Supertop & Gas Mileage.....

Aw, GEE..... Do I Have Too ???? ( L o Frigg'n L )

 

Thanks for ' Pondering ' , Cap !

 

I also got a reasonably consistent 18 mpg highway mileage out of my 5 with 4.88's and the OD. That was with the Holley 450 cfm 4 bbl and headers. I actually think most soft tops would be more aerodynamic than the hard tops. Most soft tops are not vertical on the butt and that has to reduce drag somewhat. That and they have a reduced frontal area as they terminate at windshield height. They are also 150 lbs lighter than a Meyers. Geeeze I hated that Meyers--------

On a more recent note, 18 mpg is also what I got the the 381 sbc that was in my 3B, with EFI, 4.09 gears and the OD. However, one of those Jeeps had a decidedly larger grin factor.

 

I never checked my mileage but it seems like about 12- 14 mpg I would guess. I drive it around 20mph most of the time at the Ranch, speed limit on the Ranch is 20mph in the hunting area, or in the cabin area. On the black top I drive it 50-55 mph and everyone passes me, the speed limit on the blacktop is 65mph now.

 

Always the first Requirement, 'aint it !

 

Yep. Don't know about the shape and aerodynamics, but the steel cab starts off with this clear disadvantage.

With Lockman's numbers, I figured you must be able to guess at the absolute top MPGs possible with zero wind resistance at about 60 mph. You can make a linear approximation - it's a simple problem with two equations and two unknowns: m - 52^2 * n = 20, and m - 75^2 * n = 13. Here n is the unknown drag factor, and m is the 'baseline' MPGs without any drag. The solution is left as an exercise for the reader but I'll tell you the answer which the reader can confirm. The baseline MPGs at 60ish is 26.5 mpg. With the drag of 52 mph, you lose about 6.5 mpg to drag. 26.5 - 6.5 = 20. At a drag of 75 mph, you lose about 13.5 mpg to drag, 26.5 - 13.5 = 13. Pretty close to double the loss to drag, as predicted above.

Thinking about this a little more, it's really a second-order equation in velocity, so the thing to solve would be m - (n*speed)^2 = mpg. Again, two equations with two unknowns... solvable. Tried this - it gives the same result.

 

The price of pleasure... Every 100 km I fill the tank. No more calculation.

 

I would assume that the worst aerodynamic configuration would be windshield up, and top down. The seats and tailgate are going to catch a lot of air. It would be interesting to compare the drag of windshield and top down vs windshield and top up.
Of course, this discussion is all about a vehicle with the drag coefficient of a safe falling from a high window....

 

.... In which I have witnessed , but was not involved with, some years ago ! ( 3000 # Mosler went from the third floor to 1 1/2 feet into old concrete. )

 

Tim that makes my head hurt but math isn’t my strong suit I’m more of an artist. I fill my Jeep once a week and it uses about ten gallons. If I just go to work . It’s almost twenty miles one way so twenty mpg? Seems high but my speed is low.

 

That makes sense to me, since other than the aerodynamics of the vehicle, which relates to engine load, engine speed is another major factor in determining the most efficient vehicle operation. I seem to remember reading or hearing that an engine operates at its most efficient when being run at the midpoint of the engine's torque curve. Does that sound familiar to anyone else?

 

Update : ( My final 2 cents on this )
I am averaging ~ 16 MPG around town & a little Hi- Way driving. Pretty good for A CJ5 that has the every day potential to do 0 to 60 in 8 seconds. I just have to keep ' My foot out of it '

 

but the pipes sound soooooo good.

 

OK read your profile
1975 CJ5

1984 Buick Turbo V6 SFI, 700R4 Auto w/ Overdrive
so how does this relate to a stock early cj . as to gas mileage
it's like comparing apples to oranges . although it's a sweet set up

 

I love a good engineering approximation.

I agree with Tim's earlier assessment that 2 data points with multiple confounding factors is not suitable data to make a statement about the efficiency of the jeep. Now, if you track every tank's mileage, along with air temperature, wind speed, humidity, average speed, and elevation changes for a few years.... Then we can make a nice big spreadsheet and do a real data analysis and look for primary and second order effects.

of just keep filling it up and enjoy the jeep for what it is, and inefficient brick of fun.