Toyota recommends to use regular(87) grade of gasoline, but tested cars with the gasoline grade 91(see user manual). Somebody know why they recommend to use grade 87 if they did not tested cars with this grade?
This claim is my opinion. I have no facts to back the claim.
This could even be urban legend
I've read a plethora of articles recently, due to this question, regarding this subject. It seems to be the consensus of some somewhat reputable sources that manufacturers will do testing with the lowest grade fuel to ensure that the vehicle does not knock/ping/diesel. They do validate that the lower quality pump gas will work in there mass consumable vehicle. If the vehicle didn't run on the lowest available pump gas for the lower end cost market, it would effect sales considerably and due to increase fueling cost they would run a high risk of having a negative effect on their margins.
That being said, they do all of their published testing using a higher grade fuel to eek out another mile per gallon in order to publicize and market against. If their MPG was 1 mile higher than the competition then that could have a positive effect on their margins.
So, to answer your question
Somebody know why they recommend to use grade 87 if they did not tested cars with this grade?
They do this to have the highest publishable and marketable MPG against their competition.
Again, this is my opinion. I have no facts to back my claim.