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Agree with Mike Mercury above. As for the burning more gas, I can only relate my personal experience.
I took a two hour trip a few weeks before installing my Vararam VR-1B. Got 29 to 30 mpg. After installing the 1B, I took the same two hour trip and got 31.5 mpg. I even sent an email to Vararam about the mpg increase.
The two trips were on the same roads (Interstate highway) with near identical road, tempature, humidity, and traffic conditions.
Was only able to check this once because I installed a 3.42 rear diff and TC shortly thereafter.