eli5: Why does changing tires to a larger size make my truck gas mileage go down?
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It’s heavier. Just getting bigger tires don’t always affect the mpg greatly, but if you get a lift in addition to big or bigger tires than what would’ve fit with or without rub without the lift, it’d most likely be even heavier.
A simple way to look at it would be bigger tyres = more contact with surface = more friction = more power needed to spin tyre = mileage gets affected.
Could be added weight, could be increased power requirements to get the larger wheels turning, could be increased friction from the surface area of the larger tire in contact with the road or an increased amount of friction from a different tire material. Could also be that you drive differently with larger tires and aren’t aware of it.
If by larger tyres you mean a bigger circumference then you will actually be travelling faster and further than your Speedo is showing. Your Speedo will be calibrated based on the tyre size when the truck was sold. A larger diameter wheel and tyre will actually travel more distance for a single revolution and so you are actually travelling faster and further than you think. This could add 5% to the distance so when the Speedo says you have done 100 miles then you have actually done 105 miles. Download a GPS speed app to your phone and check it against your Speedo at different speeds.
Larger tyres = fewer wheel revolutions per mile due to the larger circumference of the wheel. The engine may have to put a little more power into getting the wheel to revolve, but each revolution you go significantly further.
Weight, surface friction and aerodynamics.
Many large tires are also wider. 35s on my Jeep are 12.5” wide. That requires a rim of at least 8.5”. Heavier rim, heavier tire, requires more fuel to spin. I want to say one tire/rim is almost 80-100lb as my tire is 64lb alone.
12-14mpg city and 16 highway. We don’t drive it often
If you don’t recalibrate/reconfigure to compensate for larger tires, it is possible you only think your gas milage is different as the distance measurements from the odometer/car computer will be wrong.
If you mean fuel consumption:
Look at your vehicle when static, you can see the bottom of the tire copies the shape of the tarmac, it’s not a circle, the bottom part is flattened. Wider or bigger diameter tire will increase the amount of rubber that bends to make the contact surface. When you drive the rubber of the tire so continuously bending in contact with the tarmac. When you bend an object, its resistance to bend will generate heat; its called histeresis. More bending more heat more power loss. You can counter it by inflating the tires more but there’s a safety limit to how much you can inflate tires.
Another completely different factor is extra friction from wide tires during turns. The outboard side of each tire and the inboard side of each tire are doing the turn, but on different turning cirlcles, one is closer to the turn center than the other. So the tire has to partially slip to compensate for the different turning radius performed by different parts of the same tire. Given most roads include turns, this factor will also add up to lower your mileage.
Shoulder height plays a role too. Having thin tires on big wheels may increase or decrease mileage according to the shoulder behavior. Thin tires tend to have a stiffer shoulder which will bend relatively more, with more resistance to bending, meaning a lot of heat. On the other side, it improves the vehicle response by a lot. The opposite, soft high shoulder will give more “floaty” feeling to the vehicle. It’s ok for comfort and economy, bad for sport use.
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