All this summer and last, my 89 automatic routinely got mileage in the high 19 to low 20's. Now that it is getting cooler,(30ish) at night, my mileage has dropped to the high16s and low 17s. I know more fuel at start up to get up to temp, but it seems like a big drop. My trip to work is exactly the same. BTW summer driving is with the a/c on. No codes show up, and it did the same last winter. Any thoughts?

 

how hard are you driving it? That is horrible gas mileage. I have never checked mine, but I would say how I drive I get upper 20s. I would think it would get better in cold weather, but im not sure on that.

 

that's not horrible, if I drive anywhere above the 3800 my gas mileage drops down to 10-15, otherwise it's 18-20 normally, I don't go highways. Could be increase in amount of power usage? I'm not really sure though a 1 mile drop isn't too bad.

 

driving style is the same overall, summer or winter I may be a little late for work so I drive a little harder. I've tried driving like grandma for a tank and tried driving like I don't care for a tank and have'nt noticed that much of a drop in mileage. My FB was the same way. I wondered if my coolant temp sensor was shot, making me run rich, but I don't know.

 

you are lucky. I still get 9mpg...

have not had a chance to fix my problem since i go to school and can't afford to miss any days and i am not sure how long my car will be down.

 

So far I only get 9mpg though today I'm going to change the plugs.. I hope that helps.

 

I would take a blind stab and say that since the air is cooler, it is more dense, so there is more air going into the engine and extra fuel is added to compensate else the AF would start getting, what the cpu would consider, rather lean.

 

most cars will acheive noticibly worse gas milage in the winter months. this happends for a couple of reasons.

The primary reason is as EJay said. cold air = dense air = need more fuel
in my VR4, in the winter, I'd always had to drop my boost down 2-3PSI or else I would out-boost the fuel system and hit fuel cut. while in the summer I could easily run 15psi, winter was 12-13psi ONLY. (too bad that car's gone now that thing ****** OWNED in the winter)

The secondary reason being that, until your engine, tranny, and rear end oils are fully warmed up, they are more viscous and result in increased drivetrain loss so the engine has to work a little harder until those lubes are up to operating temperature.

 

Our cars don't measure air density.

 

yes but it still reads AF via an O2 sensor, does it not? I didn't specifically say it knows the actual density going into the motor. Just common facts of physics.

 

Does the ECU calculate density directly, probably not. Does air temp effect fuel consumption, absolutely. Colder air = denser air = more fuel needed. There is an air temp sensor in the AFM as well as on the intake manifold...

The simple fact is that cars get crappier mileage in the winter. Cold starts are colder, the car takes longer to warm up (especially if the heater is turned on as soon as the car is started), winter gas is being used (I can tell immediately when we switch to winter gas because the city mileage of the Insight falls to 65MPG or so), etc.

Edit...Anyone who doubts how much fuel cold weather demands has not tuned a standalone. For example to start my car around zero degrees C, my temp map has to add 50% more fuel which tapers down to 0% when the car reaches about 80 degrees. Colder weather is even more extreme.

 

Duh everyone knows dense air means it’s harder for your car to plow through it in an aerodynamic aspect... J/k lol

 

This isn't neccesarily about Rx7s but I've noticed the past couple of tanks my truck has had about a 3mpg drop in mileage. I wonder if they've swithed to the winter gas here because it hasn't been consistently colder. Haven't really noticed a change in the 7 though.

 

OF COURSE they measure density, but they do it by measuring airflow and temp. Lower temp means greater density. So given a certain flow and temp the ecu does a calculation and comes up with the density of the air, then plugs that in to another equation to come up with the fuel requirements.


So colder air, winter gas that uses more ethanol (less energy=greater consumption) and longer warm up times all contribute to worse gas mileage.

BC

 

Yeah, mine has been getting worse too. I actually got below 30 in my DD, which pissed me off. I really wonder if they even use winter gas here. They probably do, but its retarded since I've never seen it below 25 in years.

 

so I'm assuming this wouldn't affect electric motors much? not that you really get any mpg with an electric motor, lol.. it's late.

 

Although I never argued that our cars get worse mileage in the winter, I do see I missed the fact that they measure temperature.

 

And let's not forget that the transmission oil thickens up, as well as the differential oil, the bearing grease, CV joint grease, etc. Cold weather has a major effect.

Don't discount the aerodynamic effects as mentioned above. They are significant as well.

 

**** it, I'm getting 15mpg out of my T2 and I've been boosting the HELL out of it. I'm happy.

 

Drag difference at sea level between 75 degrees F and 0 degrees F...

.29 Cd (aero kit) = 1611.9 watts or 2.2 hp

.31 Cd = 1723.2 watts or 2.3 hp

These are pretty close considering I don't know the exact CG height or center of air resistance.

 

Without going too far off topic, it's way more significant then that.

In my Insight, I monitor mileage very carefully and have learned to recognize the conditions for optimal mileage. To separate the effect of cold air entering the engine I installed a pipe to suck air from the catalytic converter heat shield. This results in toasty warm degree intake temps (above 50C as measured by the engine's sensors). Once the temperature falls close to zero the effect becomes very noticeable. Mileage drops at least 12 MPG on the highway and 10 MPG in the city. There are of course other factors like the weights of oils and such.

To put it another way, on a nice hot summer day it's easy to cruise down the highway at 90+ MPG with the same intake temps...

 

Those were just for drag, not for combustion differences.

 

I wasn't talking about combustion differences. Notice how I isolated any intake air differences by using a hot pipe...

 

I usually get 2-3 mpg worse in Winter....

 

I misunderstood what you did with the intake temperatures. I need to stop skimming posts.