New tires = worse MPG?

[fantastic]

Let's assume for a minute the computer calculations are correct. The fact this is a high MPG Miata 6 speed would push me to say it has more MPG to lose on any changes like tires. The more MPG, the more noticeable it would be. Recommend you to start tracking your fuel economy on Fuelly to really make sure everything is what it seems before thinking it's a loss though.

 

[bullwinkle]

More rubber=more flexing=more friction. Also winter gasoline has less BTUs, so the double whammy on MPG.

 

[kschachn]

Let's assume for a minute the computer calculations are correct. The fact this is a high MPG Miata 6 speed would push me to say it has more MPG to lose on any changes like tires. The more MPG, the more noticeable it would be. Recommend you to start tracking your fuel economy on Fuelly to really make sure everything is what it seems before thinking it's a loss though.

Measuring or noticing something is always the easier part. It’s ascribing it to one or more significant variables (especially in this very chaotic environment) that is the very difficult part, and is almost never done on here or even considered.

 

[D1dad]

My cc2s have progressively gotten better mpg as they’ve worn down. But then when Ohio switches to winter fuel it’s all for nothing. I assume SC makes a switch as well? That along with fresh tread would do it.

 

[Gasbuggy]

More tread equals more diameter which changes your speedometer/odometer slightly

Might be a different load index tire than factory (heavier)

I run the max psi in my Michelins. That would get your mpg back

 

[Astro14]

Let's assume for a minute the computer calculations are correct. The fact this is a high MPG Miata 6 speed would push me to say it has more MPG to lose on any changes like tires. The more MPG, the more noticeable it would be. Recommend you to start tracking your fuel economy on Fuelly to really make sure everything is what it seems before thinking it's a loss though.

I would not make that assumption - he’s using the trip computer, which is notoriously inaccurate in most cars.

So, we start with inaccurate data, and then find a cause for a phenomenon based on that inaccuracy?

Seems like a fool’s errand…which is why the second part of your post - actual tracking, is what I would recommend.

As to some reasons why a tire change results in a change (once the change is verified) - one of the big ones is that OEM tires are specified to be low rolling resistance. Throw a replacement set of tires on there, ones that are designed for traction and performance, not low rolling resistance, and you are likely to lose MPG. Along with tread and other differences, the change in MPG is common.

But verifying the magnitude of change is going to be challenging. Since he didn’t use Fuelly, or calculate by hand, previously, there is no accurate “before” set of data. So, even if he uses Fuelly, or calculates by hand, now, the before and after comparison is flawed because of bad “before” data.

 

[930.engineering]

I recently (~6 mo) replaced the tires on it, with the same Michelin PS4 All Seasons that it had before. ...... ......
Since I installed the new tires (nothing else has changed), it's dropped to 33.5mpg.

Six months ago, so before autumn and winter season thereafter. Temperature is one of the key
factors plainly. Like stated several times, more tread will result in more flexing (incl. better ride
comfort), so higher rolling resistance. The taller diameter is true too, but that's clearly negligible
compared to the aforementioned factors such as pressures, individual use and winter conditions.

On a sidenote, a Miata deserves UHP summer tires such as Michelin PSS or PS4S.
.

 

[kschachn]

But verifying the magnitude of change is going to be challenging. Since he didn’t use Fuelly, or calculate by hand, previously, there is no accurate “before” set of data. So, even if he uses Fuelly, or calculates by hand, now, the before and after comparison is flawed because of bad “before” data.

And in a system with a high number of uncontrolled variables the longer the test is run the more significant the noise. Often people think that if you run a test like this long enough it makes it more accurate, when the facts are that for one specific variable it will make it less accurate.

The mathematician always wins in data analysis.

 

[JamusMcFamus]

On a sidenote, a Miata deserves UHP summer tires such as Michelin PSS or PS4S.

Agreed! I have a dedicated track set. This is my DD set.

 

[Allaned]

I only trust mph calculated this way: actual mileage driven divided by actual gallons added to tank. Don’t overfill by not adding any gas after the pump clicks off. I don’t trust the car computer calculations.

 

[Patman]

I only trust mph calculated this way: actual mileage driven divided by actual gallons added to tank. Don’t overfill by not adding any gas after the pump clicks off. I don’t trust the car computer calculations.

The one good thing about the car calculations is consistency though. Even if the numbers are off by a few percent, the consistency is what is key. Keeping a record of what it’s saying and watching it go up or down will give you an indication of what you might need to change (or keep doing)

My problem with calculating it at the pump is twofold. Number one is the accuracy of how much actual gas you get at the pump, especially in Canada where the volume is corrected to 15C. Second problem is how do you know that you have filled up the tank to the exact same point each time? You would have to go to the same pump each time and always fill it up at the same flow rate so that the pump is stopping at the same point. And that’s no guarantee either, as I notice big differences in the flow rates, so even if you always set the nozzle to the same point (highest or lowest setting) it’s not going to stop at the same level every time. I think the flow rate changes at a station where many people are filling up at once and it might change depending on if the underground tank is near full or near empty.

So for me I like to double check the accuracy of my onboard computer by hand calculation over at least 4 or 5 tanks of gas. That will give me a better idea of how accurate my car is. But once I have established that error rate then I don’t feel the need to check it manually all that often.

FWIW, I have determined that my Honda is only optimistic by about 1%. My Corvette is 5% optimistic.

 

[fantastic]

I'll add some context to help you understand my comment. I saw this post & wanted to rationalize an explanation how a vehicle could lose a good chunk of mileage, like 3-4 mpg, by a seemingly small change like tires. Not necessarily that the dash is showing "inaccurately".

I understand tires can cause slight deviations in fuel economy, but I'm skeptical it's the reason for the 3-4mpg difference as you're reporting. I'm open to being educated on the subject if I'm mistaken here.

Which prompted me to say.

The fact this is a high MPG Miata 6 speed would push me to say it has more MPG to lose on any changes like tires. The more MPG, the more noticeable it would be. Recommend you to start tracking your fuel economy on Fuelly to really make sure everything is what it seems before thinking it's a loss though.

You can disregard my first sentence I edited out simply b/c it was not the core focus of my statement. However, Let's look at my first sentence of that post.

Let's assume for a minute the computer calculations are correct.

It was a "temporary" hypothetical if you will "for a minute" to reasonably assess why a noticeable amount of fuel of 3-4 mpg can be lost. In which I stated that high MPG vehicles have more to lose on a change like tires. That is not an incorrect assumption but a reasonable one.

I would not make that assumption - he’s using the trip computer, which is notoriously inaccurate in most cars.

So, we start with inaccurate data, and then find a cause for a phenomenon based on that inaccuracy?

Seems like a fool’s errand…which is why the second part of your post - actual tracking, is what I would recommend.

Moving on...
I don't think you can say OP's noticeable MPG avg decrease is inaccurate. There is no other data to backup that it's incorrect, as you imply, nor data providing that it's correct but I'll reasonably say the computer should do a relatively decent job of calculating OP's MPG & show a decrease if there is one. It's a 2017 not 1997 so I give the computer more credit overall for probably being close. Along with the fact they changed tires & then noticed the change which is also reasonable. But yes, at the end of the day that's why I followed up with Fuelly comment since it could be a more precise measurement & to double check if indeed the MPG avg is incorrect in which it may be.

 

[Astro14]

I'll add some context to help you understand my comment. I saw this post & wanted to rationalize an explanation how a vehicle could lose a good chunk of mileage, like 3-4 mpg, by a seemingly small change like tires. Not necessarily that the dash is showing "inaccurately".

Which prompted me to say.

You can disregard my first sentence I edited out simply b/c it was not the core focus of my statement. However, Let's look at my first sentence of that post.

It was a "temporary" hypothetical if you will "for a minute" to reasonably assess why a noticeable amount of fuel of 3-4 mpg can be lost. In which I stated that high MPG vehicles have more to lose on a change like tires. That is not an incorrect assumption but a reasonable one.

Moving on...
I don't think you can say OP's noticeable MPG avg decrease is inaccurate. There is no other data to backup that it's incorrect, as you imply, nor data providing that it's correct but I'll reasonably say the computer should do a relatively decent job of calculating OP's MPG & show a decrease if there is one. It's a 2017 not 1997 so I give the computer more credit overall for probably being close. Along with the fact they changed tires & then noticed the change which is also reasonable. But yes, at the end of the day that's why I followed up with Fuelly comment since it could be a more precise measurement & to double check if indeed the MPG avg is incorrect in which it may be.

Well, I think we can say it’s an accurate measurement.

First, he’s using a trip computer, which is intrinsically inaccurate.

Next, he didn’t control for any of the variables, like speed, temperature, hills, route, length of trip, etc.

So, when you have several variables that affect an outcome, and you didn’t accurately measure the outcome, and you failed to control for all, but one of them, then ascribing the outcome to that one known variable is specious at best.

A much more logical question, and frankly, more accurate thread title, would be “why did my mileage change on a Thursday?”

 

[SC Maintenance]

I was going to mention it's winter time and gas mileage suffers in the cold, but then I checked and saw that you live in South Carolina where it never gets cold.

Yet they adjust the gasoline blend here in winter - it does get "cold" at night. I find my mileage drops in winter - as much as I pay attention to it - and I have a palm tree growing in my front yard.

 

[Patman]

Well, I think we can say it’s an accurate measurement.

First, he’s using a trip computer, which is intrinsically inaccurate

As I mentioned above, as long as the trip computer is consistent that’s all that matters, even if it reads a bit optimistic compared to the hand calculation. It will still show you when all of a sudden things change.

I’m a bit of a MPG nerd with my Civic and my trip computer has a separate display that shows the fuel economy for every trip I make. So I’m in the habit of always watching that and it’s very consistent on the routes that I regularly take (for instance I always get roughly the same fuel economy when I make the 50 mile drive to my girlfriends house) So if something drastic happened and there was a problem in the engine causing a drop in fuel economy, I would definitely notice it the first time I drove the car.

 

[fantastic]

Well, I think we can say it’s an accurate measurement.

First, he’s using a trip computer, which is intrinsically inaccurate.

Next, he didn’t control for any of the variables, like speed, temperature, hills, route, length of trip, etc.

So, when you have several variables that affect an outcome, and you didn’t accurately measure the outcome, and you failed to control for all, but one of them, then ascribing the outcome to that one known variable is specious at best.

A much more logical question, and frankly, more accurate thread title, would be “why did my mileage change on a Thursday?”

With that in mind I see why you think it's all wrong. Let's look at my own example.
Even my ancient XC90 does a pretty good job of MPG avg. Here's the last 10 fill up average.

Now let's look at what the XC90 shows.

The XC90 has been reset, in the recent past, so it doesn't track exactly perfect with the all time mileage on Fuelly but it's not far off even still.

 

[TheOilWizard]

As I mentioned above, as long as the trip computer is consistent that’s all that matters, even if it reads a bit optimistic compared to the hand calculation. It will still show you when all of a sudden things change.

I’m a bit of a MPG nerd with my Civic and my trip computer has a separate display that shows the fuel economy for every trip I make. So I’m in the habit of always watching that and it’s very consistent on the routes that I regularly take (for instance I always get roughly the same fuel economy when I make the 50 mile drive to my girlfriends house) So if something drastic happened and there was a problem in the engine causing a drop in fuel economy, I would definitely notice it the first time I drove the car.

Same experience here. Trip computers consistently 2-3 MPG optimistic for me across multiple brands of vehicles. But I know if the computer is telling me 30 MPG it’s realistically in the 27-28 range.

 

[Patman]

Same experience here. Trip computers consistently 2-3 MPG optimistic for me across multiple brands of vehicles. But I know if the computer is telling me 30 MPG it’s realistically in the 27-28 range.

And it’s still useful for letting you know that there could be a problem. My girlfriend texted me yesterday saying that her hybrid RAV4 has been getting worse fuel economy this week, and it was a slightly warmer week and her driving routine hasn’t changed. All winter her RAV is typically around 7-8L per 100km but this week it was 10. It has never been that far off. So when I get to her place today I’ll start seeing what I can figure out, maybe one of her tires is low on air or the brakes are dragging. There isn’t a check engine light on so it’s not a sensor.

 

[TheOilWizard]

And it’s still useful for letting you know that there could be a problem. My girlfriend texted me yesterday saying that her hybrid RAV4 has been getting worse fuel economy this week, and it was a slightly warmer week and her driving routine hasn’t changed. All winter her RAV is typically around 7-8L per 100km but this week it was 10. It has never been that far off. So when I get to her place today I’ll start seeing what I can figure out, maybe one of her tires is low on air or the brakes are dragging. There isn’t a check engine light on so it’s not a sensor.

Sure thing... I think I was agreeing with the idea that once you have a baseline for real-world fuel economy vs computer reported, it seems to be fairly consistent. I can take the reported MPG and have a decent idea of the actual fuel economy. The most accurate I've seen was 1 MPG optimistic, but that is fairly uncommon.

 

[ripcord]

And it’s still useful for letting you know that there could be a problem. My girlfriend texted me yesterday saying that her hybrid RAV4 has been getting worse fuel economy this week, and it was a slightly warmer week and her driving routine hasn’t changed. All winter her RAV is typically around 7-8L per 100km but this week it was 10. It has never been that far off. So when I get to her place today I’ll start seeing what I can figure out, maybe one of her tires is low on air or the brakes are dragging. There isn’t a check engine light on so it’s not a sensor.

With our Toyota hybrid, all it takes to kill the mileage is a bunch of short trips with the heater blasting which makes the computer keep the engine running a lot more.