My father has a 97 corolla with a 3 spd auto tranny. He's getting around 25 mpg in town mileage and roughly 30 on highway which is very poor for a economy car. I have a question: if I change the tire from the stock 14-65-175 to 14-70-175 or even 14-75-175, will he be able to get better mileage ?
Here are some of my thoughts, not sure if they are valid:
- Tire with bigger circumfrunce have a higher final gear ratio between the engine and the tire thread, thus lower rpm when crusing on highway and therefore better gas mileage at least when crusing on freeway
- But because of bigger tire, the transmission MAY shift later at least when driving in town because of the lack of torque and hence leads to poorer mileage.
So in theory whether it works or not depends on whether factor number
1 or 2 dominate. Any real world experience that somebody can share ? And will he ruin the suspension by using bigger tires ?