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Discussion Starter · #1 ·
So I recently upgraded from the stock 15x6 +52.5 wheels wrapped with 195/60R15 Hankook Optimos (OE tires) to 18x8 +35 Team Dynamics Pro Race 1.2 wrapped with 215/35R18 Nitto NeoGens. I've been experiencing a severe decrease in fuel economy lately and the only explanation I have left is the increased unsprung weight from the new wheel and tire combination.

I currently attend college at Auburn University in Auburn, Alabama and I am originally from Maryland. One way from my apartment in Alabama to my home in Maryland is 798 miles and a round trip of 1,596 miles.

I made the trip home in August during the break between summer classes and fall classes and I averaged about 42 MPG on the stock wheels and tires. I then made the trip again a few weeks ago for Thanksgiving break on the new wheels and tires and could barely muster 31 MPG.

The stock tires were inflated to Ford factory specifications of 32 PSI (I believe that is the spec) and the new tires have been constantly inflated since installation to 46 PSI (per recommendation from the seller, users on this forum, and my engineering/car friends). I doubt the tire pressure has any play in this situation, as all of the users on hypermiling forums recommend maximum sidewall pressure for best fuel economy results.

I had recently been getting pretty terrible fuel economy while at college. Primarily, because it is a college town with a large population and a lot of stop and go driving. The last tank I had before I left Auburn garnered 16 MPG! I have never had a tank that horrible since I've lived here. The lackluster numbers on the ride home confirmed that I have a problem somewhere.

I initially thought it was an issue with my tuner, that I had input the wrong tire revolutions per mile value or that I should have left it alone to begin with. The stock tires have a manufacturer reported revolutions per mile value of 851; but if using a tire size calculator, the value is 833. The new tires make 843 revolutions per mile (I went with the value from a tire size calculator, rather than looking for manufacturer values). The tuner only uses increments of 5, so I had to choose between 840 or 845. The first time I changed it, I used the higher value of 845, then I used the lower value the second time. There was an increase in fuel economy, but not by much and it wasn't the same type of interstate driving as before. The only thing I never did was calculate fuel usage with the stock tire value because I changed it as soon as I got the new tires mounted.

The only explanation I have left is the change in unsprung weight from the stock wheels and tires to the new ones. The stock wheels and tires weigh 33.2 lbs per corner, with a total of 132.8 lbs. The new wheels and tires weigh 40.8 lbs per corner with a total of 163.2 lbs. That is an increase of 7.6 lbs per corner for a total of 30.4 lbs for the whole car. It also just occurred to me that the reduced sidewall may have an effect, but I'm not sure how much.

Is 30.4 lbs of weight really reducing my fuel economy by 26%? I'd kind of expect it if I had a large sedan and put some 26" wheels on it, but not under these circumstances.

Thoughts and opinions are appreciated on this matter. I hand calculate every tank and I literally have every receipt from every tank of gas that I use to report my fuel consumption on a graphing tool on a hypermiling website, so this is really irritating to me right now.

TL;DR Increased unsprung weight by 30.4 lbs, fuel economy decreased 26%
 

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Strichmädchen & Koks
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Could be winter gas reducing mileage, plus a change in driving habits. I would also make sure tires/wheels aren't rubbing on anything, brakes aren't dragging, etc.
 

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Normal tire size to match would be 235/40-18 according to the sticky for stock sizes here, so you're spinning noticeably more revs. (that'll hurt mpg, even with speedo correct)

Have you checked that speedo vs. the GPS mentioned in another thread for accuracy?
 

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Discussion Starter · #5 ·
Normal tire size to match would be 235/40-18 according to the sticky for stock sizes here, so you're spinning noticeably more revs. (that'll hurt mpg, even with speedo correct)

Have you checked that speedo vs. the GPS mentioned in another thread for accuracy?
I'll be honest, I never looked at that sticky when I went to find a new tire size. I went off of the Tire Rack upsize calculator and other sources. But I just used another tire size calculator and it says the size equivalent to stock tires in an 18" wheel is 225/35R18.

The speedometer with the new tires is about 5-6 mph more than what my GPS reports, which is about what it was when I had stock tires. I guess I could figure what revolution value would give me the closest accuracy. I was wondering if there was a direct option though, I was reading a tuning bulletin for some other kind of tuner addressing the issue and was wondering if the SCT X3 had the same feature.

I'll be going home again on Friday, so I will adjust it before I leave and see what fuel economy I get on the way home.
 

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Discussion Starter · #7 ·
Could be winter gas reducing mileage, plus a change in driving habits. I would also make sure tires/wheels aren't rubbing on anything, brakes aren't dragging, etc.
As far as winter weather, the temperature at home was between 25 and 60 while I was home, which isn't too different from what it is here. I doubt it has anything to do with driving habits, I set the cruise control and I also do the majority of my driving overnight to avoid traffic. I did take a different way home last time than I normally take, but the beginning of it is the same and I only got 30 mpg on that part.
 

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Sorry lacole, not used to seeing 18's on anything but MkIII's so it's not off as much as I thought.

I KNEW you had a '10, didn't even think of that when I looked at the tire size & made the stupid assumption.


I've seen 5 mpg difference between tire types/brands in the same size, that still wouldn't be enough to account for the difference.

Large wheels aren't an automatic mpg killer, 15's to 17's on my Focus doesn't have an appreciable affect.

5-6 mph off for a speedo is unusual, a couple over is more typical with some coming real close. Ever get a chance to track miles by freeway mileposts? MPH & recorded miles aren't necessarily coordinated in instruments. When showing the same with different tires it wouldn't be an issue, but wasn't that after your adjustments?

More questions & thoughts than answers, puzzler so far.
 

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Your wheels are not causing the huge mpg decrease. Only thing you should notice about larger diameter tires is braking takes a bit more effort/ distance. Acceleration and mileage should be very close to stock. My 19s don't really change my car's behavior much at all from the 16" stockers, and my 19s have noticably taller tires than my stock rim/ tire combo.
 

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So mote be it
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Id think you'd notice a worse mpg in city driving, all highway shouldnt be too much of a difference. You'll use more gas having to start and stop the car often in city driving but at a constant speed on the hw, it shouldn't be much of an issue. (not the huge difference you saw on your long trips.) The 18's have also moved more weight farther outside of the wheel from center, that will use more energy to speed up and slow the car.

I noticed about a 2-3mpg hit when going from oem 15's to svt 17's wheels. I'm now using other 17" wheels that a little lighter than svt's. I can still get above 30mpg when i do mostly highway but in everyday city driving, i see 26-28mpg, which has decreased with the switch to winter gas here in Ohio. I did a couple long trips earlier this year and at all highway tanks, i was at just under 35mpg.

When was the last time you changed air and fuel filters? Those can both cause a hit to mpg (not saying its the real reason your mpg went down but it could help) I'm thinking it has to be something other than just wheel weight/size for you to take a 10mpg hit on the hw, just doesnt sound right
 

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Discussion Starter · #11 ·
Sailor, I fixed the speedometer. It's now dead on.

I have noticed a difference in braking, I posted a thread a while back about it. Was planning on going with big brakes, but just gonna do an EBC upgrade.

I have a CAI, cleaned the filter not long ago. Mk2 has a lifetime fuel filter (not even sure if it is accessible). I'm sure I would've seen a more gradual decrease in fuel economy rather than such a sharp decline. I just got my oil changed, so I will see if that improves it any when I go home Thursday night.
 

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Speedo on is always nice, mine is 1 high with the stock size 15" Winters & dead even with SVT size 17's. It had been a couple high with the original speedo.

See how it does now for sure, I've noticed a little difference with larger rims on initial braking but mileage never took a noticeable hit. I actually wondered how much was my own impressions on the braking just because I did some harder driving to play after doing the suspension & Wheels/tires.
 

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Discussion Starter · #13 ·
Speedo on is always nice, mine is 1 high with the stock size 15" Winters & dead even with SVT size 17's. It had been a couple high with the original speedo.

See how it does now for sure, I've noticed a little difference with larger rims on initial braking but mileage never took a noticeable hit. I actually wondered how much was my own impressions on the braking just because I did some harder driving to play after doing the suspension & Wheels/tires.
Yeah, If I ever put my stock wheels back on I'll readjust for the speedometer to be accurate with them.

I had the same problem. I drove a lot more 'spirited' with my new suspension, so kind of thought it was me when I noticed braking wasn't as great as it could be. But I may be getting out of the Focus game sometime next year (either S197 or PSD), so I'm not gonna spend the money putting on a BBK.
 

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The stock tires have a manufacturer reported revolutions per mile value of 851; but if using a tire size calculator, the value is 833. The new tires make 843 revolutions per mile (I went with the value from a tire size calculator, rather than looking for manufacturer values).
Your tire calculator is estimating rolling circumference just using the actual circumference of the tire diameter. This is why the rev/mile answers are off.

Is 30.4 lbs of weight really reducing my fuel economy by 26%? I'd kind of expect it if I had a large sedan and put some 26" wheels on it, but not under these circumstances.
Your car would probably see around a 65lb increase in weight during acceleration due to the effect of rotating mass.

The Effects of Rotational Inertia on Automotive Acceleration
 

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'Is 30.4 lbs of weight really reducing my fuel economy by 26%?'

Absolutely not possible. And 'unsprung' weight, or weight that is below the shock or strut, gives back what it takes in use as the car hits bumps. The springs collapse to absorb energy then give it back as they spring back out. More like talking 'rotational' weight increase here...........the difference the added weight changes when spinning at speed, or centrifugal force.

Hoping any of the tire size recommendations factor in the fact that no tire runs a circumference equal to its' physical size, the actual running circumference is lower due to tire sidewall compression under weight. Tread squirm alone will whack like a 1/4" off the radius there.

Don't discount fuel distributors messing with the ethanol percentages to get more profit if they use it in your area, they jack with that here all the time. You can get anywhere from under 10% to over 20% and it changes all the time. It definitely messes with fuel calculations here.

Entirely possible something has just faulted on the engine but not showing any CEL, it happens all the time.
 

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Discussion Starter · #16 ·
So I may have found my problem. Sometime during the summer I switched to Autolite 103 spark plugs (I thought I bought 104s for some reason, but apparently I didn't). The recommended max interval between changes is 15,000 miles. The plugs in my motor had about 8,500 miles on them, so I figured it wouldn't hurt to change them and they are only $7.50 for four. I changed them out with 104s and went for a short drive. The fuel economy gauge is already higher than what it was previously, but I will find out with more driving and I will know for certain when I make the 800 mile trip back to Alabama.
 

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So I may have found my problem. Sometime during the summer I switched to Autolite 103 spark plugs (I thought I bought 104s for some reason, but apparently I didn't). The recommended max interval between changes is 15,000 miles. The plugs in my motor had about 8,500 miles on them, so I figured it wouldn't hurt to change them and they are only $7.50 for four. I changed them out with 104s and went for a short drive. The fuel economy gauge is already higher than what it was previously, but I will find out with more driving and I will know for certain when I make the 800 mile trip back to Alabama.
I hope this is the fix for you, keep us posted. The gap will make a difference too.
 

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Discussion Starter · #18 ·
Well I just got back to my apartment in Alabama. I could only manage about 28 mpg over the entire 800 mile trip. I'm chalking this up to the cold weather right now, but I've still never had it this bad in the winter.
 

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I know my car doesnt fair well with mpg once it gets cold outside. I'm sure its a combination of colder more dense air/winter gas and whenever it snows, i find myself having fun spinning my tires on the empty side streets/turns of my neighborhood [hihi]
 
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