Bernie replied:

Hi Rusty
Yes it does increase, some thing to do with the pump & seals. I have a Deisel & use about 120L every 3to 4 weeks
not sure if filler set up is the same as petrol,
I fill sub 1st then main tank & use a torch to see fuel is visable in pipes,
(this may seem extreme but it is the only way to be sure tanks are actually full) I then run on sub 1st to approx empty then main tank untill light comes on. Then repeat same.

There is a good fuel calculater at www.hicloneqld.com/data.htm
Cheers

Member - Stephen replied:

Rusty,

In a word .... Yes.

I found that when doing economy measurements, I had to start and finish with both tanks full. This was because I had the habit of always using the main tank first so as to take the weight off the rear wheels before using the sub tank (a habit probably formed from my Prado-pre-suspension-upgrade days). Also my economy runs were nearly always over one or two thousand km, which were a wee tad over the range of just one tank.

Cheers
Stephen.

Mick replied:

Rusty you don't necessarily need to refill both tanks. Start with both full and run on either and then refill to calculate amount used. If the sub is full and you use the main, no fuel will run to the sub (if it did it would overflow) Similarly if you use only the sub, no fuel comes from the main. The only time fuel flows between tanks is when using the main and space available in the sub. By the way, I've found that if I empty the sub, then switch to main, there is only a litre or two in it after nearly emptying the main.