gas mileage with OD

[blubuckaroo]

1. Exact tire height (rear tires) with the vehicle on the ground.

The correct way to measure a tire, for gear ratio calculations, is to measure the radius to the ground with vehicle weight, and multiply times 2.
If you only measure the loaded height, your tire will appear too bigger than it is dynamically.

 

[DirtDonk]

For speed correction, you can also pace someone with a known good speedometer, or use a GPS application on your phone, or even a GPS device if you have one.
This way you can go at normal traffic speeds and get an accurate reading without trying to calculate all the stuff from 60mph.

But too, there's really no need for all the fear. I know how CA freeways are, and since the legal minimum speed in the right lane only is still 55mph, and that's the legal max for trucks and trailers anyway (yeah, I know... as if they were observing that!) you're not likely in danger of being run over at 60.
There are times when traveling that speed is fine. Just don't check your speedo at commute time!;D At that point it's all just "get out of my way dopey!"
Of course, nowadays "commute time" is relative anyway. About 3:38AM to 3:52AM is your only window of opportunity. So no problem, right?%)


For gear calculation without opening up the diffs, roll either the vehicle or a tire 1 rotation and see how many times the driveshaft turns. This is a relative thing, so there are multiple ways of doing it.
If you don't want to roll the whole truck while someone watches the shaft, jack it up and turn one or two tires. Lockers, limited-slips and open diffs can all act differently. If you jack both tires up and one turns the opposite direction from the other (open diff) the you need to double the number of turns of your driveshaft to calculate the number.
If you leave one tire on the ground, this discrepancy can be eliminated, but if you have a limited-slip or locker it can be very difficult to turn that one tire.

Basically though, if you get roughly 3 and a half turns of the shaft to one turn of the tire, you have 3.50s and if you get just over 4 turns, you likely have 4.11's. If you get more like 4 and a half turns, you could have 4.56 gears.

See? Simple, right?%);D

Paul

 

[blue 74]

Ok, I will try it this weekend. I know I have a (open diff) so I will leave one tire on the ground. I will post my results. Thanks

 

[DirtDonk]

Great. Look forward to hearing the results.
And if it acts all weird even with one tire on the ground, just roll with it and maybe you have to double the number anyway. But I think you should get a good result.

If you have a handy helper thought, a chalk mark on the tire and a mark, or a bit of tape on the driveshaft will let you do the roll test. Just roll forward one tire revolution while the helper counts the turns on the shaft.

Good luck.

Paul

 

[nvrstuk]

Aerodynamics is KILLED with Bronco height...taller tires, susp lift, body lift, and tire OD all really affect mileage. I kept a mileage log for 2 decades...every trip, every mod showed decrease or increase in mileage.

289 was/IS the best motor for mpg...excellent bore to stroke combination...

ANY smog motor 302 from 71-77 will get poor mileage...

Seems like everybody is a carb hater now...getting worse every year as less people know how to work on them...that said, my Quadrajet powered, 351W, ZF, 203/D20 35's, 4.11 got better mileage on the road AND on the Rubicon than all 8 other vehicles. We drove side by side so the comparison is accurate. 2 were Exploder EFI, 2 were Mustang EFI. I averaged over 14mpg the entire trip. We were loaded with gear, spare parts, cooler(s), etc

Engine has to be built for mileage...or at least not be a smog motor...

 

[rjrobin2002]

I got 15mpg with 5.o efi, 4.56, 35s and c4 at 60mph driving for a tank.

My friend with 5.0 and 4r70w, 4.11 and 35s got 18 mpg same drive 60mph.

Maybe you should gear it down to 4.10 or 4.30s and start driving 60mph instead of hauling ass so your OD dont downshift. You will pick up several mpgs.

I always liked about 2300rpm at my desired cruise RPM which was usually 60mph.

 

[rjrobin2002]

I

Aero is an interesting beast. Little things can make a huge difference. You know you are dealing with really bad aero when you see water drops on the hood pushed forward when driving at highway speeds.

I never thought about the water doing what it does on our hoods. I guess the way the hard top over hangs and the windshield angle is the issue.

Who is gonna be the first to add a air dam and change the windshield angle and doors to match?

 

[nvrstuk]

On that same Rubucon trip we grenaded the "long believed" theory about high and low pressure areas on the hood at the base of the windshield...ya get bored eating trail mix for 10 days without tossing one on the hood..then another...then seeing how long they stay on the hood at 65mph...fun trip!!

 

[DirtDonk]

The roof line and overhang are certainly not helping, but even smoother cars of the era with more laid back windscreens had the same effect. Remember the rear-facing hood scoops? Especially on early Trans-Am and the Cowl Induction setups? Rear facing for a reason.
And all those heater and vent intake grilles at the base of the windshield? Same thing.

When the vent scoops for Broncos were a popular thing, you saw them oriented in both directions all the time. Lots of people never quite wrapped their head around the high pressure area formed there at the trailing edge of the hood. Of course, maybe even facing forward helped direct some additional air in there anyway, so everyone was happy.

Paul

 

[Rustytruck]

Remember back in the early 80's when front hood air dams we all the rage. My Bronco got better mileage with one back then. You could see it work every day. As soon as you got up to freeway speed the windshield wipers no longer blew up over the windshield and on to the window gasket. Back then I was running a hard top and stocked geared running 31" tires and trying to go 60 mph without getting a ticket. It was worth almost 2 mpg at 60. My California tuned totally stock 1974 Bronco really liked 55 mph after that it all went down hill. But that was the worst motor Ford ever put in a Bronco.

 

[DirtDonk]

So better with the "bug shield" then, eh? That's good to know.
On trucks with better aero to begin with, the opposite was often the result.

The only comments I got from customers (non-Bronco types) was that their mileage went down after installing one.

And the Jeep guys with soft tops had to forgo the straight-up ones altogether. At anything over 45mph the soft tops would flap to beat the devil! Very annoying, and in extreme cases where they left them on anyway, the top materials wore through at the bows and other tension points near the front.

Glad we never run into any of those kinds of problems...%)

 

[surfer-b]

So better with the "bug shield" then

I can see that, the small deflection raises the draft above the overhang of the top to the windshield frame, would not have thought about it until it was mentioned

 

[Broncobowsher]

Aero is such a weird beast. I remember stories about the 70's big wagons that got the best fuel economy with a luggage rack and a rear window air deflector. Either one alone hurt, but paired together they helped. Not the most logical.

 

[bronconut73]

Aero is such a weird beast. I remember stories about the 70's big wagons that got the best fuel economy with a luggage rack and a rear window air deflector. Either one alone hurt, but paired together they helped. Not the most logical.

Yes. Very true.

Hydrodynamics is far more predictable

 

[DR650]

Yep...and the aerodynamic drag at 70 is twice..2 times...what it is at 50.

Source?

 

[nvrstuk]

Source?

Ask any bicyclist!!

Every time you lift your rig, tires, body lift, suspension lift your coefficient of drag increases and when I kept accurate mpg logs EVERY time I raised my rig iver rhe decades it dropped my mileage.

If I remember correctly if you double the speed you increase the aerodynamic drag by a factor of 4. Could be wrong, that's what I remember.

 

[73azbronco]

11 is normal

 

[73azbronco]

Source?

google.

Or, aero engineering,

D=coefficient x density x velocity squared/2 x A

drag=d, A = surface area.

Simply put, all things remaining constant, going from 50 to 70mph, is a 20 velocity increase, squared divided by 2 = 400/2=200, this does not mean its 200 times more drag, it means

the formula sees 200 more at that factor than before. Google search says 2 to 3 times more drag going from 50 to 70-. But is it drag or power required which are not the same thing. You can see the difference in your MPG and range.

 

[nvrstuk]

Back on OD and mpg... I forgot to mention that when I tried a Ford 3 spd with OD in a vain attempt to get better mileage with my 289, I gained nothing, zip, nada. The 289 didn't have the torque to allow me to cruise in OD and get better mileage and a lot of that is tire rolling resistance, wind resistance/aerodynamics. That's when I decided why not have some fun when rowing thru gears and I put in a toploader, wide ratio from a sbf, Also put built my 351W that I racked up 220K miles on. Trans lasted until SOB3 (have to look it up) and the heat treating that AA had done failed. That's when I put my first doubler in. ZF/203/D20 in '01.

351W tho is a different story. It has enough torque to allow the engine to be able to cruise in OD. Ran the 351W with a ZF with OD and a 4r70W with them behind the same 351W. The 351W did fine. Like I mentioned earlier it got better mileage with a Qjet than 4 other Broncos on a 2,000 mile trip and they had EFI. Pretty good comparison apples and apples.

 

[m_m70]

Seems like everybody is a carb hater now...getting worse every year as less people know how to work on them..

yup.......I just did a trip couple weeks back and on the 140 mile "flat stretch" got a solid 17.5mpg.

That's with 33's, 2.5"SL, 1"BL, 4:11s, 3spd manual, 302, TFS heads, dual exhaust and small Holley 465cfm 4 barrel. Speed I kept between 60 and 65 (around 2800 rpms) and like I said this was a long flat stretch with no traffic.

I like my carbs. I run a afr gauge in the cab and have the carb tuned between 15.5 and 16 at cruise which I feel is why I can get some impressive mpg. Even with the tune though, anything over 65 drops that mpg like a stone. Second leg of that trip going over Donner Summit dropped to 13 but that had a lot to do climbing to 7k feet.

No carb hater here!