Rear end gearing

I thought about getting a tuner to fix the shift points in my '02.  But I really didn't like the truck anyway, so I wasn't thrilled with that idea either.  After owning it for about 2 years I started seriously looking to replace it.  And three years later I compromised on my desire to get a truck with a manual trans and ended up getting my current '97.

1986F150Six wrote

Please understand that I am not being argumentative...

It's a discussion forum - if we all agreed on everything, it would be pretty boring & pointless.   But none of us will ever learn anything if we don't all voice our opinions.  Say what you think - that's what I'm doing...

Nothing Special wrote

With a gas engine lower vacuum (more open throttle) gives more efficiency...

Yeah, I took a few semesters of Engineering w/Mechanical Option & Mech. Engineering, too.   So I've worked those formulas a few times, and I realize that's what the numbers say.



But I've also seen it put to the test on MythBusters & Top Gear (the original British series), and their real-world numbers proved that a big engine driving in normal traffic or even on a race track gets better fuel economy than a smaller engine driving the exact same speeds on the same course at WOT.  (On Top Gear, they used a BMW 7-series sedan to follow a G-Whiz at its top speed.)  I've observed the same thing with my vehicles.

The reason many people THINK gears have a big effect on MPG is that most people who regear forget to recalibrate their odometers, which throws off their MPG calculations.

In any case: you're driving an antique truck - it's never going to be an econobox.

Nothing Special wrote

...pumping losses at low "throttle"...

I'm not sure what exactly you mean by that term.
 At closed throttle, the engine is pumping nearly NO air, so there is no loss - the vacuum in the cylinder pulls the piston back up.  It's at WOT that the engine loses the most energy because it has to not only move the air through the cylinder - it also has to compress it.

If you're talking about NET power (including the power produced by combustion), then there can never be a significant loss, or the engine wouldn't run.  It would stall.  When you let off the gas & it slows down, there's only a tiny loss in the engine.  Nowhere near enough to affect long-term (trip vs. instantaneous) MPG.

Gary Lewis wrote

And, in fact the 2 mpg difference is...

Negligible, IMO.  It's so small that it's hard to tell if that's a real difference, or just a normal variation due to calculations, or weather, or driving style, or average speed, or...  The gear difference was 4.10 to 3.15, which is an RPM increase of nearly 30%.  But the MPG change couldn't have been that high, unless the trucks in question were only getting ~7MPG to begin with.  And the quote said that tiny change in MPG only appeared at high speeds - was that due strictly to gearing, or were aerodynamics & hills contributing to those 2 MsPG?  I bet I can get more than 2MPG lower than you if I drive your truck for a day.

Ray Cecil wrote

...its impossible to do a proper scientific expirement in the real world on real roads while any or all of these conditions can change.

The closest I could imagine would be similar drivers in identical trucks (other than gears) in convoy for a few thousand miles of varied driving.  But who has time or $$$ for that?

Gary Lewis wrote

...it should be very indicative of the kinds of MPG change that would be seen by changing axle gears.

I don't think many people are changing their axle ratio by 30%.  In the context of the original post, that would mean switching from 3.54 to 4.60 or 2.73 gears.

One of the more interesting things I discovered years ago, and was really PO'd when they put the national 55 mph speed limit in was that some engines do better at higher rpm. Two specific examples, first my 1966 Shelby GT350, at that point 289 with dual Holley 465 cfm 4 barrel carbs. at 55 mph car got 18 mpg average, at 70 mph it climbed to 22 mpg. Engine basically "came on the cam" and most of the mechanical only ignition advance was in at 3500 rpm in 4th for 70 mph at 55 it was turning 2750 rpm. Second was my 1963 Jetfire, the turbocharged F85, 215 ci, 215 HP (according to Oldsmobile) it had the Roto-Hydramatic model 5 and 3.31 gears with 15" tyres. I removed the spring from under the power enrichment piston in the Rochester RC one barrel side draft carburetor. At 55 mph it got 18-19 mpg, a friend borrowed it to drive to and from Richmond VA from Newport News for medical appointments, he came back and told me he was getting 23 mpg running 80 with it. At 70+ mph the boost gauge would be just under 0, maybe 1/2" of manifold vacuum, Garret turbo was doing most of the pumping and with the spring removed it stayed in the cruise mixture condition, but probably no vacuum advance, just mechanical.

Steve83 wrote

Nothing Special wrote

...pumping losses at low "throttle"...

I'm not sure what exactly you mean by that term.
 At closed throttle, the engine is pumping nearly NO air, so there is no loss - the vacuum in the cylinder pulls the piston back up.  It's at WOT that the engine loses the most energy because it has to not only move the air through the cylinder - it also has to compress it.

If you're talking about NET power (including the power produced by combustion), then there can never be a significant loss, or the engine wouldn't run.  It would stall.  When you let off the gas & it slows down, there's only a tiny loss in the engine.  Nowhere near enough to affect long-term (trip vs. instantaneous) MPG.

The way my ICE professor explained it was that diesel engines get better mileage than gassers for three reasons, which all contribute about the same amount.  The most obvious one is that diesel fuel has a higher energy density than gas.  More BTUs per gallon means fewer gallons for the same number of BTUs.

The second is that diesels run a higher compression ratio.  The higher the compression ratio (to a point) the more efficient an engine is.  High compression gas engines are in the 10:1 range.  Diesels are 17:0 - 21:1.

The third reason is what I'm talking about here.  It takes energy to pull air past a closed throttle plate.  Gas engines don't have this loss at WOT, diesels never have it.

And no, we didn't do experiments to prove that, and maybe he was all wet.  But overall he really seemed to know his stuff.

And to further clarify, in this discussion I'm talking about ENGINE efficiency, not VEHICLE efficiency.  Vehicle efficiency is what we most often care about: how far can I get on a certain amount of fuel.  Engine efficiency is how much power can I get out of a certain amount of fuel.  This is one of the factors that play into vehicle efficiency, but definitely not the only one.

What we did learn, both theoretically and experimentally in those courses, was that the best engine efficiency came at WOT and at the engines torque peak (a detail I hadn't included before).  That's where the engine does the best job of turning the energy in the fuel into power.

However, running a truck engine at WOT at the torque peak is not a good way to get good vehicle mileage!  You will be accelerating like crazy unless you are either already going really fast, going up a steep hill, or otherwise pushing against a very large load, none of which is good for mileage.  So for best vehicle efficiency you do have to sacrifice some engine efficiency.  We typically do that by closing the throttle, but in the case of the Ford six on the lab dyno we saw that, at least in that engine, keeping the throttle open but decreasing power output by dropping the engine speed gave better vehicle efficiency.

We did go through the "thought experiment" at that time to ask if that vehicle would be reasonable to drive, and came to the conclusion that probably no one would want to drive the car if it actually had that tall gearing.

 So for best vehicle efficiency you do have to sacrifice some engine efficiency.  We typically do that by closing the throttle, but in the case of the Ford six on the lab dyno we saw that, at least in that engine, keeping the throttle open but decreasing power output by dropping the engine speed gave better vehicle efficiency.

We did go through the "thought experiment" at that time to ask if that vehicle would be reasonable to drive, and came to the conclusion that probably no one would want to drive the car if it actually had that tall gearing.


My son's 1984 F150: 4.9L 4 speed manual O.D. with 2.47 rear gears did return 26+ MPG on the highway [55-60 mph], but was a dog and not fun to drive. It idled @ ~8 mph and was very sluggish. "At 65 mph in O.D., if there was the slightest hill in the county it required shifting to 3rd." [edited per suggestion from Gary ]


The truck received 3.55 rear gears and the speedometer was calibrated to that change. With this change, the truck was much more enjoyable to drive and the highway gas mileage dropped to 22-23 MPG.

Going from 2.47 to 3.55 was a 43.7% change.

Ray Cecil wrote

Factors in mpg:
Torque @ rpm
Vehicle weight
Vehicle speed
Wind direction
Wind speed
Humidity
Air temperature
Elevation
Tire pressure
Tune/timing
Condition of engine/trans
Wind resistance/ drag coefficient
Vehicle height
Tire circumference
Tire tread pattern
Suspension alignment
Condition of bearings
Proper release on brake shoes and calipers
Driver habits
Ac and accessories being engine driven, on or off
Fuel grade/mix
Engine modifications
Engine air/fuel ratio and its devices that control it. (Carb vs injection)

Sometimes, while being involved with a group discussion, one might think of something which might be considered a "rabbit trail", but has the possibility of adding breadth if not depth to the discussion. If nothing else, it might be humorous.

Looking at the above list, some of these items play into the forthcoming illustration. Back in 1991 - 1994, GEO [Suzuki] manufactured the Geo Metro which was sold by Chevrolet. This was a 1.0L 3 cylinder econo-box. The standard one with 5 speed manual transmission was rated @ ~46 MPG. The engineers put together a model to be the highest rated automobile offered in America, at that time. The name of this model was Metro XFi and it was rated @ 58 MPG! I drove a 1992 model for years and was able to see 57 MPG @ 55 mph. The horsepower was 48.5 vs. 55 for the standard engine.

There were several changes made to greatly enhance the fuel efficiency:
   
  The camshaft was specific to the XFi engine [better torque?]
  The final drive ratio was 3.79 vs. 4.11
  The ECU was specific to this model
  The pistons had 2 rings vs. the normal 3 [1 less compression ring; reduced internal friction]
  The XFi had only the driver's side exterior mirror [decreased wind drag]
  The interior was very spartan with little insulation [decreased weight]

Food for thought.  

 
 
 

David - Would it be fair to revise your statement to be "At 65 mph in O.D., if there was the slightest hill in the county it required shifting to 3rd"?  That's the way I've heard it stated.  

As for the Geo, that's incredible MPG.  But, at quite a cost of creature comforts and, to some degree, safety.  (I remember the day I got my Super Bee.  I was going to change lanes and there was nothing "in the passenger's mirror", so started moving over.  But, there was something there.  There just wasn't a passenger's mirror!)

My philosophy about vehicle fuel consumption: If I paid cash for the vehicle, and I love driving it, I dont care what fuel mileage it gets. Its way easier to earn a few more dollars to put into the tank, than it is to try to squeeze every last 1/10th of a mile from it. Id rather drive it like I stole it, ride it hard and have a good ole time. Im not old yet. Probably never will be.

Some put their money in monthly payments and insurance, mine goes in the gas tank!

Nothing Special wrote

The higher the compression ratio (to a point) the more efficient an engine is.

I've never heard that one, or done any calculation that would suggest it.  But basic entropy graphs show that about TEMPERATURE - the higher the engine's peak combustion temperature, the more-efficient it is (thermodynamically).



Your quote implies that the cycling (rotating mass of the engine components) is the greatest loss, and that an engine with no moving parts (like a ramjet) must necessarily be the most-efficient.  A ramjet is certainly thermodynamically-efficient, but it's not fuel-efficient (MPG), which is what we're really talking about.
 And all things being equal, the most-efficient mechanical engine is a Stirling, which has very low compression ratio.

Nothing Special wrote

It takes energy to pull air past a closed throttle plate.

I disagree strongly with that...

Fluids can't be "pulled" - only pushed.  And the engine doesn't push the air outside - the earth's gravity does that.  The engine has to push the air in the crankcase out of the way for the piston to go down.  That's irrelevant of the throttle or its size.  But an open throttle allows air to flow into the engine, balancing out the pressures above & below the piston, making it easier to pull the piston down.

That might seem semantic, but it's a physical fact: the engine uses NO energy to move air past a closed throttle plate.  Only to push the air around inside the crankcase, to compress it into the combustion chamber (and higher CR means more loss there), and to push it out of the cylinder during the exhaust stroke.

Nothing Special wrote

Vehicle efficiency is what we most often care about: how far can I get on a certain amount of fuel.

That's what the MythBusters & Top Gear experiments were about (although the TG one was intentionally slanted to produce the result they wanted, for entertainment reasons).

Ray Cecil wrote

...try to squeeze every last 1/10th of a mile from it. Id rather drive it like I stole it...

That has always been my philosophy when I'm harassed about driving such a large, heavy, vehicle getting such low MPG; I'd rather pay for that extra gas than lose the safety, durability, & utility.

I'm not trying to shut down the discussion, but Sideflop may be getting confused.  So to cut to the chase, regardless of what was said in engineering classes or what we can read on line, the bottom line is that gearing does matter.  Bob proved that for us with his test of different gear selections in the same truck, with a diesel engine, on the same day in the same weather conditions.  To reiterate, his instantaneous readings, given by the vehicle's ECU, were:

55 MPH:  In 4th the engine was turning 1579 RPM and got 21 MPG.  But by shifting to 3rd it was turning 2224 RPM and dropped to 16 MPG.  That's a 24% reduction in MPG for a 29% increase in RPM.

77 MPH: In 4th @ 77 MPH it was turning 2224 RPM, the same as in 3rd at 55 MPH.  And at that RPM it was getting 16 MPG.  But by pulling it down into 3rd at that speed it was turning 3113 RPM, which reduced the MPG to 11, which is a 27% reduction in MPG for a 29% increase in RPM.

Granted, those were not the average MPG's he got with the truck.  In fact, they are much higher than what he got.  But they indicate that changing the engine's RPM w/o changing anything else does change the vehicle's economy.

And then back to the original question:

Sideflop wrote

In my F250's current state, it has axle code 73 (pretty sure that's a Dana 60 rear end with 3.54s), and the truck gets about 15-18 mpg going 65-75 mph. I'm sure that I'll be using it to haul or tow stuff occasionally. Would it be good to keep 3.54s, or is there a better rear end I should know about?

To that I'll say again essentially what I said earlier to answer that question:

"15-18 mpg going 65-75 mph" is pretty doggone good for a heavy truck, and I'd be delighted with it

3.54 is a decent ratio for towing as well as cruising light since it has the engine at an RPM where it has the torque to tow in 4th gear

However, the limiting factor is the lack of an overdrive gear in the transmission.  If a ZF5 were to replace the T19 then you could tow in 4th just as you do today, but when running light shift into 5th and enjoy better MPG as well as reduced engine noise due to the reduced RPM's.

Note that I'm not quantifying the increased MPG.  But a 24% reduction in RPM will yield some increase in MPG.

As interesting as all of the science behind fuel efficiency is, I wanted to sum up what I have gleaned from years of hanging out on IDI forums. Owners of 6.9/7.3 Report the best mpg with a final drive ratio of about 2.75-2.50, go much above that and you don’t gain much, if anything, at the pump, but may have higher EGT’s and less ability to hold gear on any kind of grade. In a 8 lug truck, 3.55’s are about the highest axle gears you will find. 3.55 (Ford tooth count) 3.54 (Dana tooth count) with a ZF-5 gives you a final ratio of 2.73. This is just about perfect for the 6.9 and stock wheels and tires. It will give you 20+ if you keep your speed down around 60-65 and your IP and injectors are healthy and in time. An auxiliary overdrive behind the T19 will accomplish the same thing, but unless you find a used one it will cost more than a ZF swap and is clumsy to use. The diesel T19 already has really close ratios so splitting gears is redundant and not a benefit. I have one and I’m not crazy about it. I need that and a ZF to overcome my 4.10’s. My 6.9 pulled the strongest around 1800-2000 rpm’s and seemed to get the best mileage there. Mine (crew cab 4x4) got 17 mpg with a final ratio of 3.20, so if you are hitting 18 with 3.55’s you are right there with what can be expected. In terms of dollars and cents you will probably never recover the cost of a ZF swap in fuel savings, but in pleasure of driving it will pay for itself in the first mile.

Well summed, Jonathan.

I particularly like your last sentence, and fully agree. The reduced engine noise because the RPM at 65 MPH on Dad's truck went from 2800 to 1900 was well worth the cost of the swap.

And that doesn't even address the 2 MPG hoped-for increase in MPG due to the T19 to ZF5 swap on Big Blue. Currently Big Blue gets 11 MPG, so a 2 MPG increase gives 13 MPG. Assuming gas is $3/gal my math says it'll take 37,750 miles to make the $1500 cost of the fully-rebuilt ZF5 up. I expect to drive BB that far, so I suspect I'll save money eventually with the swap. However, the cost of the ZF5 was a one-time expenditure and filling BB up is a very frequent event, and it is that frequent reminder of the very poor MPG he currently gets that is galling.

Can you tell I'm looking forward to getting the ZF5 in?

Gary Lewis wrote

...instantaneous readings, given by the vehicle's ECU...

Was the ECU reprogrammed for the change in gears?

Gary Lewis wrote

...a 24% reduction in RPM will yield some increase in MPG.

Not necessarily, because you're ignoring the engine load.  An engine screaming along at redline in Neutral doesn't burn nearly as much as one barely holding 1000RPM going up a hill towing its GCWR in OD.  That's the extreme example, but the same thing happens to a lesser degree when the transmission is shifted up, or the drive axle gears are lowered (numerically higher), or the tire size is increased, or...

Steve83 wrote

Gary Lewis wrote

...instantaneous readings, given by the vehicle's ECU...

Was the ECU reprogrammed for the change in gears?

The truck was running stock gearing, so no change needed.  But even if the gearing had been changed, all of that testing was done over less than 30 minutes in a single road trip, so very few variables between runs.

Steve83 wrote

Gary Lewis wrote

...a 24% reduction in RPM will yield some increase in MPG.

Not necessarily, because you're ignoring the engine load.  An engine screaming along at redline in Neutral doesn't burn nearly as much as one barely holding 1000RPM going up a hill towing its GCWR in OD.  That's the extreme example, but the same thing happens to a lesser degree when the transmission is shifted up, or the drive axle gears are lowered (numerically higher), or the tire size is increased, or...

Not ignoring engine load, specifically testing for it.  55 mph in 3rd and 4th, 4th gave much better mileage (21 mpg vs 16).  70+ mph in 3rd and 4th, 4th gave much better mileage (15 vs 11 mpg).  So no change in load, only changing engine speed made a big change in fuel mileage in that truck.

And even when changing load the mileage hardly changed.  70+ mph in 4th gear was the same engine speed but much higher load than 55 mph in 3rd, and the mileage wasn't much different (15 vs 16 mpg).

Again, this was one real-world test with one truck, a 2002 F-350 with 7.3L PowerStroke, 3.73 gears, stock tire size (~32" OD?) and a 4 speed auto trans.  All tests were about 2 miles of flat, level steady cruise, with the torque converter locked.  As they say, your mileage may vary