Kia Soul / Bad MPG / Tire Change?


#1

In 2010, I purchased an almost brand new Kia Soul ! that had one previous owner. That person had replaced the factory rims / tires with 18" Alloy rims with 40" low-profile tires. At the time, I averaged 26 MPG which, while not amazing, wasn’t bad enough to concern me. Almost every person I spoke with said I could probably chalk it up to the low-profile tires. This year, when I needed to replace my front tires, I opted to replace all four so that I could go back to the 45" factory size. The tire salesman assured me that the ride would be much better and that the MPG would not be affected, and would actually improve (albeit, marginally) since it was the intended tire size.

However, since replacing my tires, I have actually experienced lower MPG than I had before. At the moment, I am averaging 24 MPG. Now, I know that may not seem like a lot, but it ends up being a loss of a full gallon of gas or more each time I fill up. I have been religious about keeping the tires at the recommended setting (33 PSI). I check the pressure each time I fill up the tank and log it, along with my mileage. I change my breaks, oil, plus and filters like clockwork and I drive like a grandma! I rarely drive over 65 / 70 MPG and when I accelerate, I make sure that I do it slowly and smoothly so that the “Eco” light stays on, but nothing has improved. I have an app that I have used to calculate the mileage per gallon for the last six months (before the tire change and after), and I’ve even gone back to manually check their numbers to make sure there wasn’t a fluke in the programming that was rendering incorrect totals. Nothing!

I’m at a loss and have no idea where to start solving this riddle, but it’s annoying me to no end. I feel like I’ve been duped into spending more money than I intended to when I replaced my tires. Has anyone had this experience or know of any issues that may cause this drop in MPG? I’m a total car novice and don’t really have anyone in my life who can help me with these things, so please be kind.


#2

The size for your Soul is 225/45-18 - is that what you got? What size were the 40 series (not 40") tires you had before? If they were different in diameter the speedometer odometer might have been thrown off, and you might not actually be getting any different mpgs. 18" wheels (the largest that go on your Kia) will give the worst mpgs because they’re heavier than smaller wheel/tire combinations. But it’s not worth the big money to change both wheel and tires, unless you’re damaging the tires or rims on bumps.


#3

texases meant the odometer, not the speedometer. Let us know the full tire sizes so that we can do the math on that.

In addition, tires can vary quite a bit in rolling resistance. Did you get a low-rolling-resistance tire? You seem to be the type of driver that these were designed for.

What part of the country do you live in? If your weather is getting cold now, that will reduce your fuel economy a bit.


#4

“tires can vary quite a bit in rolling resistance. Did you get a low-rolling-resistance tire?”

Bingo!
Rather than focusing exclusively on the size of the tire, the OP should also consider the rolling resistance rating of the old tires vs. the new ones.

More than likely, the tire salesman unloaded some high rolling-resistance tires on the unsuspecting OP. That is one of the reasons why it is so important to do a lot of due diligence online (Tirerack.com, 1010tires.com), and via Consumer Reports prior to purchase.

If, as I suspect, the new tires have high rolling resistance, there is really no solution to lower mpg numbers other than replacing the tires with LRR (low rolling resistance) tires.


#5

In addition to the great advice already stated…make sure you check the air pressure in your tires. It’s a rare tire shop that ever gets the proper air pressure right after changing tires. Low air pressure will kill fuel economy. It goes without saying…but I’ll say it anyway: Air up the tires to the specs on the vehicle placard not the tire sidewalls.


#6

duped? how so? you needed new tires. you bought new tires. i installed tires that were 10% smaller and my mileage increased by 20%. was that wind resistance? lower rolling resistance?


#7
i installed tires that were 10% smaller and my mileage increased by 20%. was that wind resistance? lower rolling resistance?

More like your odometer thinking you went farther than you really did.


#8

yes, my odometer said i went 10% farther. trip to work was 10 miles. now it says i drive 11 miles to work. but my gas use dropped by 20%. but thats only based on odometer and amount of fuel added to tank. gas pump says i bought XX gallons. maybe it is wrong.


#9

@Stoveguyy maybe you’re unconsciously driving a little slower in reaction to the higher speedometer reading.


#10

The larger diameter of the 45 series will actually give you better gas mileage, but the smaller diameter 40 serie will trick your odometer into measuring more miles than you are going which increases the apparent gas mileage.


#11

Actually, Texases was right before he lined out the first part of the term… both the odo and the speedo would be affected.

Anyway, in addition to knowing the rolling resistance rating it would help to know the full size of what he installed (if he changed the rim sizes too, that counts). The tire size will be on the sidewall and look like 225/45-18 or something similar. Just the aspect ratio (the correct term) and the rim size are not enough.

Knowing the traction rating would help too. Winter tires have greater rolling resistance than OEM tires. I’ve added a link to tell you how to “read” tire sidewalls. The more information we have is the better we’ll be able to advise you.


#12

Every time I get a new set of tires, my MPG goes down a bit. I’ve gotten to where I just expect it.

Just recently I got a new set for my Honda Fit. On the old worn tires, it was routinely getting around 37-38 MPG. Now with these new tires, I’ve been getting around 33-34 MPG. Oh well.


#13

Don’t tires continue to vulcanize as they age and are subjected to heat cycles? I’ve always heard that tires harden as they age, handling worse but getting better MPGs.


#14

Correction: your INDICATED mileage goes down.
A new tire will typically have about 9/32" if tread.
A worn out tire (2/32" left) will have lost 7/32" in radius, or 14/32" (1.37445") diameter.

There are 63,360" in a mile.

There fore,
a worn tire with a 20" OD (outside diameter) tire will roll 3.168 times in a mile.
a new tire with a 21.37445" diameter will roll 2,964.29 times in mile.

That’s a reduction of 203.71 rotations, or 6.43% between the worn out tire and the new tire.

In short, if your odometer accurately reads 200 miles with the new tires,
that’ll be the equivalent of having traveled 212.86 miles with the old tires.
Ergo, your mileage will appear to have been reduced.

That lost rubber matters…

PostScript: I’m open to corrections in the arithmetic. I’m old.