It's a 2.4. It doesn't get great gas mileage to begin with. It's rated 21 MPG City from the from the factory. I've been getting close to 24 with nothing on, in nice weather. Also, it has a spring-loaded automatic tensioner.
Significant drop in MPG with AC on. Normal or something wrong?
- Thread starter Jonas
- Start date
I can imagine there being more mpg loss in city driving with AC.
The AC needs 1-2 hp or so, regardless of whether the car is moving or not.
Cruising on a flat highway the engine might be delivering a steady 15-20 hp to the transmission, so the AC is a 5-10% bite.
Low speed city driving will use a lower average hp to move the car, so the AC power is a bigger percentage.
Plus, there's the increased fuel flow to maintain idle speed.
The AC needs 1-2 hp or so, regardless of whether the car is moving or not.
Cruising on a flat highway the engine might be delivering a steady 15-20 hp to the transmission, so the AC is a 5-10% bite.
Low speed city driving will use a lower average hp to move the car, so the AC power is a bigger percentage.
Plus, there's the increased fuel flow to maintain idle speed.
According to epa short trips with ac use may reduce mpg by 25 percent
www.fueleconomy.gov
Fuel Economy in Hot Weather
Find out how hot weather affects fuel economy and what you can do about it!
www.fueleconomy.gov
Makes sense. I can feel it, though. I really have to lean on the gas to maintain the same acceleration/speed sometimes. RPMs are higher on average.
If you drive slow enough that aerodynamics aren't a factor, like in a big city, you could notice AC requiring more fuel. When you drive faster, like ex-urban it's often lost in the noise, except for economical vehicles. I never run without AC so I can't compare and fret about it.
My car will choose a lower gear at cruise to keep the rpms a smidge higher (8 spd trans) while the compressor is engaged. AC easily gives the Lexus a 2 mpg hit, and significantly more if I idle it. my volvos and f150 are reasonably thrifty at idle; the Lexus is not. 3.5L v6.
ps- AC can require 5-7HP iirc. The 1-2 figure above is a little short. The chrysler minivans for instance carry enough tonnage to cool an 1800 sq ft house.
ps- AC can require 5-7HP iirc. The 1-2 figure above is a little short. The chrysler minivans for instance carry enough tonnage to cool an 1800 sq ft house.
your mechanic is a hack
Also keep in mind, city driving is going to consume more energy than highway driving with the AC on for a few reasons I will spell out.
1) Cooling fans, in the city cooling fans will be running all the time. Cooling fans will be putting additional load on the alternator, as alternators get hot (because of cooling fans) they are even less efficient needing more power from engine.
2) Hot under hood temps. As intake air gets hotter, the pcm will pull a couple degrees of timing advance out of engine to prevent detonation, in extreme temps the pcm may even richen the mixture, all of which decreases efficiency
3) High condenser (high side) pressures. When its hot with inadequate airflow through the condenser your high side pressures will be higher, thus requiring more power to spin the compressor.
This all requires more power of the engine and you as a driver will press the accelerator further just to accelerate at the same rate you are use to.
The mileage you are seeing for a 4 cylinder vehicle in city driving seems reasonable. It takes a lot of power when its hot to get cool air, can't short circuit physics.
1) Cooling fans, in the city cooling fans will be running all the time. Cooling fans will be putting additional load on the alternator, as alternators get hot (because of cooling fans) they are even less efficient needing more power from engine.
2) Hot under hood temps. As intake air gets hotter, the pcm will pull a couple degrees of timing advance out of engine to prevent detonation, in extreme temps the pcm may even richen the mixture, all of which decreases efficiency
3) High condenser (high side) pressures. When its hot with inadequate airflow through the condenser your high side pressures will be higher, thus requiring more power to spin the compressor.
This all requires more power of the engine and you as a driver will press the accelerator further just to accelerate at the same rate you are use to.
The mileage you are seeing for a 4 cylinder vehicle in city driving seems reasonable. It takes a lot of power when its hot to get cool air, can't short circuit physics.
