33mpg VX, A/C on, highway miles going 80mph

[Lsturbo91]

I've got a 92 vx, its been tuned up to oem specs with a new o2 sensor. Have a cali ecu L00. Right now getting about 33mpg all highway miles. 80mph w/ ac on. Does this sound right or should I be getting more? Whats everyone getting using their a/c?

[vxdude]

Wow whats your rpms at? 3,000? I run about 55mph and get 55mpg. But yes that does sound about right.

[thisisntjared]

its a little low... but not really bad. whats your tire pressure like?

[1993CivicVX]

RPM at that speed should be around 2700RPM. 33MPG seems a little low, but if you are running the A/C on high than it could be normal. Do you keep it in 5th gear while driving at 80mph? I don't use my A/C enough to give you numbers, but I think if you have it on low (1 or 2 fan settting) it shouldn't effect the numbers too much, but I suppose at higher speeds it could have more of an impact as its sapping some much needed power at those speeds.

[nowhhs]

I agree with notjared, I would expect 10-20% better than that. Its a little hard to compare because you have one of the few Cali versions here. If it runs and idles ok otherwise, I don't think I would worry about it. But check those tires.

[Lsturbo91]

Yea, I have 15" 99 SI rims right now and the previous owner put power windows on it so its a little bit heavier. my rpm at 80mph is around 2700 rpm. I also have to downshift when the a/c is on going on slight uphill.

[thisisntjared]

Originally Posted by Lsturbo91

Yea, I have 15" 99 SI rims right now and the previous owner put power windows on it so its a little bit heavier. my rpm at 80mph is around 2700 rpm. I also have to downshift when the a/c is on going on slight uphill.

hmm....downshifting to go up a slight hill? thats not right. what is the tire pressure????

fyi the si wheels are maybe 1 lb. lighter than steelies. and the power windows wouldnt effect your mpg on the highway by any noticeable measure.

[Lsturbo91]

Originally Posted by thisisntjared

hmm....downshifting to go up a slight hill? thats not right. what is the tire pressure????

fyi the si wheels are maybe 1 lb. lighter than steelies. and the power windows wouldnt effect your mpg on the highway by any noticeable measure.

I could stay on 5th gear no problems without the a/c on. If it's on I need to downshift.

[thisisntjared]

that is weird... was the unit ever replaced?

[Lsturbo91]

Originally Posted by thisisntjared

that is weird... was the unit ever replaced?

no, but I think its a product of the a/c compressor taking up power from the motor not necessarily a/c unit being defective. After all we do have a dinky 1.5l with tallest gearing ever

[thedaemon]

I get around 35-38 MPG with the A/C on, not driving as fast as 80 though. More like 55-70. I do have to downshift on a slight hill with the A/C on, and sometimes with it off. The gears are super tall and not much torque as I would like. I usually end up turning the A/C off if I am in a hilly area and just sweat it out, because the car apparantly puts most of the torque into working the A/C unit!

[JESSE69]

Well, my 99 Civic HX M/T gets 51.15 mpg at 70 mph on 508 miles with 50 miles A/C. Did about 330 miles at 70 to 80 mph with heavy A/C and got 39.6 mpg.

[civic lover]

JESSE69- You should add your car to your garage. I just got an HX and there seems to be not many follower of the HX around.

When does the Vtec-e disengage. Is it possible it is disengaging at 80mph?

[R.I.D.E.]

vtec-e should be disengaged at 80, or over 2500 rpm. At that speed the VX's advantages actually become disadvantages.

80 in VA is the threshold of a reckless driving charge, I probably don't see 70 MPH 1% of each tank of fuel.

60 mph think 6 sqaured or 36 units of drag
80 mph think 8 squared or 64 units of drag

Drag is exponential meaning the resistance is a sqaure of your speed, since 70% of your energy cost (to maintain a constant speed) is aero, even 70 to 80 mph is an increase of 64/49 or 30% more energy required.

My car gets about 60 at 55 and 55 at 60 MPG, do the math understand the exponential increase in power required and your mileage sounds right for 80 MPH.

A Pontiac Mantana Van at 105 MPH went through 20 gallons of gas in one hour. Thats worse gas mileage than a restrictor plate NASCAR car at 180MPH.

High speeds like 80 MPH can only be achieved efficiently with significant aero drag improvements. Drop the VX CD of 32 to 19 and the total energy cost at 80 mph would drop by about 13/32X.8 or almost 33%. Multiple your 33 MPG by a 4/3 factor and you have 44 mpg.

regards
gary

[JESSE69]

Originally Posted by civic lover

JESSE69- You should add your car to your garage. I just got an HX and there seems to be not many follower of the HX around.

When does the Vtec-e disengage. Is it possible it is disengaging at 80mph?

In an HX, lean burn is still good at 70 mph < 2800 rpm. Over 70 mph the Vtec kicks in and you get more power / less FE.