gas mileage: diminishing returns?

[thaxman]

I've had my MZ5 5MT for a few months now, and while city mpg seems bearable, my hwy trips are getting downright awful. For example:
258 mi over an elevation gain of 1500 ft, mostly flat, few small foothills, 2 adults, 3 small kids, small weekend bags (maybe 550 lbs max?) A/C on full; and I had to put in 12.26 gal, yielding 21 mpg. My last trip from Yuma, AZ, was similar, so I thought it was bad calcs on my part, so I was extra careful, fuelling up the morning after I arrived and again the night before I left. But While my first trip there turned abt 26 mpg, this weekend saw only 23.5 on the way there.
I know not to expect much from city driving (I drive like an auto journalist) but with the hwy trips, no stops, using cruise, I expected far better. This trip down we kept it even at 84 mph, but after the 23.5 mpg, I slowed down to 78-79 for the return trip. I don't like the fact that at 85 I am cruising @ 4000 rpms, but from the posts on the mpg thread, others are getting great results over 80 mph with 5MTs. What gives?

 

[Rocket]

Perhaps you bought gas mixed with ethanol? My mileage dropped 10% when ethanol was introduced here.

 

[BLACKOUT]

well i have a MZ5 GT and the best i've seen is about 20mpg hwy and hwy is all that i do.and this is just me and sometimes the family and no a/c and the speed limit is 55 but i might go 65 from time to time. all i have on mine is a CAI and 18'' wheels i know bigger the wheels, it will give you a little bit of bad gas mileage(dunno) I have noticed the rpm's will drop every so often then pick back up like nothing happened, almost like you dropped a gear or something like that.

 

[joeym509]

Wow, I have never gotten below 24 and that tank I was driving my 09 5 GT hard, having fun and seeing what I could get out of it so I would have a better idea for passing, cornering, etc... My last 3 tanks were 30, 31, and 29 mpg, at about 65%/35% hwy/city. I mostly drive mine it the manumatic mode and shift around 2k-2.5k just to improve my mileage. I have found that if I leave it in auto mode it doesn't shift until about 3.5k. I don't get so zoom zoomy when I drive like this but it sure is nice to pass the gas station more than I used to be able to.

 

[5zoom]

I have never gotten below 22mpg, and that was full AC and a car load of people and stuff. I have an 08 5AT. The AC definitely kills mileage, especially when there are hills involved, but it sounds like your main issue is something that you have no control over. The 4k RPM's at speed will kill mileage, in fact I've found anything over 3200 or so makes the MPG go down in a big way. Since you have the 5 speed manual, I know its hard to avoid driving over 3k because of the short gearing. This is why the manual cars seem to do better in the city, but worse on the highway. I've gotten 33mpg out of mine on the highway and average 26-30ish most tanks.

 

[thaxman]

My warranty is up in less than a year. I am weighing the possibilities of drivetrain work, like changing out the final drive ratio if I can get a factory setup that gets me closer to 3.8:1. Besides the 3, is our 5MT used in anything else? I really was considering the Speed3 setup, not so much for the power (OK, that WOULD be nice) but more for the added torque that would allow lower gearing. I miss the supercharged T-Bird I had, & the cruising at 85 with 2500 rpms on the clock.

 

[straw10]

That is interesting. My lowest has been around 26 mpg, with a couple of tanks at 31-32. I drive consistently 70-75, sometimes a little more. I drive about 80 % of my miles on the highway. I'm sitting just above 28 mpg for the first 21k I've had the car.

 

[rscottg]

I have a scanguage in my current car and I'm going to move it to the 5 as I'm curious about gas mileage. And also I want to see the differences depending on my highway speed.

I should be doing this on my next out of town trip and will report what the gauge shows.

 

[opus]

My mileage has dropped recently as well. We go to Hilton Head Island in SC every year, it is about an 850 mile trip with everything from dead flat plains to the Smokey Mountains going between 70 and 85 with AC on almost all the way. The past 2 trips I averaged about 28-29 mpg, sometimes as low as 27 and as high as 32. This year I averaged about 26.5mpg with a high of 28 and a low of 25. I upsized the tires from 205 to 215 but otherwise made no other changes. I expcted a little drop in milage but not between 8 and 10 percent. I'm wondering if the ECM needs to reset. I have done this on my wife's Jeep Compass by disconnecting the battery cable, and her mileage improved about 10 percent. Has anybody else tried this on the 5?

 

[MD5]

I just came back from a 2,800+ road trip to Chesapeake VA and back, and I averaged about 27 mpg.

That's with the cruise set at 80 and the A/C on.

I started with 77k+ on the ODO and ended with over 80k miles.

Before I went I changed the spark plugs with Autolite AP104 plugs, oil with Mobil 1 5w20 and a Motorcraft filter, Mazda air filter and Mazda cabin filters ( know, no affect on performance with these).

I am absolutely satisfied with my car. I couldn't ask it to do any more.

 

[GoFast]

I've had my MZ5 5MT for a few months now, and while city mpg seems bearable, my hwy trips are getting downright awful. For example:
258 mi over an elevation gain of 1500 ft, mostly flat, few small foothills, 2 adults, 3 small kids, small weekend bags (maybe 550 lbs max?) A/C on full; and I had to put in 12.26 gal, yielding 21 mpg. My last trip from Yuma, AZ, was similar, so I thought it was bad calcs on my part, so I was extra careful, fuelling up the morning after I arrived and again the night before I left. But While my first trip there turned abt 26 mpg, this weekend saw only 23.5 on the way there.
I know not to expect much from city driving (I drive like an auto journalist) but with the hwy trips, no stops, using cruise, I expected far better. This trip down we kept it even at 84 mph, but after the 23.5 mpg, I slowed down to 78-79 for the return trip. I don't like the fact that at 85 I am cruising @ 4000 rpms, but from the posts on the mpg thread, others are getting great results over 80 mph with 5MTs. What gives?

I cant believe you are upset about this. It's almost like this is a joke.....

I mean really?? Slow down a hair and I am sure the MPGs go up.

 

[thaxman]

GoFast, when all your trips are the equivalent of driving from Chicago to Omaha and back, let me know of you aren't interested in getting there in less time.
Instead of cornrows though, all I have to look at is bare ground and a few hardy (read: leafbare) scrub brushes. Few cars on I-8, too. And I typically am being passed at these speeds. No one wants to be in Hell's front yard.

BTW, last trip btw PHX & TUC yielded 27 mpg. Just 2 adults, same speeds. So apparently 3 kids & clothes for 2 nights is past this vehicle's limits at highway speeds.