Hmmm, I beg to differ. The higher the ambient temperature, the more efficient engines become due to superior fuel atomization. Yes, A/C does take it's toll on fuel economy, but I've never had a car which didn't get better fuel economy in the summer, even with the A/C blasting, than it did in the winter.

 

I record my fuel & mileage everytime I fill up. ( Yep, I'm a nerd) I can confirm 100% my fuel economy goes down from long term average of 7.3 ltrs/100kms to about 7.8 ltrs/100kms during summer.

Also I have A/C running almost year round.

An average Max temp during winter' day for me is 20 degrees centigrade. Summer average is 29 degrees. ( Mid 30's common. Today was 31)

 

Still very good. Is your car 2.5 gasoline and what year model?

 

The thing is, physics and rigorous scientific testing says otherwise.

I've tracked my miles for both of the cars my wife and I drive for the last 25 years; so far we've logged just a hair under 600,000 miles (a bit over 950,000 km), hey, I'm a nerd too (and an engineer). Over those years the ambient temperatures encountered have ranged from a low of -30°C to a high of 39°C, and when plotted, the fuel economy increases with OAT in nearly a linear fashion (there is a bump around the 40°F/3°C threshold).

Regarding your driving environment; one factor you may have missed is the A/C in your cars engages the compressor far more often when the OAT is at 29°C versus 20°C; said another way, just because you had the A/C engaged, does not in any way mean the toll on fuel economy was the same.

 

I may have missed it, but if we are talking about range per tank without verifying mileage, the prior fill up could have stopped early with a splash back shutting off the pump. Is the problem continuing with subsequent fill ups?

It was my understanding that the tire pressure monitoring system in Mazdas indicated a change in the relative pressure in the tires. One tire looses pressure the light goes on. All tires loose pressure at the same rate, there is no warning.

When mine goes on, I find one tire 2 lbs or so below the others.

 

Update on mileage range on fill up's. Last fillup range showed the "normal" range of 400 miles. Must have ben a fluke from fillup that showed range of 300 miles. Like MazdaCPA said, maybe pump shut off early.

Took car in yesterday for 45,000 service, everything okay. They recommend brake flush next service in June.