I have a lot of this on another thread but thought it was more appropriate here and maybe someone would be able to help me out.

Long story short: I am looking at changing a 3.08 rear end in my 2000, F150, 2wd, 105k miles to a 3.55 or 3.73 for towing a 4200 lb travel trailer.

Question:
Why is it so bad to tow with a 3.08 compared to a 3.55/3.73? How will this contribute to burning up the trans? I have some ideas but I am pretty green in this area.

Problem:
If I do change my rear end gear ratio, is there any effect on my speedometer readout and on the trans shift points? I have been told 6 different things so far!

Thanks!
Matt

 

The transmission doesn't have to work as hard to move the same load with a numerically higher gear ratio. In essence the tranny/engine will have to spin less to get the same action reducing wear and unwanted heat buildup. If the speed sensor is locaed on the rear diff you will not have to worry about it messing up your speedometer, if it is in the tranny, you will have to get the PSOM recalibrated for the new ratio, not sure which style it is on the 2000.

 

FordPerf300,
As I stated, am a bit green...so bare with me.

I thought a 3.73:1 ratio meant the driveshaft spins 3.73 times for every one time the axle spins. So in 4th gear there would be a 1:1 ratio between the engine and trans which means the driveshaft spins 3.73 time for every one time the axle spins. If this were the case then a with a 3.08, the driveshaft spins 3.08 times for every one time the axle spins...the 3.08 spins the trans less than the 3.73.

See my confusion?

 

Right.

Well, there is always a 1:1 ratio between the engine and the tranny input shaft. The relationship between the tranny input shaft and the tranny output shaft will depend on the tranny gear ratios. If fourth gear is direct drive, it'll be 1:1, and if its an overdrive gear, it might be like .68:1 (which means the engine is turning s-l-o-w-e-r than the driveshaft, and said another way, the driveshaft is turning f-a-s-t-e-r than the engine). And typically, the ratio between the tranny output shaft and the driveshaft is 1:1.

I find it easier to think in terms of revs/mile, which takes into account your tire size. (And a mile a minute just happens to be 60 mph ;-)

To do this start at the back of the truck. First determine how many revs your tires make per mile. You can look this up in the tire mfg's tire spec, or just divide 63360 (inches in a mile) by the measured diameter (in inches) of your tire times Pi.

Lets say you have 28" tall tires so your tire revs/mile would be 63360 divided by (28 x 3.1416) = 720 tire revs/mile. (And at a mile-a-minute, your tires will turn 720 revs/minute (rpm's)

Now figure out your driveshaft revs/mile by multiplying your tire revs/mile by the rear gear ratio, or, 720 x 3.08 = 2218 driveshaft revs/mile. (And at a mile-a-minute, your driveshaft speed is 2218 rpms.)

Finally, calculate your tranny input shaft revs/mile (aka, engine speed) by multiplying your DS speed by your tranny gear ratio. If your in fourth gear and its a 68% overdrive, multiply 2218 x .68 = 1508 engine revs/mile. (And at a mile-a-minute, your engine speed is 1508 rpm.

Let's recalculate the revs/mile (or revs/minute) with your 3.73s:

Your tire rpm will be 720 rpm with either gear set.
Driveshaft rpm will be 2218 rpm with the 3.08s and 2686 rpm with 3.73s.
Engine speed will be 1508 rpm with the 3.08s and 1826 rpm with 3.73s.

So changing from 3.08s to 3.73s will increase your rpms at any given mph by 21%.

Another way to look at it is, a V-8 has four power strokes per revolution. If your hauling down the road and you come to a mile long hill, your engine will put out 6032 power strokes pulling that hill, and with 3.73s, you'll get 7304 power strokes.

Which would you rather have when your hauling a heavy load?
And which would you rather have when you pull up to the gas pumps?
E-v-e-r-y-t-h-i-n-g is a compromise.

Hope I haven't made this more confusing, but there is a lot to it.

 

Wow, that was a great explanation PDQ! I'll remember that formula for calculating revs/mile from tire dia too.

One thing to consider that PDQFord didn't mention is that with an auto trans without a lock-up torque converter, the engine spins about 200-300 rpms faster than the trans input shaft due to torque converter slip.

In more layman's terms, what PDQ was getting at with the change in axle gearing is that a lower ratio (higher number) will spin the engine faster at any given vehicle speed, which *could* result in decreased gas mileage, depending on the engine's powerband, whether or not the engine is operating within that powerband with the new gear ratio, and how hard it is working to pull the load, but can also result in increased available pulling power, again depending on the rpm range of the engine.

Typically, the lower factory optional axle ratio (higher numerically) results in more towing power and better towing mileage, when towing at a reasonable speed, but poorer mileage when driving on the highway with the truck unloaded, unless you decrease your speed to lower the engine rpms and save gas. That's what I do on my truck that has 4.56 gears and 33" tires (4.30 effective gear ratio considering tire size change) with no overdrive in the transmission. If I had overdrive, I'd be set . I just go slow and it gets descent mileage. Towing power was greatly increased over the stock 3.55 ratio I switched from and mileage when towing or driving around town in stop and go traffic and when pulling long grades loaded or unloaded went up some with the new lower gears.

Hope that adds some to PDQ's great analysis.

 

your right. the 3.08 does spin the trans(and engine) less than the 3.73. thats why the 3.73 has more power.

your current 3.08 doesn't have to spin as many times (better gas mileage empty), but the engine has to push a little harder. when you put it under load, it lugs.

on a 3.73, the engine doesn't have to push as hard, but it has to spin more times. you have to find the sweet spot for whatever your application.

imagine if you had to pull an ATV out of a ditch w/a hand crank. if you turn the crank once, the rope would move 1 foot. now imagine if you had to turn the crank 4 times to make the rope move 1 foot. you would have to spin more times, but each crank would be much easier. the 3.73 is making the tranny and engine turn more times, but each turn is much easier.

 

Gentlemen,

Thanks for all the help and the explainations! I was thinking about it last night...everyone I was talking to kept saying the trans/engine spins less with the lower ratio and that was really confusing me. In actuality it is more like a pulley system where (assuming all the rope angles are perpendicular to the ground, eliminates trig if you have one fixed pulley and a load the distance the load moves is the same distance your hand moves when a force is applied to the rope. In addition the force exerted on your end of the rope has to be the same as the force exerted by the weight.
Now attach one end of the rope to the ceiling, through a movable pulley with the load, to a fixed pulley hanging from the ceiling. The force applied to move the load is divided by 3 but for every foot you pull the rope, the distance the load travels 1/2 a foot. You sacrifice distance traveled for force applied. Or like using a chain fall to lift an engine.

Same with a 200lb, 36", solid oak door opened 90 degrees to the jamb; if you apply pressure with your hand only 5 inches from the hinge you will have to apply a large force (about 640 lbf) to get the door to close because the moment arm is small (moment=force*distance). But the distance your hand will travel is only 7.85 "(arc length=radius*angle in radians=5"*3.14/23=7.85"). Now move your hand 36" from the hinge. The moment arm is much bigger so you only have to apply 89 lbf to get the door to close...but your hand has to travel 3.14/2*36"=226". Sacrifice distance for force applied.

So it is just as PDQ stated: Which would you rather have when your hauling a heavy load? A 3.08 offering 6032 power strokes up a hill or a 3.73 offering 7304 power strokes up the same hill?

Makes sense now! Sacrifice gas milage because the engine turns more but you get more power at the rear wheels with a lower ratio.

Now what about the speedometer read out? I have a chilton manual that says my year truck has a gear in the trans for the speed sensor. The picture in the book looks like it is located on the side of the trans at the top, toward the rear (where it necks down and couples to the drive shaft). I looked and I can't see a thing. I'll look again. I do have a sensor in the top of the differential that the manual calls a vehicle spped sensor in the manual but is says only 1997-8 F2/350's have them. More confusion. Is there a better manual than this chilton?

 

Except, what SoCalDesertRider stated. With the lower gearing (higher numerically), its possible to improve the mileage. For example, if crusing at say, 60 mph @ 1500 rpm with the 3.08s, vs. 1800 rpm with the 3.73s, the engine maybe in its "sweet spot" at 1800 rpm. Air flow through the intake valves may cause better turbulance for better fuel atomization and a more complete burn, cam timing may be optimized at that rpm to prevent blowing some of the intake charge (and fuel) out the exhaust valve during valve overlap, the spark timing curve may be just a tad more "perfect" at that rpm and engine load, etc, etc.


If you have the sensor in the top of the differential, I'm pretty sure that it is reading the speed of the ring gear (which turns at the same speed as your axle/wheels) and probably reports the signal to the PCM which calculates the vehicle speed for your speedometer. (Please note: I'm guessing here becasue I don't have any experience with these newer trucks.) So changing the rear gear ratio wo'nt change the speed that the ring gear turns, and your speedo should be okay.


As far as I'm concerned, you can't beat the factory manuals!

 

I think we discussed the speed sensor issue in another thread, but anyways, it is as PDQ stated, the rear axle speed sensor is the one your speedo reads from and the gear change does not effect the speedo because of the tone ring on the ring gear that the VSS reads from. The Chilton manual is wrong about what they said. I have the speed sensor in the rear axle of my F350 and it is a '92. My gear change did not affect my speedometer. The tire size change did affect the speedometer though, but that could be recalibrated if I was interested in doing it.

 

I think the VSS was on the other thread too but I wanted to see if there was a new crowd here that would know for sure. I heard that I have to do nothing, I have to change the gear in my trans, I have to change the VSS, I have to change the Tone ring, I have to re-flash the PCM, I have to buy a $350 programmer, and I have to buy a $150 PCM add on.

One other thing I was wondering, will my shift points be messed up?

Thanks again for all the help!

 

I looked in the Ford Workshop manual for my truck. The Sensor in the rear end is the antilock brake speed sensor then there are 2 sensors in the transmission (no where near where the chilton shows them) both called the TSS/OSS sensors (Turbine Shaft Speed/output Shaft Speed). Looking at the exploded view of the trans, I can't tell what it reads off of but it definately is a sensor (not a gear/manual device). Based on that, I will probably need my computer reflashed, huh? My friend knows a guy who can do it, since the dealer doesn't know how.

 

What did the factory manual say about reflashing?

I recall on some models you could change the speedometer conversion factor by simultaniously pressing a combination of buttons (seems like it was the cruise control buttons or something like that) and enter a new conversion factor. I do remember that you could only do this a maximum of 10 times - then you would have to replace the instrument cluster. Anyone heard of this?

 

pdq,

The only thing I can find about flashing is in the Electrical Distribution-Module Configuration Section. None of the verbage in the differential section refer to flashing the computer because I assume that they assume you are replacing the ring/pinion with the same ratio. I didn't see anything about shift points besides the general verbage describing upshift, downshift, kickdown, etc. I'll keep looking though.
<TABLE cellSpacing=0 cellPadding=3 border=1><CAPTION>Programmable Parameters Index </CAPTION><TBODY><TR><TH vAlign=bottom align=middle>Module</TH><TH vAlign=bottom align=middle>Calibration Updatable</TH><TH vAlign=bottom align=middle>PMI</TH><TH vAlign=bottom align=middle>Programmable Parameter</TH></TR><TR><TD vAlign=top align=middle>Generic Electronic Module (GEM)</TD><TD vAlign=top align=middle>No</TD><TD vAlign=top align=middle>Yes</TD><TD vAlign=top align=left>· Speed Dependent Wipers</TD></TR><TR><TD vAlign=top align=middle>Powertrain Control Module (PCM)</TD><TD vAlign=top align=middle>Yes</TD><TD vAlign=top align=middle>Yes</TD><TD vAlign=top align=left>· Fuel Octane

· Tire/Wheel Size
· Axle Ratio
· Transmission Type
</TD></TR></TBODY></TABLE>

As a side note, I haven't seen anything in the manuals describing the diagnostics contained in the odometer (push trip reset before starting vehicle and hold, start vehicle, odometer says test, etc). Any idea what that is called?

 

I thought they would have something in there about changing to different tire size, and if the PCM is picking up the vehicle speed from the TSS (turbine shaft speed?), a change in rear gear would be similar to a change in tire size. B-U-T, if'n you have ABS and its figuring the rear wheel speed from the tone ring in the diff, you might not want to change rear tire size for fear of upsetting the ABS.

I believe in the newer trucks its called a "Hybrid Instrument Cluster", but for some reason Ford abreviates it as HEC. What you are describing is the method for putting the HEC into self-diagnostic mode, much like you can put the EATC (Electronic Automatic Temperature Control) in a self-diagnostic mode.

 

I'm no pro but it doesn't seem like changing the gear ratio would alter the antilock brakes (as long as the PCM is reflashed to referance the new ratio) because the manual says the sensor in the rear end compares the average rear wheel speed (hence the need for reflashing) with the rate of deceleration:

"The rear anti-lock brake system (RABS) continuously monitors rear wheel speed with a rear anti-lock brake sensor. When the teeth on the rear anti-lock brake sensor indicator, pass the RABS sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency proportional to the average rear wheel speed. In the event of an impending lockup during braking, at vehicle speeds above approximately 8 km/h (5 mph), the anti-lock brake control module senses the drop in rear wheel speed. If the rate of deceleration is excessive, the anti-lock brake control module activates the RABS valve, causing the isolation valve to close."