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It's final drive gearing.
For example I replaced the clown tires on the Dodge Dakota wich were 215/70r16 with proper 30x9.5r16, it lowered my highway RPMs by over 200rpm at any given speed. It gets about 1mpg better.
OEMs can run clown car tires because they run numerically lower final drive gear. So never expect to go smaller and get better fuel economy it's a trap.
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1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.
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