That's really not that much extra rubber and the Toyo MTs aren't super aggressive so you shouldn't have dropped off by 1/2. Are you being heavy-footed? What psi are you running your tires at? Is the weather especially hot and you're using the a/c alot?
I've run 37" Iroks and MTRs. The Iroks, being a super aggressive tread and a heavy tire, offer lots of extra rolling resistance and unsprung weight. They knocked off 2-2.5mpg from highway driving. The MTRs are a relatively aggressive tread and weigh several pounds less. With the MTRs I usually see maybe 1-1.5 mpg difference from the stock BFG ATs.
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