Seems impossible but anything can be done.
Do any of you remember about 10 years ago Ford had a prototype F-150 that essentially was a stock 5.4L with modified tranny and basically a well bladder tank? The bed was 6" shallower and under it had what was essentially a big well bladder tank. It held something like 50 gallons of ATF.
On the highway it got the same MPG as any other F-150 but around town it was getting something like 60 mpg. As you drove the trannys pump simply charged the bladder tank. When you left a stop light its pressure was used to get you rolling. Once the stored pressure was used up it simply charged it again for the next start. The other cool thing was it produced something like 800 FT TQ for that start so pulling a load such as a 10,000# boat up a ramp would be a walk in the park.
The tank was Stainless Steel and I think the fluid change intervals were 200,000 miles due to the large volume it held.
I always thought this was a interesting approach compared to what the others were doing with electric motors, batteries, ect.
They are bringing it back again but using a hydrogen motor instead of a gas powerplant. They are claiming 40mpg with this current setup.
Non-electric Ford F-150 hydraulic hybrid could get 40 mpg - Drive On: A conversation about the cars and trucks we drive - USATODAY.com
Chris