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Discussion Starter #1 (Edited)
I am agonizing over whether to switch from my 2014 SV to an SL or Platinum because of the sunroof and rear power vent windows, AND to get the color I truly want, charcoal.

All SV's have the 2.9 rear drive gears and I get 21 MPG when measuring with 100% highway driving at the pump (not the dash computer). All SL's and Platinums automatically have the 3.3 rear drive gears (2010-2015) and I have read many posts on here where everyone seems to get 16-17 on the highway - even worse.

Could this be because of the 2.9 rear drive gearing and smaller wheels?

I instinctively know that the 2.9 gears will get better MPG's if all other things are equal. But they are not, because ALL SL and Platinum wheels are increased to 20" versus the SV's 18". So shouldn't the larger tires pick up the slack? ...since we're getting 20" of distance instead of 18" with each shaft revolution?

The 2.9 gears turn 2.9 revolutions to travel 57" (18" * Pi = circumference) and the 3.3 gears take 3.3 revolutions to travel 63" (20" * Pi = circumference). With some quick arithmetic, we find that:
2.9 gears/18" wheels require 1,112 drive shaft revolutions to travel 1 mile;
3.3 gears/20" wheels require 1,006 drive shaft revolutions to travel 1 mile
...which are almost identical (considering how many revolutions there are)! MPG's should be about the same, and the EPA reports that it is!

One last thing: it's instinctive that increasing the size of the tire would give you better MPG on the highway at stable speeds. But when I researched this online, I found that the larger tire WOULD except for one thing: the gain is canceled out by something called Rolling Resistance. Since the larger tire has a larger footprint, the resistance and friction are increased too much to realize the gain from the larger diameter! The energy needed from an engine to overcome this added rolling resistance basically yields an overall LOSS of MPG's from upgrading just tire size (besides a little added weight)!

So I wonder if SV's do indeed get better MPG's than SL's and Plat's, is it because they have 20" tires instead of the 18" tires? If anyone has an SV, I'd love to know what your 100% highway MPG's are.

Thanks! ...any input is welcome.
 

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First thing that comes to my mind is that the 20" rim and wheel package overall diameter vs the 18" wheel and tire package would be the same. The width of the Rims are the same. You shared distance traveld is slightly different in the Plt and SL. Now the weight of the wheels are a factor here.
I shared in another thread that Enki makes a lightweight wheel that is 18x9 and weighs 23lbs. Others shared that it would improve acceleration and I agree. I did raise the question that wouldn't it save fuel economy too? Short answer was No basically. This has been a hot topic shared on many forums for many years.
Weight loss is said to be the surest way to improve fuel economy. That is for our gas consuming vehicle. I would love to order those wheels and try it out. The are $400 each and that adds up. Not sure how long it would take to recoup my money with fuel savings if there is any. Good news is fuel prices are falling. (y)
 

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Discussion Starter #3 (Edited)
Thanks Tanks!
OK, I just realized my calculation isn't correct, because using 18" and 20" for the wheel diameter doesn't account for the tire height, just the rims! Sorry!

But now a new issue is raised with Tanks comment. Is the OVERALL diameter of the 18" and 20" wheel + tire the same? I assumed the 20" overall diameter of the wheel+tire was 2" larger than the 18" wheel+tire. Forgive my ignorance, but I just got an education on this matter. The 18" and the 20" diameter of the actual tire ARE about the same! ...32.6" vs. 33.0" The difference between the tires is only a .4" difference, or like 3/8th of an inch.

Now, if I recalculate, then:
One AXLE shaft revolution for the 18" wheel+tire will travel 102.4" (32.6" diameter * Pi = 102.4" circumference), and
One AXLE shaft revolution for the 20" wheel+tire will travel 103.7" (33.0" diameter * Pi = 103.7" circumference)
so,
18" wheels require 619 AXLE shaft revolutions to travel 1 mile (63,360" / 102.4");
20" wheels require 611 AXLE shaft revolutions to travel 1 mile (63,360" / 103.7").
...but since it takes 2.9 or 3.3 DRIVE shaft revolutions to make 1 AXLE shaft revolution, then:
18" wheels require 1795 DRIVE shaft revolutions to travel 1 mile (619 * 2.9 = 1795);
20" wheels require 2016 DRIVE shaft revolutions to travel 1 mile (611 * 3.3 = 2016).

Wow, so it takes 2016 shaft revolutions to go a mile for the 20" wheels - that's 12.3% more revolutions than the 1795 it takes for the 18" tires because of the 3.3 gears, mainly. So it'll take 12.3% more energy (or gasoline) to travel every mile! That means if I get 21 MPG on the highway with my SV's 2.9 gears/18" tires, then applying a 12.3% reduction, we get 18.7 MPG with the 3.3 gears/20" tires!

So it DOES look like the SV will get better mileage with the lower final drive gears (with everything else being equal).

Other Notes:
(1) My reference to Rolling Resistance above would also be incorrect if indeed the overall diameter of the 20" and 18" tires are the same (33.0" vs. 32.6"), BUT I just realized that the width of the 20" is 3/8" wider, so the rolling resistance will be worse for the 20".
(2) The 20" tires are 3/8" wider than the 18" but I don't think that would change MPG with wind resistance since the air is hitting the front of the car so violently and 3/8" isn't going to reduce MPG by 3!
(3) Tanks, yes I believe there's a difference in weight with wheels. But even if it's a whopping 20 lbs per wheel (80 lbs. total), would that affect MPG's? If I take a 160 lb. person with me as a passenger, my MPG's don't drop by 3 MPG's; it doesn't drop at all with my experience. So I am not a believer in wheel/tire weight affecting MPG's, unless I am missing something.
 

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So in my limited time to calculate the variables said above, I don't think the little difference in gear (2.9 vs 3.3) would be that significant than the effects of sprung / unsprung weight to mpg. Not to mention, when talking about rev per mile you'd also have to take into account the amount of 'sag' on the tires as it will affect the overall rolling diameter.

Also IIRC, unsprung weight is approx. equal to 3x the equivalent sprung load - e.g. the 50lbs you'd save from, say, going with lighter wheels and tires choice would be almost equal to unloading a 150lb passenger.
 

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Thanks Tanks!
OK, I just realized my calculation isn't correct, because using 18" and 20" for the wheel diameter doesn't account for the tire height, just the rims! Sorry!

But now a new issue is raised with Tanks comment. Is the OVERALL diameter of the 18" and 20" wheel + tire the same? I assumed the 20" overall diameter of the wheel+tire was 2" larger than the 18" wheel+tire. Forgive my ignorance, but I just got an education on this matter. The 18" and the 20" diameter of the actual tire ARE about the same! ...32.6" vs. 33.0" The difference between the tires is only a .4" difference, or like 3/8th of an inch.

Now, if I recalculate, then:
One AXLE shaft revolution for the 18" wheel+tire will travel 102.4" (32.6" diameter * Pi = 102.4" circumference), and
One AXLE shaft revolution for the 20" wheel+tire will travel 103.7" (33.0" diameter * Pi = 103.7" circumference)
so,
18" wheels require 619 AXLE shaft revolutions to travel 1 mile (63,360" / 102.4");
20" wheels require 611 AXLE shaft revolutions to travel 1 mile (63,360" / 103.7").
...but since it takes 2.9 or 3.3 DRIVE shaft revolutions to make 1 AXLE shaft revolution, then:
18" wheels require 1795 DRIVE shaft revolutions to travel 1 mile (619 * 2.9 = 1795);
20" wheels require 2016 DRIVE shaft revolutions to travel 1 mile (611 * 3.3 = 2016).

Wow, so it takes 2016 shaft revolutions to go a mile for the 20" wheels - that's 12.3% more revolutions than the 1795 it takes for the 18" tires because of the 3.3 gears, mainly. So it'll take 12.3% more energy (or gasoline) to travel every mile! That means if I get 21 MPG on the highway with my SV's 2.9 gears/18" tires, then applying a 12.3% reduction, we get 18.7 MPG with the 3.3 gears/20" tires!

So it DOES look like the SV will get better mileage with the lower final drive gears (with everything else being equal).

Other Notes:
(1) My reference to Rolling Resistance above would also be incorrect if indeed the overall diameter of the 20" and 18" tires are the same (33.0" vs. 32.6"), BUT I just realized that the width of the 20" is 3/8" wider, so the rolling resistance will be worse for the 20".
(2) The 20" tires are 3/8" wider than the 18" but I don't think that would change MPG with wind resistance since the air is hitting the front of the car so violently and 3/8" isn't going to reduce MPG by 3!
(3) Tanks, yes I believe there's a difference in weight with wheels. But even if it's a whopping 20 lbs per wheel (80 lbs. total), would that affect MPG's? If I take a 160 lb. person with me as a passenger, my MPG's don't drop by 3 MPG's; it doesn't drop at all with my experience. So I am not a believer in wheel/tire weight affecting MPG's, unless I am missing something.
A slightly faster way to reach a similar conclusion is to get the percentage delta between axle ratios, assuming all else is the same. You already correctly calculated that at just over 12%. So, given a certain speed at, say, 2000 RPM with the 2.9 ratio rear end, the 3.3 ratio would require 2240 RPM for the same speed. I don't think MPG would be 12% worse, but certainly internal friction is higher at the greater RPM.
 

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Discussion Starter #6
ppointer: That's good info! ...thanks! The increased friction never ocured to me since I'm a bit of a novice with this stuff.

I am just wondering why the MPG's reported by the EPA are the same for all models despite the obvious difference? Were they mistaken or lazy? If I were Nissan, I'd be pissed because 21 MPG's on the highway is a great number for such a large vehicle.

My wife's 2004 Buick Rainier (same as a trailblazer) has a 5.3L V8 gets 21 MPG too (100% highway), so it's not far fetched that a 2014 5.6L V8 would also get 21 MPG.
 

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I am unsure if 21 mpg is possible. I am able to at 70 mph in a 2010 titanium 4wd with cruise control eek up to 19 mpg on a tank. Maybe if its a 2wd model and you really babied it on acceleration and set cruise control low thought.
 
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