Quote:
Originally Posted by Big Dave
Increasing tire OD almost always backfires and MPG is reduced.
It might work if you were a long-haul trucker who can maintain the same speed hours on end, but most of us have to deal with stop-and-go situations.
Rotational moment of inertia goes up with the square of diameter. That means those four "flywheels" have to absorb more energy to increase their RPM. Until the tire RPM increases road speed doesn't increase.
Another evergreen myth that deserves a sticky.
|
Big Dave makes a good point that many people miss.
Yes(as many hear already realize), if you were to drive at a constant speed, you will travel a longer distance given the same rpms but with a larger diameter tire.
However,
imagine holding your arm out with a bag of apples in your hands. If your rotate your body and swing the bag around you, you will find it takes some strength (torque) to ACCELERATE it to speed for a full rotation.
Now imagine holding a stick that is the length of your arm with the bag of apples on the end. You will find it takes MORE strength (torque) to get the apples to ACCELERATE up to speed around you.
If you increase the diameter of your tires, the engine now has to deal with FOUR wheels that have increased in diameter and the car will require more torque and rpms to accelerate the same amount to get to speed.
Given that this is gas saving forum, it's safe to say that everybody here is already accelerating slowly. Assuming all other variables are the same, to get the SAME mpg, you would have to accelerate EVEN SLOWER. These are just laws of physics.
You will however gain mpg at steady state speeds. Hope this makes sense.
To actually gain mpg with taller tires, one would have to drive a higher ratio time at
steady state than to
acceleration.
Each person would be different and it's complicated to say if one did or did not increase that ratio.
To complicate matters more, tire rolling resistance (likely brought up in other threads here) make a difference too and most people change their tire type when changing wheel size so it makes it even more difficult to gauge which made the difference in mileage. Let's not also bring up how much the weather difference changes it too -->
http://john1701a.com/prius/images/da...ic-Compare.png
I am considering to upgrade the 14" wheels of my '01 Prius to 15" so I can purchase the ultra fuel efficient Michelin Energy Saver A/S. These have been proven to reduce rolling resistance and increase gas mileage -->
Tire Test Results : When Round and Black Becomes Lean and Green Knowing the downside potential of larger wheels, I'm only considering the lightest wheels I can buy. Super lightweight Enkei RPF1 -->
Enkei Racing RPF1 Bright Silver Paint
Other downsides to larger wheels in a hybrid that I'm chewing on:
1. Given the same distance traveled on the highway, I likely will get less battery charge from the lower engine rpm.
2. Given the same distance when decelerating, I will likely charge the batteries less.