[aerohead]

Originally Posted by itsmedc

Thanks for the update and inspiration; have been reading all the info old and new on the T-100 and a lot of other materials. Look forward to the next update.

Have a question for you.
I have a 72 Dodge D100 Shortbed which is the first year of the redesign after the style from the late 60's and was used in its basic form up into the 80's. This is a 3rd vehicle used for some hauling and occasional pulling of a pop up camper. I check every tankful of gas but it does not get driven a lot. Also have picked up several sheets of aluminum sign material, for FREE, since they were throwing the old signs away. After a lot of reading, thinking, and studying that includes "SAE 881874 Pickup Truck Drag Reduction...Without Limiting Truck Utility" my plan is to combine a
- 24 inch long cab width wing at 8 degrees down angle
- with a partial bed cover.
The truck came with a 24 inch long over the rail toolbox behind the cab that I use; also has tubular bed rail protectors attached to the box.
The bed cover plan includes a removeable 12 inch extension past the tailgate in the up position, with sides and bottom similar to the bed extension you did but, with an open back. This is after looking at the ATDynamics Trailer Tail for Class 8 truck trailers. Will also be working on a tray to go under the rear bumper and forward to the back edge of the rear wheel well.

My question, finally, is about the length of the tail extension;
? based on your experience, is a 12 inch open back extension, with the proper angles likley to provide a measurable benefit or would it need be longer?

Note I am also planning several other mods before the tail extension including the previously mentioned wing and bed cover as well as:
- partial grill blocks, angled from the bumper to the hood, that includes improved control of the air flowing to radiator
- and a front tray / splitter from bottom of bumper back to the low hanging frame / suspension points with deflectors for the front tires
- and partial belly pans from the edges of the cab and bed over to the frame running from front wheel wells all the way back to the rear wheel wells.

itsmedc,being that your's is a shortbed truck,I think the extension is a good idea.Since you're starting from"square"architecture,and she's not very long,you're not going to alter the wake significantly,as 12-inches won't support much curvature,although you will approach the long-bed length with better chop off.Sepp reports from Europe that an open tail loses some mpg vs a "closed" unit with a back.---------If you stay within 10-degrees for your downslope and say 7-degrees at the sides,and 2 1/2-4-degrees coming up from below you should have attached flow if you run at least a 1/2-tonneau ahead of the tailgate.Your partial bellypan will help your underside flow.--------- As to estimating an improvement,calculate the new wake area of the added tail and compare to the present area of your tailgate/rails/bumper valance/spare-tire and support structure.------------ Your rule of thumb will be a 5-6% mpg improvement at 55-70mph,respectively,for any 10% wake area reduction,as drag is an arithmetic function of wake area.Obviously,the longer the tail the greater the wake reduction but then length can be an issue.24-inches is good for 1.7-mpg,interstate with the T-100.------------- I have a 1962 D-100.I swapped a 4-spd OD and 3.50 rear axle out of a 1977 Dodge and pushed mpg from 11,up to around 18.6 mpg at 70-mph.A 6-inch nose extension with fractional airtight radiator inlet,plex headlight/turn signal covers;partial front wheel well gap-fillers,hubcaps,and aeroshell allows 21.5 mpg @ 70 mph,with a very weak six-banger.