Squeezing some more mpg from a brick
 

Hey all, I have a 1968 Jeep M715 that I’ve rebuilt with a modern powertrain and drivetrain that I use as my haul truck for work. It went from 7 mpg to 12 (mixed city/highway) after the swap. I’m now building a 45 degree windshield frame (I’m an aluminum fabricator) and I’m wondering how significant the difference will be. The current window is a vertical 5’x2’ flat surface because they were never meant to go faster than 45 mph. Main goal is to make it a little more economical to drive I’d be pretty happy if I could get to at least 15 mpg and the windshield mod is pretty far along already so it’s happening in the next couple weeks I just find my curiosity getting the best of me as I build it. Not sure how this post will be received as I’m sure it’s fairly different than what usually comes on here but I just figure “what better place to ask?”. Eventually I’ll also build an aero cover over the bed.

This could have gone in Introductions. Anyway...

What we called a 5/4 ton? It's hard to picture a 45° frame that won't impact headroom. Are you chopping the top?

Until you have enough posts to include pictures, I'll just sugggest a three-piece windshield with bevels (Tara, Clark Cortez, others) might be best. What will you do about the fenders?

It is the 5/4 ton, I've heard them called many things lol. This one also has an all aluminum bed too. Your right that a 45 degree is steep, really during mockup it seemed like 20 was more reasonable to expect. But I tend to be ambitious and it usually pays off, the key to making it work is moving the windshield forward about 5". This allows the slant to extend up high enough without interfering with the driver. The roof will also descend down to the windshield to help retain head clearance. The top might end up 2" shorter at the peak but not much to lose before it becomes an issue for someone on the taller side. No doubt this project is more than just the windshield, it also means a new roof bow setup, side window glass modification, new roof and back panels. No plans for the fenders, can't delete them for legal reasons (width of the wheels). Nor am I interested in building a "brodozer" with wheels completely sticking beyond the body. I have been searching around to see if there's any kind of diverter that might help in that region. The important thing is to get engine load down as it is going down the highway the engine is loading up to the point that its pulling timing and adding fuel (entering WOT settings off manifold vacuum). Taking that load down would allow it to stay lean and in its cruising timing which would provide much better efficiency and I think slanting the windshield would go a long way towards that. I have considered a multipiece windshield, it would allow certain parts of the glass to angle even further back, but I'll need to experiment as I have some concerns about potential blind spots. Anyways Thanks for the reply freebeard!

It's what we do here. :)

One picture I'd love to find again was of a WWII Army Jeep with the windshield and roof of a B-17 grafted on.

https://images34.fotki.com/v1212/pho...DuVall2-vi.jpg
images34.fotki.com/v1212/photos/0/223020/10957908/oCalPlating31FordTruckDuVall2-vi.jpg

Two examples of the DuVall windshield.

For the fenders, I was thinking pontoons, or time attack wide body.

Found this while looking for something else:

https://i.ebayimg.com/00/s/MTIwMFgxN...RhFKd6/$_1.JPG
A three-pice slant-back windshield.

45-degree windshield
 

I went looking for 'blueprints' of the 'brick' and the first thing to show up was the M1715 Ambulance and civilian Gladiator ambulance, which has a raked windscreen, shown together.
While fuel economy for the M1715 was given at 7.6-mpg, at unspecified conditions, I couldn't find mpg data on the Gladiator.
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Today's Stellantis-JEEP Gladiator incorporates the slant-back screen, so that's a clue. Some owners of the diesel-powered JEEP are reporting over 30-mpg.
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It's possible that adding edge radii to the sides and top of the windshield frame ( header and A-pillars ) could achieve fully-attached flow around to top and sides ( like the '32-Coupe at A2 Wind Tunnel ).
Dr. Paul MacCready, of AeroVironment received a US Patent for an aerodynamic fairing for the JEEP 'Wrangler' windshield frame, which amounted to attaching miter-cut sections of 8-inch PVC pipe, cut in half, top and sides ( blind spots would be an issue ). Greyhound buses have been modified likewise.
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The DODGE military trucks which the M1715 replaced, had a slanted, split-vee screen, and the A-pillars appeared to possess ample curvature ( MASH 4077th ).
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The JEEP is 85-inches wide. As little as 3% ( about 2.5-inch radius ) of width has been shown to provide drag reduction, compared to a 90-degree transition. 7% shows 'remarkable' drag reduction ( around 6-inch radius ).

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Even with the raked screen, you'd be strongly encouraged to 'soften' the A-pillars.
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Some 'cardboard and duct tape' edges and yarn tufts would tell the tale.:)

I would think aero is the Least important thing for a jeep

What modern powertrain and drivetrain have you fitted to your truck?