I have a 2006 Ex Coup Manual transmission. I have stock 16" rims, stock tires. With that setup I average about 350 miles per tank. I also use in the summer, 17" SI rims, with Kuhmo Ecsta SPT tires, and i've noticed a significant drop in range, and I get anywhere between 270 and 310 miles to the tank.

Also, driving in the freeway, often with the 16" rims I can put it into neutral and coast a bit on the flat spots to soak up as many mpg's as I can without losing much speed. When I have the 17" on, Its a noticeable deceleration when in neutral on the flats, and sometimes even noticed on downgrades.

Any idea whats up? I know that the larger tire / rim will reduce mpg, but isn't that a bit dramatic? My driving style is consistent, and just because I have the larger rims doesn't mean I drive all speed racer'ish.

 

Tire pressure.

 

I just had them changed this last weekend. The tire pressure should be fine. Are low profile tires generally set to a lower tire pressure?

 

The increased diameter pushes the mass away from the center of the wheel, therefore requiring more HP to spin said mass. I'm assuming since you went +1 with your rim size that your tires also went up in sizing. Your contact patch should now be 10 mm wider than your OEM tires. More contact with the road leads to more friction.

 

Wow!!! Learned something new!!!

 

no way and not even the slightest chance can such a minimal change in size make such a great difference,we are talking about a 15-20% here for the OP guys,there is definatly more than just that modest difference in friction to the equation here!

I went from eg si rims/tires at 185/65/14 to 215/40/17 and had not more then 5% max 10% a difference just to give you an idea!

perhaps the wheel bearings are worn or the brakes are touching? did you check that out yet?

 

Are you just going to ignore the whole concept of weight an rotational mass?

 

Wider tires will have noticeable rolling resistance in a Civic imho. (very light car)

I have 205/55R-16 grippy summer tires (Firestone Wide Oval)
and 195/65R-15 for winter, Michelin X-Ice 2's
The 195/65R-15's are actually 0.2" taller. (about the same height while supporting the weight of the car)

Michelin's X-Ice 2 was designed for low rolling resistance.
Firestone WideOvals are designed just for grip, wet and dry traction, soft compound; long-life and low rolling resistance weren't priorities in these tires.

I see a subtle drop in economy switching from MY winter to summer tires.
On the highway, I gain about 25-40 miles distance-to-empty with a full tank on winter tires, or about 5-10% fuel economy.

The difference in resistance is extreme turning the wheel below 5 mph.
I feel at least double the resistance by steering effort, my summer tires vs. winter tires.

 

Rotational mass (on top of friction) also probably affects my car, especially while accelerating.

My 195/65R-15's wheel combo (on old CR-V alloy rims) weigh 6 lbs less
than my 205/55R-16's mounted on factory 16" alloy rims.
The 15" alloy rims themselves weigh 4 lbs less than the factory 16" alloy rims.

*I wanted narrower tires for winter because it's better to bite into shallow, slick snow.

 

I probably wouldnt have graduated years ago(mech eng) If I were to ignore the effect of extra weight on rotational mass,I obviously agree with you on that.

However the OP has gone from an OEM 16 to an OEM 17,meaning the difference in weight at max would be 5-10lb per wheel including the tire thus the effect would be minimal if any and far from making a difference of 20% loss in mileage if you ask me!

there are a 101 articles on this and unless written by some 20 year old HT member they almost all state a difference of 2-5% on modest wheel size differences and 10% on gigantic ones. again 15-20% difference in mileage wont come from such a slight difference.

what I want to ask the OP is to give us the rest of the diameters on these new tires ,and also did you have these wheels balanced?

 

The tires are 215/45 zr 17. I believe that they were balanced. I took them off last December, and had them put back on a few weeks ago. Never taken off the rims.

One other thing... I had my rear brake pads replaced, and I have started to hear a strange pulsating noise every once in a while... is it possible that the calipers are not aligned correctly? They were replaced when I still had the stock 16", and I didn't notice a change in mileage, but its an annoying woo-woo-woo-woo-woo noise thats audible now and again.

 

wider tire means more friction as well, and yes, the rotational mass is higher. add it all up = less MPG.

 

ya but wider tires look so swwet and give you sick acceleration

 

Actually, no.

Wider tires, when ran with the same pressure, have the same sized contact patches, just in a different shape. The narrower tire ends up with a longer but narrower contact patch. The wider tire ends up with a shorter but wider contact patch. Same total area though, as long as the same pressure is used.

Since the narrower tire must deform the sidewall more, the rolling friction is reduced for a wider tire, made of the same compound, and ran at the same pressure.

However, a wider tire does present more aerodynamic drag than a narrower one. This is actually the primary cause of reduced mileage from wider tires.

What makes the biggest difference in rolling resistance, is the compound of the tire. This is why the Prius (and other high economy cars) come with special "low rolling resistance" tires. Higher performance tires have more grip, which increases rolling resistance, which reduces mileage.

The OP is using Kuhmo Ecsta SPTs as summer tires, which are "ultra high performance summer" tires. Mileage should go down with grippy tires such as those.

I lost a good 15% of my city mileage when I mounted a set of Falken RT-615 Azenis on my car, in comparison to the crappy Michelin all seasons it had stock. And, just as the OP reported, the car doesn't coast anywhere near as far in neutral with the performance tires. All just part of the price of performance.