This is a good website to get information, such as fuel economy, on all Toyota vehicles for all model years. Prius HEV and Prius PHEV are under the Ha line. Toyota Japan full vehicle catalog (all years) | トヨタの車両情報 | トヨタ認定中古車 | トヨタ自動車WEBサイト
Going out on a limb here, the fuel consumption figures are going to be from the official tests of Japan. The current one will be WLTP-J, but earlier models will be on the JC08 or 10-15 test, which were the easiest official tests in the world. The gen1 has the same efficiency as the gen5 on that site.
