Strange decrease in mpg with VX when driving 65 mph instead of 72
 

Attention all VX owners. I noticed a very strange phenomenon with my mpg when traveling to Chicago last weekend. I purposely slowed down to 65 to 67 mph (in 5th gear with the cruise control engaged) to attempt to get higher mpg. Just the opposite happened - it dropped from 45 to 41. On the way back from Chicago to Detroit I got 48 mpg at 72 mph. The wind direction was from east to west on the return trip - no wind assistance obviously. I have also noticed a slight decrease in mpg in my 54 mile daily roundtrip from home to work when I drive at 65 mph in 5th gear instead of my usual 70 to 72 mph to keep up with traffic.

What the hell happened? I'm thinking that strong lean burn at 65 to 67 mph results in higher NOx in the exhaust, which prompts the ECM to increase EGR to take care of the NOx. Increasing EGR results in a loss of power due to inert gas being added to the intake air/fuel to lower the combustion temperature. The ECU might have been forced to enrich the mixture to compensate for the loss in power to maintain constant speed (I had the cruise control on the whole time). This could be taking place in repeated cycles of strong lean burn/EGR/richer air fuel/strong lean burn/etc.

While traveling at 72 mph I could have still been in lean burn mode but with a air fuel ratio of 16 to 18 instead of 20 to 22. If this is true, then there would not be a demand for EGR to "stabilize" the combustion temperature to take care of the NOx emissions. At 72 mph, I'm still only at 2300 rpm which should keep me in the 3 valve/cyl operation mode (the two exhaust valves are always operational).

Does anybody have a PLX air/fuel meter attached to their VX? If I could get one cheap, I'd install it just to find out what happened this weekend. Has anyone else experienced this? It is really weird. Does the supermid show this tendency at all?

I would think that the key to your mystery lies in this ^^^ boldfaced statement. I'll leave you to ponder that if it isn't immediately clear. :)

Unrelated... what year is your VX? I think the final ratio of the 1994 and '95 models is lower than the '92 and '93 models which is why the '94 and '95 get better FE on the highway and less in the city than the '92 and '93.

I think the reason you got better gas mileage at higher speed is because you were keeping up with traffic. Why would keeping up with traffic be good for your FE? Drafting. You were probably driving behind other cars, which would cut down on your wind resistance. So think of it as having the wind resistance of driving at 65mph and therefore using about the same gas as going 65mph, but you have the added benefit of going 7mph faster (72mph).

What do you think?

I had this hapen once where I seemed to be getting a higher mpg going faster. This only happened once though (would have been too good) and I think it probably had to do with wind directions and such; who knows. Fact is though that if you drive more often with lower speeds you will definitely find out that the slower you go the better your mpg will be. I only wish it would be the other way around :)

I agree, there are many factors to determine what will happen in one tankful.

I did replace all four of the oem size tires recently from 165/70R13 to the next size up that is readily available 175/70R13. I know that will decrease the mpg slightly. The tires I just put on the front are Toyo Spectrum touring radials and these tires have a directional tread on them. They are really interesting to drive on so far since I have noticed they want to follow certain grooves in the road that make it feel like I have cruise control steering. The new tires work well in the rain however. I was to the wear bands on the old tires and I was hydroplaning with them - no fun at all.

I will keep an eye on the mpg with these new tires. I have been logging my mpg by the tankful for the past two years and will find out what gives.

BTW - I was driving home today in the center lane of a three lane interstate at about 68 mph keeping up with the prevailing speed of the heavy afternoon traffic when a middle aged lady in a Ford Explorer tailgated me and motioned for me to move over into the right lane. I returned her suggestion with the same motion - I pointed and recommended that she use the right lane if she wanted to get by me. She then proceeded to give me a double barreled middle finger salute which I promptly returned to her (LOL !!!!). Then, I blew her a big kiss before she stepped on the accelerator and blasted past me only to find out she needed to slow down to 70 mph not too much later. Does everyone now understand why it is dangerous to drive under the speed limit in Michigan. On a one lane road, I can understand how someone following me could feel frustrated, but I refuse to move out of the way under the circumstances just described.

I've had a lot of luck with the tailgating drivers. Often, when someone comes up on me-- a few minutes later they'll just slow way down and be several hundred feet behind me. Sometimes cars tailgate, but most of the time they are pretty respectful. I find it interesting tho when they fall back. I feel like maybe they got why I was going slow and decided they too could do to save some gas. :D

Sometimes I feel like I confuse them, because I will be coming up on a down hill and will accelerate before the hill and then pop it into neutral and accelerate in neutral going down the hill, and then after the hill and I've resumed 40-45mph they'll stop following me close.

Jadziasman: your tach is at 2000rpm at 66mph. Mine is at 2000RPM at 60 mph. Why are our tachs different?!?

Perhaps you two have different tires and or random flucuations in tach and speedo goodness. :p

I don't know Jacinto.

The tire circumference is a little larger which should make the speedo read slightly slower for the same rpm. Of course the tach get its signal directly from ignitor pulses and thus is independent of the VSS attached to the teeny gear the gets turned by the differential to tell me how fast I'm really going -wheeeee!. My speedo seems accurate though - I tested it on my 600 mile roundtrip last weekend and it's within 1 or 2 mph. My guess is that the ignitor pulse from the distributor is being misinterpreted by the tach for some reason so the tach probably reads lower than actual. There are laser tachometers now that are pretty cheap that can read the crankshaft pulley rotation directly. I was thinking about getting one but I'm not motivated enough to buy one - at least this year.

I have a new distributor I bought as a spare if/when the TD-42U distributor that's on the cylinder head right now takes a dump.

I have been collecting spare parts for the last two years for my VX. I have two slightly used L1H1 5 wire sensors from a tuner shop that I have in reserve. Got them pretty cheap too. I couldn't pass up the price on the last one I bought even though I already had a spare. VX ownership makes you do some strange things but it's one of my good hobbies.

I used to drive a 79 Camaro that got the same mileage weather I was driving 70 and 100 if I had the cruise on.