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Old 03-18-2010, 07:26 AM   #1
si_monkey
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Default Are my tires causing bad gas mileage?

This may seem obvious to some or most of you, but I'm just wondering if my tires could be the cause of the 23 mpg I'm getting right now? A while back I totaled my EP3 with 205/55-15's so I just popped them onto my new car (95 EG). I just swapped in a SOHC ZC, but thought I would be getting better than 23 mpg. I'm thinking about buying new 205/50-15's. How much of a difference would it make?
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Old 03-18-2010, 08:50 AM   #2
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Default Re: Are my tires causing bad gas mileage?

None at all.

Unless you are running super low tire pressures, your tires shouldn't be causing you trouble.

I run 195/55/15's in the summer and consistently get over 35 mpg with a z6 in an eg.

I'd look elsewhere for your fuel consumption.

Are you trowing any codes?
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Old 03-18-2010, 09:09 AM   #3
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Default Re: Are my tires causing bad gas mileage?

Thanks! I'm not throwing any codes that I know of. I'll have to check it out. Just replaced a leaking fuel pressure regulator that was severely reducing my mpg. I was hoping that would be the end of it, but you know how it goes.
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Old 03-18-2010, 10:50 AM   #4
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Default Re: Are my tires causing bad gas mileage?

Um, stevieteg is wrong, tire size-especially the overall diameter-can have a big affect in your MPG. 205/55/15 is way too big for your eg. The correct size for your EG is 195/50/15, so I'd get a set in that size and you should see a vast improvement in your gas mileage.
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Old 03-18-2010, 11:09 AM   #5
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Default Re: Are my tires causing bad gas mileage?

That's what I was thinking. Thanks for the input. I've done the measurements and 205/50-15 is the widest I can go without messing up the speedo, so I'll definitely be going with that size. They're going on the same 15x7" rims, so 195s would look a little stretched in my opinion. I'm sure I won't get the best mpg with that combo, but hopefully it will be better than my current.
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Old 03-18-2010, 12:45 PM   #6
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Default Re: Are my tires causing bad gas mileage?

Quote:
Originally Posted by p nut View Post
Um, stevieteg is wrong, tire size-especially the overall diameter-can have a big affect in your MPG. 205/55/15 is way too big for your eg. The correct size for your EG is 195/50/15, so I'd get a set in that size and you should see a vast improvement in your gas mileage.
You're crazy if you think 10mm is going to make a substantial difference in mpg. That's only 1 cm difference in overall diameter.

Getting 23 mpg in an eg is really ******* it, unless it's an auto. There is clearly something else wrong with the car.

I have run many different tire sizes on my eg, all with very minimal effect on my mpg.

Let's see here:

185/60/14 (stock)
205/50/15
205/55/15
195/55/15
185/65/14

All consistently getting over 35 mpg.


How is 195/50/15 the correct size for eg anyway? No US eg came with that oem tire size. Even the oem tire size for a gsr or em1 is 195/55/15.

The only true 'correct' tire size for a US eg is the oem 185/60/14 (Si) or 175/70/13 (Cx, Dx, Vx)


OP, you can drop $300 on new tires but you're going to be very disappointed when your mpg stays the same.
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Old 03-18-2010, 02:48 PM   #7
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Default Re: Are my tires causing bad gas mileage?

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Originally Posted by stevieteg View Post
How is 195/50/15 the correct size for eg anyway? No US eg came with that oem tire size.
It is about the same diameter as the original tire sizes.

Width isn't that important, but keeping the total tire diameter the same is.

My GSR came with 195/55-15s. I currently have 205/40-16s on it. When I calculate my mileage, I need to subtract 4% from my recorded mileage, as the tires are 4% smaller in diameter than stock, and my odometer will read 4% more miles than I actually traveled. Basically, when my odometer shows 100 miles, I've only actually gone 96.

If you have larger diameter tires than stock, you need to add the difference to the odometer reading to get the actual miles traveled, before calculating mileage.
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Old 03-18-2010, 03:00 PM   #8
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Default Re: Are my tires causing bad gas mileage?

Quote:
Originally Posted by stevieteg View Post
You're crazy if you think 10mm is going to make a substantial difference in mpg. That's only 1 cm difference in overall diameter.

Getting 23 mpg in an eg is really ******* it, unless it's an auto. There is clearly something else wrong with the car.

I have run many different tire sizes on my eg, all with very minimal effect on my mpg.

Let's see here:

185/60/14 (stock)
205/50/15
205/55/15
195/55/15
185/65/14

All consistently getting over 35 mpg.


How is 195/50/15 the correct size for eg anyway? No US eg came with that oem tire size. Even the oem tire size for a gsr or em1 is 195/55/15.

The only true 'correct' tire size for a US eg is the oem 185/60/14 (Si) or 175/70/13 (Cx, Dx, Vx)


OP, you can drop $300 on new tires but you're going to be very disappointed when your mpg stays the same.
First, you need to learn how to read tire sizes. You obviously do NOT know what the numbers mean. It is not just a 1cm difference in diameter. Here is a good article.

http://www.tirerack.com/tires/tirete....jsp?techid=46

Here is a breakdown of the overall diameter on the various tire sizes:

185/60/14 = 22.74"
195/50/15 = 22.68"
195/55/15 = 23.44"
205/55/15 = 23.88"

205/55/15 is over an INCH bigger. Do you realize how big that is? Even on my truck, when I went from 31" tires to 32", my mileage dropped by 2MPG. And that's with a V6, where weight and tire size doesn't have as big of an effect as with tiny 4-cylinder engines with no torque.

And as far as your EM1/GSR comment---EG's tire size is SMALLER than EK/DC/EM1's (about half an inch). This is a widely known fact. Again, refer to the chart above. This is so sad that all this misinfo is coming from an 03 member.

OP, you should see a good bit of improvement if you switch to the correct tire size (185/60/14 or 195/50/15). It might not get you up to 35-40MPG, but should be a drastic improvement coming from 205/55/15's. You should also notice a little more pep, because turning a smaller wheel takes less energy than a bigger one. I know of some ITR owners that have gone down to 195/50/15's to essentially "gear down".
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Old 03-18-2010, 03:46 PM   #9
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Default Re: Are my tires causing bad gas mileage?

Quote:
Originally Posted by p nut View Post
First, you need to learn how to read tire sizes. You obviously do NOT know what the numbers mean. It is not just a 1cm difference in diameter. Here is a good article.

http://www.tirerack.com/tires/tirete....jsp?techid=46

Here is a breakdown of the overall diameter on the various tire sizes:

185/60/14 = 22.74"
195/50/15 = 22.68"
195/55/15 = 23.44"
205/55/15 = 23.88"

205/55/15 is over an INCH bigger. Do you realize how big that is? Even on my truck, when I went from 31" tires to 32", my mileage dropped by 2MPG. And that's with a V6, where weight and tire size doesn't have as big of an effect as with tiny 4-cylinder engines with no torque.

And as far as your EM1/GSR comment---EG's tire size is SMALLER than EK/DC/EM1's (about half an inch). This is a widely known fact. Again, refer to the chart above. This is so sad that all this misinfo is coming from an 03 member.

OP, you should see a good bit of improvement if you switch to the correct tire size (185/60/14 or 195/50/15). It might not get you up to 35-40MPG, but should be a drastic improvement coming from 205/55/15's. You should also notice a little more pep, because turning a smaller wheel takes less energy than a bigger one. I know of some ITR owners that have gone down to 195/50/15's to essentially "gear down".

I see, you are correct. The side wall # is a ratio to the width, not a measurement in mm. So my #'s are not correct.

I would be shocked if the op saw a significant mpg increase through. As stated before, I have run these sizes and still gotten over 35 mpg.
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Old 03-18-2010, 04:15 PM   #10
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Default Re: Are my tires causing bad gas mileage?

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Originally Posted by stevieteg View Post
I see, you are correct. The side wall # is a ratio to the width, not a measurement in mm. So my #'s are not correct.

I would be shocked if the op saw a significant mpg increase through. As stated before, I have run these sizes and still gotten over 35 mpg.
Well, it depends on what kind of conditions he's driving in--city, stop/go, lots of idle, etc.

Still, should see a drastic increase with the correct size.
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Old 03-18-2010, 04:51 PM   #11
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Default Re: Are my tires causing bad gas mileage?

I would not expect a huge difference in mileage due to tire size, other than due to the odometer error introduced by a tire with a different diameter from stock.

The outer diameter of 205/55-15 is 5.4 percent larger than that of 175/70-13. So if your odometer says that you drove 100 miles and it is perfectly accurate with stock-sized tires, then you actually drove 105.4 miles with 205/55-15 tires, assuming that the tire dimensions are the same as the size on the sidewall. If you calculate your mileage as 23 mpg, adjusting for the tire size means that it was actually 24.25 mpg. So your mpg is lower by a mile and a quarter due to using the oversized tires. I would not call a difference of 1.25 mpg "drastic", but whatever word you want to use, that's how much it is.

If your tires aren't rubbing and you want to use them up, go ahead, but when they need replacing, get 195/50-15 tires for your car. In the meantime, in addition to affecting your mpg calculations, it's also going to throw off the accuracy of your speedometer, so don't get a speeding ticket because you forgot that your speed is 5.4 percent higher than indicated due to the oversized tires.
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Old 03-19-2010, 10:33 AM   #12
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Default Re: Are my tires causing bad gas mileage?

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I would not expect a huge difference in mileage due to tire size, other than due to the odometer error introduced by a tire with a different diameter from stock.

The outer diameter of 205/55-15 is 5.4 percent larger than that of 175/70-13. So if your odometer says that you drove 100 miles and it is perfectly accurate with stock-sized tires, then you actually drove 105.4 miles with 205/55-15 tires, assuming that the tire dimensions are the same as the size on the sidewall. If you calculate your mileage as 23 mpg, adjusting for the tire size means that it was actually 24.25 mpg. So your mpg is lower by a mile and a quarter due to using the oversized tires. I would not call a difference of 1.25 mpg "drastic", but whatever word you want to use, that's how much it is.

If your tires aren't rubbing and you want to use them up, go ahead, but when they need replacing, get 195/50-15 tires for your car. In the meantime, in addition to affecting your mpg calculations, it's also going to throw off the accuracy of your speedometer, so don't get a speeding ticket because you forgot that your speed is 5.4 percent higher than indicated due to the oversized tires.
It's not just the odometer being off, it's the size and weight of the tires that also affect the MPG. On my Tacoma, going from 31 to 32's (3% difference) resulted in 2MPG drop. People that go from 31 to 33's (6% difference) notice even a bigger drop (~6MPG). It just takes more energy to turn heavier and larger wheels. The difference between 185/60/14 and 205/55/15 is 5%. That's a big difference and a lot more weight that the engine has to turn.

I'm almost positive the OP will gain back a lot more than the 1.25MPG if he goes down to the stock size.
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Old 03-19-2010, 10:41 AM   #13
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Default Re: Are my tires causing bad gas mileage?

I'm definitely going to look into other potential causes for the bad mpg. Any suggestions on where to start looking? Thanks for everyone's input though. New tires (205/50-15) were ordered yesterday since the current ones rub on minor dips and I'm getting new suspension soon that will lower it a bit and make the rubbing worse. The current tires have a lot of tread left on them (90%ish), so pm if you need 205/55-15s. They are General Exclaim UHP's and I love them so much that the new ones are the same.
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Old 03-19-2010, 10:44 AM   #14
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Default Re: Are my tires causing bad gas mileage?

You do realize that's still larger than stock, right? Should have gone with 195/50/15's.
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Old 03-19-2010, 11:14 AM   #15
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Default Re: Are my tires causing bad gas mileage?

Yeah, I realize this now. My mechanic buddy runs this size on his EG with no problem. Hopefully I won't have any problems either since it's too late to change the order. Do you think I'll need to roll my fenders even if it's not tucked?
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Old 03-19-2010, 11:26 AM   #16
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Default Re: Are my tires causing bad gas mileage?

I don't think you'll have problems with clearance. Just will affect the odometer and MPG slightly. But for future reference, you'll be quicker with 195/50/15's and usually, they're cheaper.
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Old 03-19-2010, 11:49 AM   #17
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Default Re: Are my tires causing bad gas mileage?

Thanks. I'll keep that in mind for the next set. I like the idea of being quicker.
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Old 03-19-2010, 12:19 PM   #18
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Default Re: Are my tires causing bad gas mileage?

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People that go from 31 to 33's (6% difference) notice even a bigger drop (~6MPG). It just takes more energy to turn heavier and larger wheels. The difference between 185/60/14 and 205/55/15 is 5%. That's a big difference and a lot more weight that the engine has to turn.

I'm almost positive the OP will gain back a lot more than the 1.25MPG if he goes down to the stock size.
That's just not true. Yes, it might mean another mpg or so. But 6 mpg because of the wheel/tire weight? That's beyond the realm of possibility. However, there are a lot of other things that can cause a variation of 6 mpg from one tank to another (as noted below).

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I'm definitely going to look into other potential causes for the bad mpg. Any suggestions on where to start looking?
There are a lot of things that can affect mileage. The first thing to realize is that not every tank is going to be the same. On one tank you might have a lot of stop-and-go situations; on another tank you might have almost all highway miles. Weather can affect the mileage - not only because of the operating temperature of the engine, but also open windows can decrease mileage from closed windows, and air conditioning on with closed windows can decrease mileage from A/C off with closed windows. Heck, maybe on one tank you were in a hurry and did a lot of jackrabbit starts. And you don't always fill up the tank to exactly the same level; no one does. So maybe on one tank you filled it all the way up, and on another tank you could have squeezed in another gallon. Your mileage will be worse when it's substantially heavier, such as if you have 3-4 adults in it rather than just the driver. As you can see, there are lots of reasons why your mileage can vary. Keep track of your mileage tank by tank and you'll see that with absolutely no changes to your car, it's still going to vary by 3-5 mpg or more, no matter what you do.

However, if your average mileage over a period of time and a number of tanks is going down, there are some specific possibilities having to do with your car. A dirty air filter is one of the most common causes. So are valves that need adjusting, or timing that's off. So if your car isn't up to date on its scheduled maintenance, that's a good place to start.

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New tires (205/50-15) were ordered yesterday
Any chance you can still return them for 195/50-15?
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Old 03-19-2010, 01:14 PM   #19
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Default Re: Are my tires causing bad gas mileage?

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That's just not true. Yes, it might mean another mpg or so. But 6 mpg because of the wheel/tire weight? That's beyond the realm of possibility. However, there are a lot of other things that can cause a variation of 6 mpg from one tank to another (as noted below).
You can argue that point all you want, but the fact is, larger tires (not factoring in the incorrect ODO reading) affects MPG. I’ve experienced this myself, friends that I wheel with around here notice the same, and look on ttora.com---people that go to 33’s experience a severe loss of power and MPG.

BTW, I keep a meticulous chart of my mileage, so yes, I do keep an "average" MPG from many fill ups. I also commute the same route every day and my commute is about 100 miles per day. This should at least negate most of the variables. Luckily, I hardly hit traffic and I am at 65-70MPH all the way.

Some of the quotes (FWIW):

"I was getting 18-20 hwy with my 33x10.50/15 BFG MTs....but...keeping the speed down to under 65 was the key...go much faster and forget it. Also, keep the PSI at optimum for the vehicle weight.
2.7 5sp that got 26 hwy when stock "

"33x10.5/15 3" OME lift, 2.7, 5spd, 4.10's, i average about 16mpg with a city/highway mix" (Stock, 2.7L 5sp is about 20-22MPG mixed).
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Old 03-19-2010, 03:03 PM   #20
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Default Re: Are my tires causing bad gas mileage?

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You can argue that point all you want, but the fact is, larger tires (not factoring in the incorrect ODO reading) affects MPG. I’ve experienced this myself, friends that I wheel with around here notice the same, and look on ttora.com---people that go to 33’s experience a severe loss of power and MPG.
Severe? I just don't believe it. I'm not saying that people aren't experiencing a big drop from one tank to the next, but it's undoubtedly for other reasons.

A few pounds in unsprung weight won't have that big an effect. They say that each pound difference in unsprung weight is equivalent to 1.5-2.0 pounds in sprung weight. Let's say each wheel/tire is 5 pounds heavier. That means that the four of them represent a difference equivalent to adding 30-40 pounds of weight in the car. Your mileage isn't going to drop 6 mpg by putting a 40-pound back of dog food or fertilizer in your car. Just won't happen.
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Old 03-19-2010, 03:18 PM   #21
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Default Re: Are my tires causing bad gas mileage?

I wonder if the larger decrease in mileage from larger diameter tires is a result of the torque reduction.

Larger tire is a change in total gearing, which reduces acceleration, which would cause most drivers to step on the gas harder (to keep the same acceleration they are used to), which would result in significantly reduced mileage. Basically, driving habits changed with the change in tire size.

Just a thought anyway.
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Old 03-19-2010, 04:10 PM   #22
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I wonder if the larger decrease in mileage from larger diameter tires is a result of the torque reduction.

Larger tire is a change in total gearing, which reduces acceleration, which would cause most drivers to step on the gas harder (to keep the same acceleration they are used to), which would result in significantly reduced mileage. Basically, driving habits changed with the change in tire size.

Just a thought anyway.
You're on the right track. Any time you place the weight further away from the center, the harder it is to turn. This is why you see even the new hybrids using smaller rims than their gas-only counterparts. (The Civic, for example). I love riding bicycles and when you go from race tires (700x23) to say more casual riding tires (700x26), the weight is about the same, but with a bigger overall diameter, there is more rolling resistence. Talk to any cyclist if pedaling on 23 vs 26+ is harder or easier. Same with our engines--they have to work harder to give the same output. This is why you see the decrease in MPG (also it's harder on drivetrain parts.) Your points about gearing changes, driving habits also add to this.
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