Mileage while behind a truck

A week into getting our Prius, we decided that our trip home from Xmas in LA would be an opportune time to find out whether & by how much we could increase our mileage if we rode behind a truck, in its "wake". Insert disclaimer here about how this can be dangerous, do not try this at home, we were always following at a safe distance, etc. Here are the conditions and the approximate results (in a nutshell, yes it does increase, and by 10-15 mpg).

THE DRIVE: Driving I-5 in California, btw Kettleman City and Hwy 152 (the exit for Gilroy), plus on 152 West for about 30 miles. On I-5, mostly flat and very straight. Outside temps were in the 40s with occasional showers. On Hwy 152, very hilly (one climb to 1400 ft), temps in high 40s with heavy rain.

THE TRUCK: A 30' moving van, like a U-haul or Ryder truck but no Gramma's attic. I'd been looking for a good truck to follow since leaving LA, but the big trucks seemed to go too slow (do they know what you’re trying to do and slowed down on purpose?). As luck would have it, just north of the exit for Kettleman City, this moving truck passed me while I was going around 65 so I knew it was THE truck and pulled in behind it.

There were two ways I compared my mileage.
Method 1: eyeballing the “current mileage†number (in the “Energy†screen) for roughly 1 minute. To do this, one of us watched the “current mileage†(in the Energy screen) while the other drove behind the truck, and compared it to what we know it’s been before the truck came along. Because “current mileage†sometimes jumps around, we disregarded the brief fluctuations up & down, so not sure how accurate this is.
RESULT: 35-45 mpg normally, 45-55 mpg behind truck, at speeds of 65-70 mph
% INCREASE: About 20%

Method 2: recording the “average mileage†number (in the “Consumption†screen) after following for a long time. I ended up following Mr. Truck for almost 2 hours. When we both exited onto Hwy 152 W, I followed him until we split at Hwy 156. So for approximately 120 miles, I was right behind him the whole time, passing when he’d pass and slowing when he’d slow, except a few times when it was safer to let a car between us; but I always got behind him again after a minute or 2.

Our speed ranged from 60-80 mph (mostly 70-75 mph) on I-5, a distance of roughly 90 miles. On Hwy 152 (roughly 30 miles, very hilly, plus it was raining & very wet) we had big ranges in speed, from 35 to 70 mph. My average mileage over the 120 miles was 53 mpg.

Based on mileage I’ve been tracking before Mr. Truck, or on my commute & elsewhere, I would guess that I get about 35 mpg if driving 65-70 for 90 miles; and around 44 mpg driving on a hilly road similar to Hwy 152. So…

RESULT: 35-44 mpg normally, 53 mpg behind truck
% INCREASE: 50% better at faster speeds, 18-25% at slower speeds

So there you go folks. Please note that my calculations may be slightly off due to inaccurate memory (it was more than a week ago), but the numbers are as best as I could remember them. Also, I don’t mean for this post to become incentive for all Prius’ers to start tailgating a truck. But it does work, folks.