Tire Size/vs Gas Mileage

[Jimmy Lavoie]

I'm confused as to why larger tires equate to lower gas mileage...



Here's my thinking: If the tire diameter is greater, wouldn't the engine have to rev less to go the same distance thus better mileage? In other words, one complete rotation on a larger diameter tire travels a father distance than a smaller one.



Now - if the lower mileage is a result of greater tire weight - that's different.



My rationale above takes into account the difference in actual speed. If you were traveling 60 MPH on a larger tire - you would actually be going faster than the tranny was tuned for.



I hope this makes sense.



Any thoughts?

 

[Dingo]

Several reasons, including weight, rotational inertia, potential-to-kinetic energy transfer, and contact patch friction.

 

[Woppy V]

larger tires weigh more, grip the road more, yes the diameter is larger but that will throw off the balance between the tranny, differential, and tire rotations..



your motor will be doing more work to spin the wheels thus eating more gas.

 

[Woppy V]

dingo your quick to the punch. u must beat my post by a couple seconds!

 

[Johnny O]

Know also that modern OHC engines do not necessarily get better mileage at lower rpm. They need to spin a little to be at their most efficient level.

 

[EddieS'04]

There is the posibility if the gas milage was mesured at a continous 60mph. Without stop and go or any acceleration. You might see better milage as the rpms are lower. But that is not the real world...

 

[Jimmy Lavoie]

Now I get it. Thanks for your help!

 

[Todd Z]

How about NOT calculating your speedometer correctly for the larger tires....



That will do it too..





Todd Z

 

[Richard L]

Assuming that you are only increasing the tire diameter to the next size or two larger, there is probably only nominal difference in gas mileage.



I would have to say that the larger tire represents a larger rotational mass and more initial roll resistance. That means it would take more power (and fuel) to get the tire turning at speed than a smaller tire. Once it actually got up to cruising speed, it would probably get you a slight improvement in mileage. If yo cannot sustain that speed long enough, you may not recover the gas mileage lost just getting the tire turning.



This would be the case if you do a lot of city driving. That would make you city milage worse with larger tires.



The other factor is that the larger tires throw off the speedometer accuracy by showing less miles than were actually driven...Since larger diameter tires turn less times per mile, the speedometer/odometer will show a slower speed and less miles than the true speed an miles traveld. So even if there were no difference in mileage, it would calculate as lee miles travel on the same amount of fuel...And that would appear to decrease your gas mileage.



Since changing tire sizes on the drive wheels is like changing the final drive gear ratio, it can sometimes return a noticable improvement in mileage if the original gear ratio wias just a bit off and then larger tires braught it back into the sweet spot. This was more common on older cars, and larger tires were also used to get mor miles out of the tires since larger tires turned slower they wore less rapidly than smaller tires.



...Rich