i have 285 75r 16's on my truck. i dont think that was stock. i am looking for MPG's and having trouble understanding what numbers make the difference. the 285 is width right? and 75 is how tall. i know width eats up mpgs but more importantly radius or diameter actually effecting mpg calculations and sort of changing your gearing. taller is better for highway and overall??and is the 75 number the percentage of height in relationship to width? in other words if i switch to 17" is a 285 75 17 the same height? -hope thats not too incoherent.

 

yeah, 285 is width and the sidewall height is 75% of the width. So I'd say a 17" wheel with the same tire size would give you a 1" taller tire. For mileage, I'd say go to a 235 85r 16, gives you a tall, narrow tire.

 

Old Dodge is correct in suggesting a narrower tire for better MPG. Any car or truck is driving around with what is essentially 4 postcard-sized contact patches! As far as fuel economy goes, you need to figure on the 2 effects that rotating tires have: rolling resistance against the road and air turbulence around the wheel wells. First, let's talk about rolling resistance. A wider wheel/tire combination results in a wider contact patch, better lateral handling (think of tires on a race car) but sadly, more drag against the road surface and more fuel burnt. A larger diameter wheel/tire combination makes for a longer contact patch against the pavement...this is good for high performance braking (again, think of a racing car)...this won't be a noticable betterment in our heavy trucks...it may actually make braking distances longer! Think of the law of leverages and then apply it to the fulcrum length from the center of the hub to the tire tread surface; it would take more braking power to do the same stopping distance. Nothing is for free, is it? LOL. On the other hand, the larger diameter tire results in fewer revolutions per mile, therefore lower RPM going down the road...and this is good to a point. I had a 97 4x4 5spd w/ 3.54s and I put 19.5s on it thinking that I could cruise down the road at 70 pulling only 1750rpm and get fantastic mileage! WRONG!!! Even with skinny tires, my economy went down the toilet as the motor was running 'behind the cam' and to get back in the 'sweet spot' I'd have to run almost 80..and then I'm pushing so much air that my mileage was even worse! Just keep this in mind for later...
Air turbulence is caused by the the tires spinning in the wheel wells. Wider tires and agressive tread patterns make more.
BC

 

I agree with bcfast but I was alway told that the second number was a ratio of the width to the height, like a 285/75/16 is 285 mm wide and the 75 means that the distance from the rim to the outside of the tire is the ratio of the distance from the outside of the tire to the wheel divided by the width, so that means that the profile is about 213 mm.

 

bcfast hit two of the big factors in mileage related to tires. One other large factor is rotational inertia. The larger in diameter a tire is, the more rotational inertia it has, so it takes more power to get it spinning. If you are only running highway, this doesn't matter since you are not accelerating and the rpm drop is good. However, if you are driving around town, a smaller diameter tire will get you better mileage.

 

RU is correct...the first number is the treadblock width in mm, and the 'aspect ratio' number is a percentile of that width...from the bead to the top of the tread (when new). Dartmouth makes a great point as well talking about that 'mass' of the tire and, at the risk of being all physics and stuff, breaking inertia and getting the mass rolling...yes, the more mass the more power it takes to get it going. The Ricksons I mentioned above would give you a freakin' hernia putting them on! The only other things about tires vs mileage I can offer are: 1)air pressure...run them closer to the max, even when you're empty. 2) ..and this isn't so important in our heavy trucks as it would be in your car...the treadwear number. This number is embossed on the sidewall and represents the 'hardness' of the rubber. Simply put, the higher the number ie. a #600 will be a longer-lasting tire (and worse-cornering) tire than say a #220...or on a high-performance sportscar tire a sub-200 rating...or even a sub-100 'R' compound tire will be great at the racetrack and stick like glue for a few hours and then the cords start showing! Good job analyzing this, guys!
BC