I believe the NX300h will mirror the MPG of the RAV4, just as my Lexus ES300h mirrors the MPG of the Toyota Avalon Hybrid.

 

That's cause that model's curb weight differs by less than 50 lbs.

If you look at Fuelly the current NX averages around 29-30 mpg real world while the Rav4 Hybrid gets 38-43 mpg real world.

The smaller Toyotas and Lexuses that are cousins of eachother usually differ wildly in weight as small Toyotas have an emphasis on Fuel efficiency (and cost) so are usually light and cheap while larger Toyotas tend to be pricier and have more standard luxury/weight (sound proofing) so the weight delta is negligible there.

So unless the new Rav4 is a luxurious soundproofed expensive monster, I doubt the weight will be the same as its new NX cousin and therefore not have the same MPG.

 

2010 Prius Curb weight: 3042 lbs
2011 CT200h Curb weight: 3131 lbs

I totally whar you mean that Toyota is going to be lighter and nosier then the Lexus counterpart but its amazing with the little difference curb weight and huge MPG difference.

 

Ct200h uses heavier and larger 17" wheels with wide tires and has worse aero (higher coefficient of drag). and the little extra weight doesn't help either.

the prius with the optional 17" wheels and tires does suffer mpg wise. the stock are 15" with 195 tires. extremely light. takes a lot more energy to spin up the heavy 17" wheels. Also the 17" wheels have spokes that create more drag. while the prius has smoothed out wheel covers. CT200h is no match to the Prius aero wise

i used to have a 3g prius. i got 50mpg. when i put 17" FRS wheels and tires (same size as ct200h) the mpg went down to around 40-42

 

It's not so much the weight difference there are other factors in play. 17" low profile wheels and wide tires cause a hit compared to the Prius skinny 15" rims. I believe the two programmed differently the CT feels peppier than the Prius in all modes by comparison. The Prius feels rather sluggish, I drove my Prius in normal mode and the CT is always in ECO Mode.

As for the RAV4 vs. NX300h...

These are the stats for the 2018 Toyota RAV4 Hybrid and the 2019 Lexus NX300h

The RAV4 has a Curb Weight of 3950 and the MPG is rated at 34/30/32
The NX300h .... Curb Weight of 4180 and the MPG is rated at 33/30/31

Here is the source for the info I provided:
https://www.toyota.com/rav4/2018/fea...4444/4456/4454
https://www.lexus.com/models/NX/spec...00h-awd#engine

So I have no reason to believe that the new 2020 refreshed or redesigned NX200h won't be within 1 MPG of the 2019 RAV4