This won't be for everyone but I suspect it will be effective for me. It's really a simple way to decrease weight by carrying less fuel but still have a way to accurately compare tank to tank data. When I first started hypermiling I was so excited to see where I was at, I kept filling it up when I reached a half tank to see the results. The first time I drained the tank down to E and filled up I noted an increase in FE and I attributed this to driving around with less fuel. Since then I've hypothesized that you could get better FE by only filling the tank half full - but the problem with that is that you can't get accurate mileage results tank to tank because without a fill constant you wouldn't know how many gallons you actually used.
I theorized that you could somehow modify the fuel tank to make it smaller but I saw two problems with this: One, I don't know how to do that, and 2 it would limit your range which would be lame for road trips (which I take semi-regularly).
It's funny but this has been one of those brain teasers for me, I really thought there would be a way to do it so I never forgot about it but i never really came up with anything either. UNTIL RECENTLY!
On a road trip return leg I was in Phx with about 40 miles left on the tank. I know that if I get gas in Gila Bend I will have just enough gas to get back to San Diego and it means on the very strenuous hill climb (which is a lot of work for a 3 cylinder car full of 3.3 people and luggage) I will be on my last gallon of gas and therefore save about 56 gallons (assuming gas weighs 8 pounds/gallon like water does). The problem was that Gila Bend is 50 miles from Phx. If I got gas in Phx I would have to fill up before the hill climb and carry an extra 50 lbs. That's when I finally figured it out
! In Phx I stopped to get gas and I put in EXACTLY 1.000 gallons and did not do a fillup in the SG and did NOT reset the trip odometer. This got me safely to Gila Bend, where I filled the tank to full as usual but when I calculated MPGs I simply did odometer miles divided by Gallons in Gila Bend + Gallons in Phx (1.000).
What I realized is that any time I want I could fill the tank partially full, note the actual gallons I put in the tank, and so long as I don't reset the odometer or calculate mileage until the next proper fillup (all the way) then make sure when I calculate MPGs I add the actual number of gallons used in both the partial fill and the top-off fill: My FE will be real world accurate and I will effectively carry less weight.
This wont' be for everyone. You'll have more limited range, have to stop for gas more often, and get less chances to log your fuel (which, let's be honest is pretty addictive)... but It should be very effective on cars like mine where every pound seems to matter. If you wanted to be really crazy you could basically try to keep your tank super low by only adding a gallon for a few refuels in a row, then maybe monthly filling it all the way to check your FE. The more partial fills the more effective but also the more annoying. I've decided to try it for a while adding only 4 gallons when E then doing a proper fillup on the next tank.
What do you guys think? Anything I'm not seeing that makes this a terrible idea?