According to this website the payload of the first model Picasso (Xsara) is 550 kgs.
http://www.automobile-catalog.com/car/2002/547955/citroen_xsara_picasso_2_0_hdi_exclusive.htmlThat would exclude driver, fuel and any accessories. That payload would be spread over both axles.
I do know that the payload for the Berlingo Multispace (first variant 1999 - 2003) was 450 kgs. In comparison the van was 600 and 800 depending on model. (The 800 had uprated tyres and rear suspension. I think the front was the same as the 600.)
There will be a plate on the car somewhere that tell you max axle weight. If you take your car down to a weighbridge they will be able to weigh each axle and then compare it to the max weight. You are interested in the rear axle. The difference will be what you can put over the axle which I would imagine will be in the region of around 300/350kgs.
if you put the tank behind the rear seat then the max weight will reduce - probably to around 250kgs.
You will need to do the maths once you get the info.
In this example, the kerbside weight including the driver is 8 Tonnes.
The load is 10 Tonnes, which in this case is evenly distributed, and the vehicle's GVW is given on the plate as 18 Tonnes.
We now need to measure the distance between the centres of the axles to find the wheelbase, and found this to be 5 Mtrs as in the picture.
We now measure the distance from the centre of the load to each axle and found that in this case it is 3 Mtrs to the front axle and 2 Mtrs to the rear axle as in the picture.
Next, we use the formula in the picture, where
P = payload,
D = distance
and W = wheelbase.
So now to the calculation:
10 x 2 = 20 divided by 5 = 4 Therefore the front axle weighs 4 Tonnes and the rear axle must weigh 6 Tonnes.
NOTE: It doesn't matter which of the 2 "distances" you use, but you should remember that the answer that you get will be for the "other" axle. [IMG]
Doing the same calculation using the "other" distance will give you:
10 x 3 = 30 divided by 5 = 6 Therefore the rear axle weighs 6 Tonnes and the front axle must weigh 4 Tonnes.