Puzzled

[m_m70]

Hey Paul,
Ya, I'm pretty happy with the MPG on the freeway.
I would give credit to good old fashion tuning........
Going with the 1403 carb (1406 too much for a 302 imo) helped and spending a LOT of time tuning (jet and rod changes until right) is key.
I upgraded to duraspark 2 and a stock '77 distributor. My stock springs in dizzy HAD to be changed to get the curve right and the spring tabs bent in a touch to get to the sweet spot.

Nothing has worked "great" right out of box and tuning takes time and patience but once set, the 289/302 is a great motor.

Just a side note, my BMW 525xi spins around 2700 rpm's @ 70 stock. MPG isn't solely based on rpm but more on where your engine is happy. My 302 is happy @ 2900/3000 and I'd go cross country in it today with no worries!

 

[Yeller]

Yep I get as high as 18 @ 50-55 at 75-80 10 is as good as it gets. It takes a lot of fuel to make enough power to push it through the air. That line is not linear either, wind resistance is an incredible force. On a graph it’s a nice gentle sweep to about 55 at 60 it starts climbing like a rocket. There’s a reason those super cars with better aerodynamics than a fighter jet make 2000hp to go 300mph. It takes it to push the air

 

[Rustytruck]

I was happy when I went to the quadrajet carb it more than doubled my off road mileage. i run with a club with allot of 4 & 6 cylinder jeeps they can go along way on 15 gallons of gas off road they went further than I could go with 30 Gallons. changing to the Quadrajet I no longer feared their mileage as was left with plenty of gas at the end of the day when they were running empty. highway was better too but nothing like off road driving in low range. that motorcraft carb just sucked gas in bumpy low range.

 

[nvrstuk]

Here's what people don't understand... it's not just RPM that affects mileage. Like previously mentioned... you have a cardboard box going down the road...

Decades ago I realized when biking what not only aerodynamics due to NEVER going faster on a bike but also how much HP it takes to move the same bike or Bronco as air drag increases exponentially!! Serious!!

Check out a wind tunnel machine and what air at speed does to HP needs to increase speed.... it will blow you away (pun intended).

Roughly to double your speed you need to roughly 3x4X the HP due to many factors but aerodynamics is the largest.

So on the chart below you can figure a Bronco is a Rectangular box... this shows that driving a Bronco is appr 4x less aerodynamic as a car... simple comparison but this is why Tour de France riders all go about the same speed downhill and on the flats but the races are won by the guy that can outpull the others going up a steep hill because nobody can do 50mph on the straights and if you did you wouldn't last...

 

[1970 Palmer]

I'm setting here reading about Bronco gas mileage, and I look up at the new T-shirt I picked up last week.

IGNORING GAS PRICES, SINCE 1989

Wild Horses, Lodi, CA

John

 

[blubuckaroo]

OK I get the center block down the Hwy, but I would think if you dropped your RPMs you should get better fuel mileage.

Sent from my moto g(7) using Tapatalk

You will get your best economy when your engine is cruising at its maximum torque RPM. With a stock 289, that is about 2200 RPM.

It's best to select gears/tires so your normal cruise speed is at that RPM, but aerodynamics become a major factor at about 60 MPH.

 

[nvrstuk]

You are dropping your rpms but if you are going at 70-75 now instead of 60 your HP demand has increased by 25-30% (guessing) so then your mileage drops significantly because your engine is wirking harder-like going the same speed but with 3,000 .ore lbs in the back

 

[nvrstuk]

I kept a mileage log for about 30 yrs with my 289, 3spd, L70x15, stock height.

I wrote it all up on CB probably 15 yrs ago.

Nutshell-

I lose more mpg when lifting my rig to 3 1/2" suspension and 2" body lift than when I bumped up from 289 to 351.

3 spd w/OD the mileage stayed exactly the same even though I dropped rpms. Like broncobowsher said, I was out of its peak torque range...

Still having doubts? Stand straight up in a 40mph wind.

Now hold up a 4x8 sheet of plywood in that wind. Take more effort? Probably 5 x as much to try to hold it up...

 

[DirtDonk]

Another example. A 1996 K1500 stock height, tires (should be 265/75/16 if I remember), 3.73 gears, 5.7/350 full EFI.
In overdrive going 55mph flat freeway (I5 in the Central Valley) I would get 17.5mpg.
Same conditions going 60mph, 17mpg.
Same conditions going 65mph, 15mpg.

Same truck, same tests, only in Drive/3rd gear rather than OD.
Same exact miles per gallon in each category.
So 400-500 or so rpm higher at each speed netted the same mpg.

Same truck, same tests, only lifted 6" with 35" General Mud-Grabber tires. So, lower rpm across all tests, but higher aero load and theoretically higher rolling resistance.
All tests netted 13-14mpg.

Same truck, same lift and tires, after a re-tune with a pre-production Hypertech tuner.
All tests 14-15mpg

Same everything, but this time with a hard tonneau cover.
A slightly more consistent 16mpg and all speeds, but in overdrive only.

Later test with General tires changed to BFG Mud-Terrain tires. Lost 1mpg under all conditions. Tires were half an inch taller on the ground. Speedo was about 1mph slower but all results were taking that into account.

All tests were done with cruise-control set and very little traffic usually late at night (I was on Hwy 5 a LOT back then with nothing better to do than check mileage!). The only thing I could not account for, or even tell if it was a factor, was any head or tail wind conditions. But the results were pretty consistent with not much wind working for or against.

My conclusion was that a few hundred rpm made little to no difference, while actual speed, aerodynamics and tire rolling resistance made the most notable changes.

Paul

 

[DirtDonk]

And as a direct comparison to just tire changes, my supercharged 3.8L '04 Buick GS can gain or lose as much as 4mpg just by changing tires models.
I've had three sets of tires since I've owned it, and the first change dropped my consistent 31mpg on the trip to Lodi down to 29. The next set dropped it to 28. Same commute, measured over many years.
Must more aerodynamically designed car when compared to the trucks, so speed was not as big a deal, but the sweet spot for that car is 66mph, where 55 does gain some, but not enough to make the longer trip worth it! But over 70 loses about 3mpg.

For the aero part, excluding tire type, following closely behind big square trailer big-rigs at about 60mph netted me an additional 4-6mpg consistently.
I think they did that on Mythbusters once, but I don't know if they were going for mpg or measured load, or what. Probably MPG though, knowing those guys.

All numbers are calculated, rather than based on the vehicle's on-board computer. Which were consistently optimistic by about 2-3 mpg, over actual mileage.

I know those are both apples to oranges comparisons compared to a Bronco. But you can see the patterns that would still apply to the even less efficient Broncos too.

Paul

 

[Rustytruck]

my 2016 tacoma is very sensitive to gas quality and tire pressure i pretty regularly drive my stock factory truck from LA to AZ about once a month. tire pressure at 30 psi tire pressure at 70 mph 18 mpg running middle grade chevron. same everything 35 psi 22 mpg. last trip back stopped at a different chevron and only got 17 mpg everything else the same. must have got regular instead of mid grade. I had to get gas on the way home where usually I would be left almost 1/4 tank. Little things can make big differences.

 

[nvrstuk]

... another example. Most of you know I'm a detail/numbers guy.

Back when the Arab Oil Embargo hit & our speed limit was decreased from 70 to 55mph, tben Gov Dixie Lee Ray in Wa had to fo better and lowered it to 50mph (I felt like I could RUN that fast.lol

Anyway, My Vette powered 58 Chev 1/2 t gained 1 mpg and my wife's '65 MGB dropped 4 mpg... consistently with every fill up till the speed limit was bumped back up.

This shows that being out of the engines torque/efficiency range can really effect mileage. Car was slippery [coefficient of friction) to begin with and losing 15 mph killed mileage.

 

[blubuckaroo]

I learned about economy and tire design when I replaced the "green rated" tires on my hibrid Lexus. The new tires weren't "green rated" and my mileage dropped.

 

[nvrstuk]

Green tires have a hard tread compound. Did a bunch of research to buy those here in our area (but still put my sticky snow tires on in Nov) but between my reading and talking to several shops the consensus was do not use them in areas with frosty roads as like the Toyo MT, the compound is so hard they wont grip a frosty road...