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Discussion Starter · #1 ·
Hello. My turbo owner's manual states that the fuel tank capacity is 12.7 US gallons (3.785 L/gal.). For the first time I let the tank run down to near-empty: the monitor showed 5 miles fuel remaining. I filled it to the auto pump shutoff point at Costco. It took only 10.6 gallons.

Does anyone know where the other 2+ gallons capacity is/went? Thanks.
 

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Hello. My turbo owner's manual states that the fuel tank capacity is 12.7 US gallons (3.785 L/gal.). For the first time I let the tank run down to near-empty: the monitor showed 5 miles fuel remaining. I filled it to the auto pump shutoff point at Costco. It took only 10.6 gallons.

Does anyone know where the other 2+ gallons capacity is/went? Thanks.
In fact, even when the monitor shows 0 mile, you can drive for few more miles...
 

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Just a guess, but I'm thinking they purposely skew the display to show you're "empty" when you really still have 2 gallons left, to save people from truly going empty and getting stuck somewhere. Also I used to hear how running very low on fuel was bad for your fuel pump. I don't know if that's still a concern anymore with newer cars, but if true it would be another reason to trick people into re-fueling sooner.
 

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Discussion Starter · #4 ·
Just a guess, but I'm thinking they purposely skew the display to show you're "empty" when you really still have 2 gallons left, to save people from truly going empty and getting stuck somewhere. Also I used to hear how running very low on fuel was bad for your fuel pump. I don't know if that's still a concern anymore with newer cars, but if true it would be another reason to trick people into re-fueling sooner.
Thanks. I don't doubt that there are a few miles left in the tank when it reads empty. But seems excessive at 2.1/12.7 = 16% for a tank so small. That's nearly 50 miles. Plus if true, I don't need Mazda being secretive about the amount; i.e., not trusting me, the vehicle owner. Now if Mazda do state the amount hidden somewhere & I haven't found it, then this is on me.
 

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2020 CX-30 Skyactiv-X MHEV GT Sport Tech
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Just a guess, but I'm thinking they purposely skew the display to show you're "empty" when you really still have 2 gallons left, to save people from truly going empty and getting stuck somewhere. Also I used to hear how running very low on fuel was bad for your fuel pump. I don't know if that's still a concern anymore with newer cars, but if true it would be another reason to trick people into re-fueling sooner.
Thanks. I don't doubt that there are a few miles left in the tank when it reads empty. But seems excessive at 2.1/12.7 = 16% for a tank so small. That's nearly 50 miles. Plus if true, I don't need Mazda being secretive about the amount; i.e., not trusting me, the vehicle owner. Now if Mazda do state the amount hidden somewhere & I haven't found it, then this is on me.
As a car ages, there can be a sediment build-up which settles at the bottom of the tank If this sediment gets into the pipes then it can cause problems with the fuel delivery to the engine, which is why most cars will read empty while there is still some fuel in the car. Also, that 50 mile low-fuel warning must be triggered with all tolerances of fuel gauge senders so that's another factor that the manufacturers design in to give some margin to the low-warning light.

You'd not want the tolerance working against you in these conditions ... :unsure:
 

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Discussion Starter · #6 ·
As a car ages, there can be a sediment build-up which settles at the bottom of the tank If this sediment gets into the pipes then it can cause problems with the fuel delivery to the engine, which is why most cars will read empty while there is still some fuel in the car. Also, that 50 mile low-fuel warning must be triggered with all tolerances of fuel gauge senders so that's another factor that the manufacturers design in to give some margin to the low-warning light.

You'd not want the tolerance working against you in these conditions ... :unsure:
Thanks. But those aren't reasons to tell me I have 12.7 gallons capacity when it's effectively 10.6 gallons. Tell me it's 10.6 gallons. Why is that hard? I mean when the gauge reads empty, I'd expect I could put at least 12 gallons in the thing. I see nothing unreasonable about that assumption.


Mazda fuel gauge tolerances are 16% these days? That's sad. Should I now assume I have another 25-40 miles driving range when the gauge reads zero miles?

And that sediment is only going to lower the working capacity further from 10.6, not from12.7 gallons.
 

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There are several reasons for the “reserve” fuel reading. If you are driving up or down hill the gauge will read differently than it will on flat ground. Gasoline expands when it is hot, contracts when cold, so there is a buffer built in. It’s a warning so you still have some miles to reach a gas station. An empty tank holds 12.7 gallons, what you do with it is your concern.

BTW, Mazda is hardly the only maker who does this, almost all do the same thing.
 

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Discussion Starter · #9 · (Edited)
There are several reasons for the “reserve” fuel reading. If you are driving up or down hill the gauge will read differently than it will on flat ground. Gasoline expands when it is hot, contracts when cold, so there is a buffer built in. It’s a warning so you still have some miles to reach a gas station. An empty tank holds 12.7 gallons, what you do with it is your concern.

BTW, Mazda is hardly the only maker who does this, almost all do the same thing.
Yes, it is my concern. That's why I started the thread. Duh. What do you suggest someone do with the other 2.1 gallons?

If a 12.7-gallon tank, new car, is only 10.6 gallons functional, the extra 2+ gallons is a meaningless number. If Mazda put a 100 gallons tank in the car & I can put only 10.6 gallons in it, why tell me it's 100 gallons capacity? Capacity is what I can pump into it, not how large the tank is. LOL on the change in volume of gas with temperature. The pump reads volume, regardless of gas density.

For the other autos I've had, I've been able to fill within less than 1 gallon of stated capacity. Net, 12.7 is a bogus number as far as I'm concerned. And yes, that's what I'm doing with it - calling it bogus.

Net still, where is the other 2+ gallons?
 

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Yes, it is my concern. That's why I started the thread. Duh. What do you suggest someone do with the other 2.1 gallons?

If a 12.7-gallon tank, new car, is only 10.6 gallons functional, the extra 2+ gallons is a meaningless number. If Mazda put a 100 gallons tank in the car & I can put only 10.6 gallons in it, why tell me it's 100 gallons capacity? Capacity is what I can pump into it, not how large the tank is. LOL on the change in volume of gas with temperature. The pump reads volume, regardless of gas density.

For the other autos I've had, I've been able to fill within less than 1 gallon of stated capacity. Net, 12.7 is a bogus number as far as I'm concerned. And yes, that's what I'm doing with it - calling it bogus.

Net still, where is the other 2+ gallons?
You're not wrong.

I've never seen anything directly from a vehicle manufacturer as why they fudge the numbers. I have seen test where a return type fuel system will heat the fuel as it pumps it, the lower the fuel level, the more trips the fuel makes in circulation, resulting in higher fuel temps. Which can shorten the life of the fuel pump.
 

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this is nothing new and this has been the case on every new vehicle I've owned regardless of the manufacturer. You just adjust to it and fuel as required - it's not a big deal. Also keep in mind that all manufacturers say to not fill the tank further after the auto cut-off, which could compensate for some of the capacity if it was filled to the neck.

As an example my previous vehicle had a tank size of 54L, I tracked all my first 49 fills and the most I ever got into it was 47.7L

My FWD CX-30 has a 51L tank capacity.
 

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Hello. My turbo owner's manual states that the fuel tank capacity is 12.7 US gallons (3.785 L/gal.). For the first time I let the tank run down to near-empty: the monitor showed 5 miles fuel remaining. I filled it to the auto pump shutoff point at Costco. It took only 10.6 gallons.

Does anyone know where the other 2+ gallons capacity is/went? Thanks.
Maybe it's like computer hard drive space... they list a drive at 500GB but you only really get 465GB
Half joking but it might be something like that.... maybe there's a section that is always has a bit of gas even when it is empty so that added to what you can actually fill (the 10.6 plus the 5 miles and a little more you had left) equals 12.7
 

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Maybe it's like computer hard drive space... they list a drive at 500GB but you only really get 465GB
Half joking but it might be something like that....
Not to get too off-topic, but that's because hard drive capacities use (sigh) powers of 10, while actual storage numbers use powers of 2. For example, 1 GB is supposed to be 1024 MB (2^10), but hard drive manufacturers represent it as 1000 MB (10^3). It's dumb.

Regarding the CX-30 gas tank, what's the lowest anybody has gone here when filling up? I assume Mazda would not have a reason to lie, but rather just programmed the meter to be very stingy.
 

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If you want to see how far you can go, even past the x miles remain, load a 3-5 gal. proper fuel can in the back and drive till you can't. I wouldn't make it a habit of running on empty though.
 

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I could write out a number of different factors as to why the capacity of the tank isn't exactly what you get to put in on a fill up. Part of it has to do with protecting the fuel pump from starvation, another part of it has to do with the evaporative emissions system, (preventing fuel or fuel vapors that could escape into the atmosphere). The stated tank volume is what you get mathematically when calculating the interior space of the tank. Some of the tank's volume has to be space for fuel vapors and air when the tank is "full". You don't want to force fuel into the tank until it is clear full, that would leave no room for thermal expansion, plus it wouldn't leave room to prevent liquid fuel from getting past the vapor management valve at the top of the tank which could in turn damage the carbon cannister. You don't want to run the tank empty, or too low for the pump module to pick up fuel. When the fuel level is very low, heat from other sources can cause the lighter ends of the fuel to vaporize in the tank. Those vapors have to be dealt with by the engine control system when they are being created, but it also means that the composition of the fuel (specifically the RVP, reed vapor pressure) can now become incorrect for proper operation. There is way more to this but it's best to just accept that it is what it is and by design it will perform longer with less potential for trouble.
 

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My online research found an estimated 30-50 mile reserve is typical for Mazda. Personally I have gone 25 miles past E due to poor trip planning.

Respectfully, regarding the sediment issue... I have doubts believing that the fuel is contaminated and Mazda doesn't want you to use it. My reasoning is that the fuel pickup is logically / likely located at the lowest point in the tank, manufactures often use a filter sock around the pickup, and lastly, as much as fuel sloshes around I am sure it stays quite agitated and contaminants are kept in suspension. We are talking fine particles.

It is just a reserve to keep the system from sucking air as you drive around corners and up and down hills, and for when people do stupid things, like poor trip planning. Manufacturers do it to protect the vehicle and give drivers a safety net. If the OP's tank really were 5 miles from being bone-dry, there would have been about 2 cups of fuel left moving around the tank. I suppose then we would be having a discussion about how cars are sputtering and burning up fuel pumps when the gauge clearly says they can go 5 more miles, they couldn't, and Mazda is lying to us. Sounds like a wonderful solution.

It is a 12.7-gallon tank, it does serve a function, and the extra 2+ gallons are not meaningless. Mazda cannot lie about the capacity just to placate owners. I know, "But they are lying about it being empty!!!". I guess......choose to get pissed about it if it's your thing. Hope you come up with a solution.
 

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Discussion Starter · #19 ·
My online research found an estimated 30-50 mile reserve is typical for Mazda. Personally I have gone 25 miles past E due to poor trip planning.

Respectfully, regarding the sediment issue... I have doubts believing that the fuel is contaminated and Mazda doesn't want you to use it. My reasoning is that the fuel pickup is logically / likely located at the lowest point in the tank, manufactures often use a filter sock around the pickup, and lastly, as much as fuel sloshes around I am sure it stays quite agitated and contaminants are kept in suspension. We are talking fine particles.

It is just a reserve to keep the system from sucking air as you drive around corners and up and down hills, and for when people do stupid things, like poor trip planning. Manufacturers do it to protect the vehicle and give drivers a safety net. If the OP's tank really were 5 miles from being bone-dry, there would have been about 2 cups of fuel left moving around the tank. I suppose then we would be having a discussion about how cars are sputtering and burning up fuel pumps when the gauge clearly says they can go 5 more miles, they couldn't, and Mazda is lying to us. Sounds like a wonderful solution.

It is a 12.7-gallon tank, it does serve a function, and the extra 2+ gallons are not meaningless. Mazda cannot lie about the capacity just to placate owners. I know, "But they are lying about it being empty!!!". I guess......choose to get pissed about it if it's your thing. Hope you come up with a solution.
Thanks.

A perfectly good solution for me would have been for Mazda to state in the manual that there is 2 gallons/25-50 miles driving fuel left in the tank when the fuel gauge tells the driver it's empty & that it's risky to continue driving using that hidden volume. It's helpful to know the true driving range for emergencies.

The CX-30's 250 miles is the lowest driving range I've ever experienced for a vehicle. Knowing it was that low may have impacted my buying choice. My previous sedan easily got over 400 miles on the highway. DW's SUV is in 350 miles range. The 12.7 volume told me a 300 miles range.
 

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I usually fill up at about a 1/4 of tank. I have let it slip past a tiny bit and I believe that I put in about 10.5 gallons in those cases.
 
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