster tells you which number to divide by the other.

3.73 divided by 3.55 = 1.0507. The 3.73 gear will run the driveshaft at 105.07% of the speed required with the 3.55's So the 3.73's will increase the rpm at all speeds by about 5.1%. If you were running 1900 rpm, the new rpm will be 1900 x 1.0507 = 1996.3.

4.1 divided by 3.55 = 1.1549. 4.10 will increase the rpms by about 15.5%. If you were running 1900 rpm at 70 mph with 3.55's you will run 2194 rpm with the 4.10's (at 70). You can do the same type of ratio comparison with tire sizes using the circumference of the two tires.
Good Luck

 

The older Z-71s have the 5.7 liter (350). The new ones have the 5.3 liter (325).

 

ScrewnStLou.
I'm not disagreeing with you. I was a typin' when you were a postin' and I didn't see your post till I was done. I took his number of 1900 rpm at 70 mph with a 3.55:1, and just plugged it in the ratio to get a rpm/speed reference. It's suprising how close the numbers came out.

 

Ok I got 2 people who agree on each others math, so I will take that. It sounds good to me. Also I talked to my friend again yesterday and he has the 350 5.7 (like what 97greenlariat said)

Now I normally drive around 80mph (shouldn't do this, great way to get a ticket and a ticket could buy a lot of gas!) With the diffrent rear end I wonder if my habit would change????

Now I don't think just because the RPMS are 15% higher that my gas mileage will go down 15%. Thats a lot!

Now I know that the SCREW puts out it top rated torque at 2500 RPMs so for towing, it would make sense to have the 4.10 for the towing. If I was to get really big tires, lift, wheels, etc. I would opt for the 4.10s as they would compenstate nicely for the larger diameter tires, if I was to keep my tires about the same I think the 3.73 would be better for me.

????????? ya know I'm really not sure, guess I'll make the decision when I have $$$$$$$$$$$$$$$$$$$$$$$$$$$$...

 

Oh and I found those specs on the Lightnings rear end. It's a 3.73.

http://www.svt.ford.com/flash/svt_pd..._lightning.pdf

Man that is a nice truck! Even faster and more powerful than the 2000 model I wonder what the 2002 will have?

 

InfernalCombustion,

Actually, we agree perfectly. I just started from a different point and used a general formula instead of your simpler, more direct approach. I took the formula for all this and found the calculated rpm at 70mph for a 3.55 rear end and 255/70-16 tires, which works out to 1950 rpm, not the 1900 specified and that you used. I then lumped all of the non-changing values together except the final drive ratio, leaving 549 x final drive ratio.

If you take your ratios (a much simpler approach for the specific problem at hand) and multiply my starting rpm of 1950, you get the same 2048 rpm for a 3.73 that I got.

Here's the formula (there's no pretty way to show this):

rpm = (speed x gear ratio x final gear ratio x 5280ft)
/(60 x tire circumference in ft)

4th OD gear ratio = .70
go to www.powerdog.com/tiresize.cgi to get tire circumference in inches then divide by 12 (duh!) to convert it to ft.

You can rearrange the formula to get the speed for a given rpm, too.

speed = rpm x (60 x tire circ in ft)/(gear ratio x final ratio x 5280)

Have fun with your calculator!!