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First, you can be pretty confident that the OE tires have at least SOME RR consideration. After all, the vehicle manufacturer is require to publish an MPG value - and tires contribute to that. However, some OEM's push the envelope pretty hard.
You can also be pretty confident that any replacement market tire has been given LESS consideration for RR - because those tires carry a treadwear warranty. Even if they don't, then they are designed with grip as a primary consideration - which is also in opposition to RR.
And the term "LRR" is a bit of a misnomer. Tire Rack applies that label based on what the tire manufacturer says - and they generally are referring to the RR compared to comparable tires, meaning similar treadwear and traction ratings. So if you are taking off OE tires, and applying replacement market tires with high treadwear ratings, you can be close to certain that you'll take a FE hit.
Tire weight is not a good indicator of RR - or, IMHO, FE. The only time weight plays into the FE value is on accelleration - and a change in weight of tires and wheels is small compared to the overall weight of the vehicle.
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