so many misconceptions about both hybrids and diesels.
First of all, lets get things straight. Modern turbo diesel is abut 1,500USD to 2,000 USD more expensive than similar petrol engine. It is also 150lbs heavier than modern petrol engine. Torque is nice and it gets an nice kick, but its power band is only 1,500 rpm (thats with great engines).
Actually, comparable European diesels weight more than Toyota Prius.
Combining extra cost of diesel, with extra weight of diesel, and the fact that they are still polluters (modern super cats will add aditional 500$ cost to make diesels cleaner, but still not as clean), you pretty much lose every good point of an hybrid, and add extra weight and significant price, for few extra mpg. Aditionally, diesels have problems with warming up quickly, so they couldnt not turn off at any time. Also, because of heavy vibrations, you car would shake heavily each time it turns off.
All of that combined explains why there is no full diesel hybrid. Toyota already started selling diesel hybrid in a Van in Japan, 2004.
As to the why no manuals - Prius's CVT serves as power splitting device that regulates power from electric and gas engines. It would be impossible to have an manual transmission. Besides, CVT is more fuel efficient. Power splitting device in an Prius is very much direct mechanical link, and that poster in that forum has absolutly no knowledge of what he is talking about

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I would personally pay more for an hybrid Rav4, than for diesel Rav4. While diesels are more advanced than ever, and I shall be purchasing an diesel Rav4 in next 2-3 months, they are still very loud, and whoever says that you cant notice the diesel is pretty much an big liar. Come colder days, all of them shake and sound like tractors. That includes 7 series and S class diesels. Thats how all of our diesels sound too, even Avensis, which is considered to have pretty quiet and refined engine. Well, at 40F today, it sounded pretty throaty.
I dont see how an diesel would handle better than hybrid, if anything, opposite is true. Diesels are nose heavy, while batteries in hybrids are at back, balancing the car.
I got pretty nice mileage from our test Rav4 diesel (4.2 model), something like 30-35 mpg average, which is GREAT for europe. Gas 2.0l engine gets about 30% less, which is still pretty good.
However, our Prius gets 48mpg. Very similar Avensis diesel gets very good 35mpg. In Winter, differences are less, Prius drops to 38-40, but Avensis diesel also drops to 30mpg. This is average driving, mix of city and highway, same routes for both cars.
So to conclude:
Hybrid Rav4:
+ Best possible MPG
+ Green
+ Fastest
+ Very Quiet
- Price
Diesel Rav4
+ Great MPG
+ Nice torque
+ Good price
- diesels are still loud and not refined.
- not enviroment friendly, DCAT models cost more.
I would pick an diesel over normal 2.0 or 2.4l engine in Rav4, but I would pick hybrid over diesel at the same time.