Quote:
Originally Posted by oil pan 4
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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I read this question as asking about wheel diameter with a fixed outer tire circumference. For example, in the 5th gen Prius, you can get them with either 17" or 19" wheels, but both come fitted with tires that result in virtually the same rolling circumference.