good mileage at high speed, bad mileage at low speed
 

hi

new on the forum. sorry for the long post.

i have an 04 MT that i bought last spring. i know the mileage on our cars is absolutely wonderful...for the gas companies...

when driving on straight highway runs, depending on the highway,
i've gotten 22.8 mpg (US Gallons) while driving anywhere between 75-90mph.
i've gotten this on two road trips, both directions.

when driving on pure highway on a different route, i'm limited to 60-65 mph, on these repeated trips (ottawa to ogdensburg) i repeatedly got 16 mpg in 6th gear.

i changed to a K&N first thing after getting the car.
new plugs/wires/coils, absolutely no difference. old plugs looked OK, new plugs looked OK and the K&N is kept clean, regular oil/filter changes. i don't think the cat would be blocked otherwise i'd be getting much worse mileage at the higher speeds.

i checked with a dealer who looked up the VIN, the car had gone in for the tests on the seals and was fine and appeared to be up to date on any TSBs. i went to a local RX7 tuner (i was originally looking for a JDM FD but couldn't fit well) but they said they "didn't have the software for the RX8"

my typical mixed city/highway mileage is 17 mpg, pretty easy driving, which is essentially what i got with cruise control at 60 mph on the highway!.

i know how to get good mileage out of car (29/39 mpg combined/highway in our G5).

i recently tried a couple of tanks with 4,500-5,000 rpm shifts and pretty stayed pretty much at 16 mpg.

what would cause such behaviour? improper functioning of valves/vanes on the intake manifold? i get no codes whatsoever....

power feels fine, just this oddly poor mileage at 60 mph and much much better at 85-90....just looking for tips on where to work next and not waste money at garages/dealers.

thanks, Mauro

Nothing seems off to me here, this is what I typically see as well. I see 18 in mixed driving, about 16 in bumper to bumper or high traffic and 22 on the highway (65-85). I dont think anything is wrong with your vehicle, just drive it and enjoy it.

Yeah man living in Austin TX to better my gas mileage I just take the toll roll to avoid those bumper to bumpers.

I'm lucky to get 14mpg in stop and go traffic.

Yet i'll easily get 26 mpg on a road trip.

our car is tuned rich at low rpms... (usually in the 12s) I've done my own tuning and the engine simply favors more fuel at low rpm / low load. I guess it also helps with lubrication a little.

You're doing good keep it up. When I had my car I could get 22mpg highway, but never better than 12mpg mixed mostly city.

hi
thanks for the interest, but i think you've all missed the point, maybe i didn't make it clear.

at 85 mph i get 22.8 mpg
at 65 mph i get 16 mpg

BOTH of those are straight highway driving, both directions. if those two were reversed i wouldn't say anything!

at 85 mph i think i'm up around 4,500 rpm
at 65 mph i think the revs are only 3,500 rpm

is there an rpm at which point the intake manifold valves flip position and one is stuck in the non optimum position?

i've heard of some valves/actuators giving trouble, but they throw a code.

thanks, mauro

I think there's two possibilities here

1. Your RX-8 is more efficient at higher rpms

2. Your rotary engine is so inefficient, that it doesn't use significantly LESS gas at lower rpms, so if you're driving on the freeway you might as well cruise in a higher rpm and travel farther for each given interval of fuel consumption.

One of the very few negatives of owning and driving an RX-8 is the poor gas millage especially on local roads at low mph. I have a 10 minute commute to work on local roads, so I feel your pain with regard to this. I rationalize it as the price for happiness.

when i bring mine out of storage in the spring i will try this experiment doing a week with the cruse control on at 60mph vs. the next tank at a higher speed as i drive the same route/traffic pattern each day

Ok i read.the content, well, are u filling up at the same.station?

The reason i ask is gas quality varies ... even its the.same brand. Some have much higher garbage ethanol than others. More.ethanol means worst.mpg and more poisonous gas.

Check your AFRs at 65 and 85 cruise and compare. Sounds like you're a bit richer at 65 or too lean at 85.

3.7K rpm in 6th gear, 22 mpg easy

hi

yes the air fuel ratios could be the culprits.

i'll be getting an engine monitoring software soon,

a buddy has a cable that will let me run this

OBD-2 Vehicle Explorer Scan Tool Browser

and i'll also be checking how quickly the o2 sensors respond...although a slow response might/should trigger a fault/CEL.

the CEL comes on a start up, but i may disconnect something to confirm faults are detected...

mauro