It sounds like a really stupid question but we all know that the higher the ratio, the quicker your truck will be off the line. I.e., a 4.10 is quicker off the line than a 2.73. But what exactly is this ratio referring to? A ratio to what? To be a ratio, is has to be compared to something! Any insight to this would be greatly appreciated! Thanks!

Neal Coppola

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'94 F-150, 4.9 6cyl., K&N FIPK, Mangecor 8mm wires. Gibson 3" Side Swept, MSD Blaster Coil, superchip. Just stolen: Pioneer DEH-P835R Head Unit, Rockford Fosgate 200a1 pushing 2 RF 10" subs. Pirelli Scorpion A/T 255/75/15. Est. 200 hp 300ft/lb. Torque! Not bad for a straight 6!! Also driving the girlfriend's '94 Mercury Grand Marquis.

 

Your rear end ratio is input turns to output turns or how many times your driveshaft is turning your axle/companion flange/pinion gear compared to how many turns your ring gear is turning your axles/wheels/tires. Lower gears = higher numbers like 4.88. So with a 4.10 gear: for every 1 turn of your axle/wheel/tire your driveshaft/pinion will turn 4.10 times. Ratio = 4.10 to 1 . You can also determine this by dividing the number of teeth on your ring gear by the number of teeth on your pinion gear. I hope this helps some.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
Class III tow/Payload #3/Convenience pkgs.,
4-wheel disc/ABS,Chestnut/Parchment 40/60,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

Max,

Ok, after almost discrediting your answer and thinking about it for a moment, it makes sense. I find it funny how much people use axle ratio's in discussion, yet most of us (I speak for myself) aren't exactly sure what it refers to! One other point that I would like to make is this (correct me if I'm wrong): There is an optimum axle ratio depending on where your engine's power curves are. Basically, you want your engine to be running where it is the most effective. Does that make any sense? Oh well, it does to me! Thanks for the info/answer Max!

Neal

 

Let me give it a try. For max performance you would want to get highest(numerically) gear without running out of rpms. For example for 1/4 mile drags you generally want to wind top gear out at the end of the drag. But for street driving (gas milage) you need the a gear that keeps your rpms as low as possible without straining the truck. So yes you need gears that are set up for your powerband.

 

Ok so we're gonna go there huh?

EVERY vehicle reacts slightly different to gear changes. There are WAY TOO MANY variables here!

For street applications its fairly simple! For performance its NOT!

Variables to concider:

Engine HP& TQ curves
DRIVER
weight distribution
TIRES, TIRES, TIRES
TRANS ratios
Suspension
weather
street conditions

and probably several other variables!


-----------------------------

FIRST POINT:
Take a 1997 Chevy Z28 LT-1 AND a 1998 Chevy Z28 LS-1. Same car right? WRONG!!!

All things being the same EXCEPT the engine! The LT-1 (chevy 350 4 bolt, 285 HP) engine in the same car reacts VERY VERY differently with rear gearing than the same car with the LS-1 (new alluminum 350, about 345 HP stock) engine.

LS-1: peak torque @ 4000 RPM peak HP @ 5200
LT-1: peak torque @ 2400 RPM peak HP @ 5000


The torque in the LT-1 comes in early where as the LS-1 it comes in later, at 4000 RPM.

The new LS-1's dont spin as bad as the LT-1's! Funny cause the LS-1 has about 65 more HP and about 40 more ft-lbs of TQ! Moreover, the LS-1 has a more even HP Torque curve that allows you to get the car moving before you get the rush of power.

SECOND POINT:

Once I bumped the HP & TQ up on my LT-1 to about 375-400 HP I had a tremendous amount of TQ at 2400 RPM. I could not stop frying the tires at the track with 2.73s. They would literaly spin about 1/8 of mile if I never let up. So I put 3.73 gears in the rear and VOILA the car would hook much better.... WEIRD HUH? Sounds backwards, but thats how it worked for me! Could be the driver?

THIRD:

I took off the stock Goodyear GSC's (255-50-16) and replaced them with the cheaper Goodyear RSA's that are about 1 inch less wide! The GSCs are supposed to be a Super High Performance tire and the RSAs are not! Funny, cause the RSA's would hold me so good that I shaved about another 2/10th's off my 1/4 with very little trouble hooking up! That LT-1 would get me 13.1 in the 1/4, feathering the throttle.


So you see the statement: "For max performance you would want to get highest(numerically) gear without running out of rpms" is not all together true.

There are plenty of LS-1 Z28s running high twelves on 2.73s that are relatively stock.

Furthermore, your 4:10 might not be quicker off the line than a 2.73. In theory thats the way its supposed to work for street cars but I've KILLED many 3.42 six speed Z28s with my 2.73 auto!!! Sure they might get a 20 ft jump on me, but that usually last about 2 seconds before I start pulling away...

Ringgggggggggggg... There goes the bell... Class dismissed!




Oops, I forgot to mention... If you take 2 identicle cars except one has 2.73s and the other 4.10s and you use the SAME professional driver AT THE TRACK, 9 vehicles outa 10, the 4.10s are going to give better performance 0-60 and in the 1/4....


[This message has been edited by Rand (edited 07-13-2000).]

[This message has been edited by Rand (edited 07-13-2000).]

 

94-F150 - I'm glad you didn't "discredit" my response. I'm glad you "now" understand it. Yes, there is a optimum gear ratio for most applications. This frequently is obtained by pinpointing what your needs are ex. towing/1/4 mile times/highway mpg/top speed/engine strain & wear, as well as many other factors like Rand said. There are some people here that have known about gear ratios, muffler/pipe flow rates, tire sizing, and actual complex automotive issues for many years (before some members were even born). I find the mix is interesting.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
Class III tow/Payload #3/Convenience pkgs.,
4-wheel disc/ABS,Chestnut/Parchment 40/60,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

All I know is if your gonna do some pullin and some highway driving get a gear ratio that is in the middle, like a 3.55.

 

Also remember, for 1/4 mile running, the less gearchanges (and time spent without the engine pulling the car) you make, the better. So if your engine has a wide torque/powerband, the taller rear end will allow (maybe) one less gearchange...

[This message has been edited by BlackLight (edited 07-21-2000).]

 

I found an interesting chart in the 2001 catalog for 4-Wheel Parts Wholesalers. If you have a store near you, I would encourage you to pick one up. It shows the effects of tire size changes to speed/engine RPM's. Enjoy!

Neal

 

Formula for figuring desired speed:
Tire size (dia.) X desired rpm=______
______divided by 336=????? ????? divided by axle ratio= XXXXX. If trans. is overdrive, divide XXXXX by % of o/d.
Example: 1600 x30= 48,000
48,000 div. by 336= 142.857
142.857 div. by 3.55= 40.241
40.241 div. by .70 = 57.54 MPH
The best I can tell, our triton V-8 (with a/t) seems to have 30% overdrive, or .70.
This is a formula which I have used for many years to gear small trucks, big trucks, race cars & passenger cars. The main objective, before you start calculations, you must have the correct numbers to plug into this formula or the findings will be incorrect. Tire dia. can be found by measuring the circumference of the tire tread and dividing by 3.14. Use caution when measuring this way, because, tire dia. will change when you load the weight of vehicle & cargo. The easiest way I've found is to take a tape measure & measure the height of the tire. Hope this will help you out in understanding & figuring gear ratios.

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1999 teal blue f150 s/c
4.6l, xlt, 3.55 L/S
Tow pkg., rhino liner, engine block heater & hard toneau cover. single c.d.
255/75 r16 Generals-owl

 

Gearman,

Thank you for that equation. I had that once but had misplaced it.

One more question for all of you. What is the OD ratio on a '94 4.9L 5-speed?

If I plug in "Gearman's" OD percentage of 30% the numbers would show that my top speed governer is kicking in at 110.6 mph, could that be true!? Sounds kinda high from what I hear from other Fords! The numbers I plugged in were as follows:

29" tire
2450 RPM's (2400-2500 usually)
2.73 rear end
and
.70 OD

Could I have a straight 6 that runs at 110 mph!? God, my Chevy buddy would hate to hear that!!

Neal Coppola

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'94 F-150, 4.9 6cyl., K&N FIPK, Mangecor 8mm wires. Gibson 3" Side Swept, MSD Blaster Coil, superchip. Just stolen: Pioneer DEH-P835R Head Unit, Rockford Fosgate 200a1 pushing 2 RF 10" subs. Pirelli Scorpion A/T 255/75/15. Est. 200 hp 300ft/lb. Torque! Not bad for a straight 6!! Also driving the girlfriend's '94 Mercury Grand Marquis.