Your car does not shift into 4th gear at 20 MPH. The shift points occur (approximately) as follows: second at 15, third at 25, fourth at 35, followed by torque converter lockup at about 45 MPH, which feels like a shift but it's actually the torque converter locking up.Originally Posted by thehunterooo
So at 20 MPH you are actually still in second and are locking in second gear. You might have more acceleration, but it won't help your fuel economy.