My mother had a 1989 Mercury Grand Marquis with the large V8 that only had a 15 gallon tank. It would get about 27 mpg on the highway, but, around town, you filled it frequently, much more than the CX5. My MX5 has a 12 gallon tank, but it does get better mileage than the CX5. My smallest tank was in a 1969 MG Midget at 7 gallons, but it would get 50 mpg on the highway and about 28 in town. The only real problem I had with that was during the gas crisis when Fort Jackson initiated a policy that you had to buy 10 gallons at a time to keep people from topping off at the on post service station.

 

What does that have to do with anything? If all the other vehicles in the segment have about the same size (even a bit smaller) it probably means this is the size tank which comfortably fits in the chassis...

The funny thing is there are several members who report getting 29 MPG in their turbos (usually it's the 22/23 models for some reason) which would get to your magical 400/tank figure...

 

On the subject of seats, the passenger seat of my wife's '19 Touring model doesn't recognize her 110 body when she sits on the needed thin cushion support. In other words the "airbag off" light stays on leaving her unprotected. I often wondered if there is a dealer adjustment for this??

 

Matters not if she is setting on thin cushion support or not.
At 110 pounds your wife's body weight will not activate the passenger airbag(s) as they could cause her physical harm.
There is no "specific weight" threshold for the airbags to activate but is "approximately" 165 pounds.

Correction: my error the 165lb I read was what Mazda thinks is the average weight of an adult (maybe in Japan but not the USA). As pointed out by hsdejong the Mazda manual says 88lb

 

Can't believe the stir my comment has caused. It's funny really "my magical 400 tank figure" What, lol? I drive a fair bit (15k-19k a year.) A mix of city hwy. All I can say is this, I put gas in this Mazda much more often than any car I have owned in recent memory. On the trip to my sister's in NC (730 miles) I have to stop one or two additional times vs the last five other cars I've owned or leased recently. And again, I'm fine with the MPG. I average 24.3 with 26-28 on hwy. The OP asked about complaints. My complaint is range. The This will be the last comment on my part.

 

Funny but that doesn't seem to matter on My CX-30!
Maybe she needs to put some dumbbells on her lap as big phone books are harder to find!

 

You could get the sensor checked to be certain it is functioning correctly.
NHTSA documentation does not specify a weight.
All they say is to turn off the passenger air bag in the "proper scenarios".
Online "some" websites say the "average" is 65lbs but in my "research" it varies by manufacturer.
Dumbbells flying around the inside the car probably wouldn't be the best thing...neither would weights

 

I found:
Mazda CX-5 Service & Repair Manual: Seat Weight Sensor Calibration
mentioning 40kg or 88lb.

But I don't know the procedure to set the sensor correctly. Just that you need two 20kg (44lb) weights to perform the calibration.

 

Yeah, my error the 165lb I read was what Mazda thinks is the average weight of an adult.
Correcting that I've seen 65lbs for average now 88lb for the Mazda and Tesla is 100lb, so it will vary.

 

The other day my girlfriend's 25-30 pound pug set off the seatbelt alert on the front passenger seat lmao. I guess it's very sporadic...