Espace 3
 

My target is lower consuption for speeds 80 - 95 mph. Consuption varies 29.4 - 23.5 mpg. Lower is typical Normandy hilly region.

Under view rear axle and tyre ( url photos.app.goo.gl/EQFnLer3jerSukSp8 )
https://photos.app.goo.gl/EQFnLer3jerSukSp8
https://ecomodder.com/forum/member-m...under-view.jpg

Did you mean KPH? If not and you mean MPH, why so fast? Your easiest and most cost effective mod will simply be to slow down a little. Aerodynamic drag increases fast per than speed. Stay under 80 and you'll definitely see fuel economy benefits...

Assuming you can't or won't slow down. A rear difuser for that underside is an idea, but you will want to design and test it before you settle on a shape. Keep it smooth (no fins) and measure the angle of the difuser precisely if possible.

What kind of car is this?

We travel long distances on highways with this car. Typically 1500 miles in two days. That is why we are keeping speed and why aerodynamics is very important.

With 4 bike bike rack behind is 19 ft long, but only five and half feet tall.

Factory built engine and body is optimized for 80 mph (130 km/h).

It is a 7 seat MPV built on protective special steel frame from glass fiber. Second and third row seats are easily removed. In Grand model there is 2 ft trunk space behind last line.

If you're getting 30 MPG at 80 MPH, it's already not too shabby.

Basically, had the Chevrolet Lumina APV been designed in France, would be just like the 3 earliest generations of the Renault Espace.

Closest France got to the Previa. :)

This is the car?

https://ecomodder.com/forum/attachme...1&d=1628654677

If so, in addition to a full undertray, see if you can keep the four bikes on the back rack within the frontal area of the van, oras close as possible. If they are sticking out into the flow, they may be a little like a parachute.

I agree with freebeard, though, 30 mpg at 80mph with four bikes hanging off the back? Amazing really.

Are those US or UK miles?

Car in these images is base model. I have Grand Espace which is about 1 ft longer.

I agree that loaded consumption is amazing in that 80 mph speed. But when you look under belly pan can achieve more. I would say 12-15% savings.

I try to give all values mph mpg in US version with calculators.

Maybe that much improvement is possible, I don't know. It would depend on how you execute. Are looking only at the underside, or are you willing/able to be more open about mods? How about rear wheel well skirts? Or a box cavity fairing at the rear to reduce the wake and maybe "hide" the bikes from the airflow? These would need to be tuned, as research has shown that box cavities, wheel well skirts, and undertray difusers interact and significantly change each other's performance when used together and in various combinations. (Don't let me talking like that confuse you as to my expertise... I have no formal engineering or aerodynamics training. I am a hobbyist.)

Europeans even got the 1st-generation Previa with a 3C-T or 3C-TE turbodiesel and a 5-speed manual transmission.