I've been a long time believer that adequate gearing can improve MPG's, and I'm sure it's true to a point. But I've never had any vehicle where I've been in a position to test it. So instead, I decided we could have a discussion about it.

This has probably been discussed several times before but I'd like to keep this MPG debate related to gearing only- I'm not concerned with engine related MPG improvements- tranny and rear end gearing are the main issue I'd like to focus on. We should probably include tires as well, as those will have an effect as well.

I'd also like to note that I don't currently have any MPG issues, nor am I looking at changing anything on my truck. This is just for kicks and friendly discussion.





Hey...





HEEEEY! I said friendly discussion!



That's better. Don't make me start thrashin' you kids!

I realize that this can depend on vehicle set up, such as an F150 vs. an F350, 5.0 vs. 7.5, short bed vs. long bed, how much towing is done with it, etc. It all has an impact. But I'm just trying to get at the basic principle here.

Feel free to list any vehicle you want and we will discuss each as we go. I'm gonna use my truck as an example for my post. It's a 1991 F150 5.8, E40D w/ OD, axle code 18 [3.08], and 15" tires.

So, to improve MPG's on gearing alone, I propose swapping the E40D for a 5 speed manual, 4.10 rear end, and larger tires [I think 31" is as large as I can go w/out a lift].

Thoughts? [If I'm confusing anyone, I'm sorry- this is only making sense to me right now 'cause I'm about to go to bed.]

 

I have a 97 F-350 Reg cab, long bed, no lift, 35" tires, Dana 60 axles... but idk what the gearing is.

I feel like my gas mileage SUCKS. A full tank will last me like 2 days if not just a bit more. But I just cut my air restrictor horns off and now it is slightly better. As soon as I fill up this next time I will make sure I find out my exact mpg but still. I see people here that tow with their trucks and get 12-14MPG.. I don't get it. I don't drive around much at all. I go from my house into town, which is 5 miles then at the furthest I drive 10 miles out of town. And I barely drive into town and back 4 times a day. Sometimes 6 times. But that's it. So even then. It's 30 miles a day just going into town and back home... :\ I hate it... Any way to improve it without extreme major modifications?

 

I don't know if the Dana 60 is a stock axle for that, but look on the driver side door jamb. There should be a sticker there listing different things on the truck, and there should be the word "Axle" with a number below it- this is your axle code, and we can decipher the gearing from there.

As for your current MPG's, if you're doing a lot of back and forth driving, starting and killing the motor [sounds to me this is a farm truck, correct?], that will definitely kill MPG's, especially when you combine starting and killing the motor with in town driving.

 

It used to be a farm truck but now it's just my daily driver for all of my errands as my other truck just **** the bed. So the turning it on and off constantly will kill it? That's sorta what I figured... But I can't do much to change that because I have things to do and I can't leave it running.

EDIT: The axle code is C9. But the axles have DANA stamped on them both.

 

Yeah, constant on and off will definitely reduce fuel efficiency. You have to consider that, besides the amount of fuel needed to start the motor, and depending on how long it's sitting in between starts, the motor is heating up then cooling off, heating up then cooling off again, sometimes starting hot, sometimes starting cold, etc. It all has an impact. And when in town driving is added to the mix, it makes it that much worse.

The only thing I could suggest, if you absolutely need a truck to run your errands, would be to get a smaller pick up such as a Ford Ranger to do the majority of your running around, then only use the F350 when you need to haul heavier loads. But even the Ranger would get limited MPG's because of the same reasons.

Otherwise, make sure to keep up on the maintenance and give it an annual tune up.

 

That's what I thought. I was going to sell this one and buy me a standard diesel because it'll help me on gas... I hate little trucks

I bring the 350 wheelin every weekend n such so yeah.

 

93 F-150 automatic, 2wd, reg cab, short box, 4.9L engine.

Trans code "E", Axle code "18"

I just bought it earlier this week and I couldn't find out where to decipher the codes in the door jamb. I am guessing it is... E4OD and 3.08 gear ratio.

I haven't had a chance to check MPG but I will soon.

What is it going to take to get better MPG from this little gas saver truck?

 

Well, from a gearing standpoint, it would take similar changes as I suggested with my truck. 5 speed tranny, higher geared rear end, and larger tires.

One of the things I'm trying to get at with this thread is what gains we could potentially see with this.

So taking mine or JPC's truck here and making identical changes with the tranny and axle, maybe tires- what kind of a gain would we be looking at? 2-3 MPG at best, or could it go higher if we continued to change the gearing for more of a mechanical advantage to the engine?

Basically, by making the engine work less to move the truck, would there really be a gain, or would you only be able to go so far with gearing before you're doing more harm than good?

 

Well I hate driving with a manual transmission. So that is not an option for me but the axle swap could happen.

Your theory sounds good to me. I makes sense that the less your engine works to turn the wheels the higher your MPG.

I might have to go and by a new axle just to find out how high. I just saw a guy in town is parting out another F-150. I wonder what his gear ratio is????

 

That's just me talking. I like manuals- they're more fuel efficient than autos to begin with and much better control for winter driving. Plus, I drive a tractor trailer for a living, so it feels more natural to me.

 

Let me just state that through many years of trial and error (over 25 that I'll admit to), I have come to some general formulas that seem to work rather nicely for good fuel economy and acceleration in both city and highway driving.

First, let's review the basic formulas for MPH, RPM, and Gear Ratio (GR):

MPH= (RPM*TD)/(GR*336)

RPM= MPH*GR*336/TD

GR=(RPM*TD)/(MPH*336)

where TD is tire diameter, and GR is gear ratio (transmission times rear gear ratio).

Now, here is how I have used these formulas to get the best mileage and acceleration out of my vehicles over the years.

1) 1500 RPM in direct gear should be in the 40 to 45 MPH range for good acceleration and city driving. Any lower MPH than about 37 MPH at 1500 RPM hampers highway MPH and fuel economy due to excessive RPM at higher highway speeds; any higher MPH at 1500 RPM affects acceleration at lower speeds and will require downshifting to get the engine into its power band.

2) Another formula that I found useful (to some extent) is one I found relating to crankshaft stroke and best cruising RPM at highway speed. That formula is this:

RPM= 7200/crankshaft stroke in inches

SUPPOSEDLY, an internal combustion engine, according to a study I read, is most efficient running with a piston speed of 14,400 inches per minute. Now, plugging the number you get from this formula into the Gear Ratio formula should give you the final drive ratio (transmission overdrive ratio, if used, times rear gear ratio). I would recommend calculating the final drive ratio that this formula recommends and using that in the 1500 RPM calculation mentioned above.

Again, all of these formulas are speculative, but I will give you an example using my current vehicle (1993 F150, 4.9, 5 speed, regular cab, short box, 2WD, 29" tire diameter, 3.08 rear end gears):

1500 RPM in direct = about 42 MPH. This is fine for acceleration and city driving.

At 70 MPH, the RPM is about 2000 RPM. Again, this is fine for most of the highway driving I do (65 to 70 MPH), and is about 2150 RPM at 75 MPH.

According to the 7200/stroke formula, the best cruising RPM is around 1800 RPM, which would be about 63 MPH.

I can say that with the vehicle I have, that cruising at about 63 to 65 MPH in overdrive, wherever possible on the highway, seems to be the best for highway MPG. I can also say that it does very well in city driving with the 3.08 gearing in 4th gear (1:1 ratio), pulling very well from about 30 MPH in 4th gear, and cruising very well at 40 to 45 MPH in the majority of the city driving I do.

- Modification to the 1500 RPM formula for other manual transmissions: Use 3rd gear if you have a 4 speed, 2nd if you have a 3 speed, regardless of what the final transmission gear ratio is.

- Modifications to the 1500 RPM formula for automatic transmissions: I would do the calculations for 1500 RPM in both direct and overdrive gears if the transmission is an overdrive automatic. For non-overdrive automatics, I would do the 1500 RPM calculation, and try to keep the MPH near the 45 MPH number for the best compromise of city and highway driving.

Again, your experience using these formulas may vary, but in my humble opinion and experience, they have been proven to me to work very well.

 

250 heavy duty 4x4 with the 2"(haha) leveling kit. 7.5 ltr 5 speed 4.10 rear and 35's. I do 12-13 open road and 10's in town. Loose about 2-3 mpg towing in the 5000lb range.

 

1987 F-150
5.0L (soon to be carbed)
Trans code T (soon to have a C6)
Axle code 19

What is the axle gearing on this? My MPGs have been really good, better than the Merc GM that we traded for it.

 

F250 HD, 4x4, 460 EFI, E40D, 31.8" tires, 3.55 gearing, all stock motor.

I manage an average of 11 mpg (about 85-90% hwy miles). I think 1600 rpm (60 mph) is too low of rpm's to pull the truck up hills without adversely effecting fuel economy (no complaints about the low end torque which allows me to manage it though ). However, the few times I've taken it down the interstate at 70 (~2100 rpm), it does surprisingly well despite the large drag from the air.

 

WOW- that's some really interesting info! Who knew math actually had advantages!?

My only issue here is that you're using 1500 RPM as a baseline. My truck only peaks 500 RPM when I'm accelerating. I don't really know why- I've owned it for a little over 2 years now and it just doesn't idle or run that high. It's always idled at about 350 RPM, and at highway speed runs about 450 RPM unless I'm towing.

When I had to do some ignition and other work with it I found the timing was advanced about 20*! I reset it to 10* BDC as recommended by our fellow FTE'ers, but those are still the RPM's it runs at, save for towing and upshifting.

But it seems to run just fine at those RPM's too, so I'm not all that worried about it.