Hii, we're planning on buying the 09' lexus rx350.l know premium fuel is recommended but it can use regular too but the fuel efficiency will go down.

l was wondering if anyone knows the MPG on regular fuel and MPG on premium?

If the premium is more efficient, then it will only cost a negligible amount more, but if not then it would not be worth it.

Thanks:)
l read that it has the same engine as the rav4? (v6 version)
l read that it has the same engine as the rav4? (v6 version)

Premium fuel is recommended because it burns hotter and faster. If your vehicle needs premium and you use regular it wo not burn as hot you will see a power loss, and sometimes a rougher running engine.

It all depends on the engine but in general regular gas will do 3 things, it will make your car sluggish, horsepower will definitely reduce as much as 10 HP. Mpg will suffer, how much varies. Pistons may very well turn to crap from the detonations, i've seen this happen.

Put in premium.

If you intend to tow, then premium is better and makes a difference.

l have an RX330. If l use regular, l get about 22 mpg, when l use premium, l get about 25 mpg and better performance. l did the math and it actually costs less per mile to use premium fuel. Should be similar for your new vehicle.

It is recomended by the factory for premiun but you can use a lesser grade but fuel mileage will drop approximately 1 to 3 mpg's

premium should get 1-3 more mpg than regular grade so it should be more efficient