94 Civic VX - Not great MPG

[paulhale]

Originally Posted by theholycow

Yikes! That not only will take a chunk out of your MPG but puts you at a major risk for explosive tire failure (if you do any highway driving) and probably made handling pretty sloppy.

I do mostly highway driving. Oh well. At least I will know to check it constantly now.

[benfrogg]

I run higher pressures because it's worth more to me to squeeze out the extra FE that I can from running over pressure. I run the 35 psi tires super high because they are pirelli P300's which I've found on the web to be bad at FE. I'm trying to burn those off so I can put on the new sumitomo HTRT4's that are sitting in the garage ready to go. I can't see putting on new tires when there's 10k left in the old ones.

There's so many things in a VX that can hurt mpgs. It's a complex car; when running incorrectly it can burn more fuel than the cx or dx easily.

My VX was recently weighed empty at 2060 #'s. That's because of weight reductions I performed. I'm using the 165's because it's what came stock. Anything bigger and your FE #'s are off because the speedo is reading fewer miles than are actually driven. For instance, the 175's mean you need to add 1.9 miles to the odo for every 100 miles driven (going from memory here and what my gps told me when I ran 175's)
So, assuming you get 600+ miles out of a tank, that's nearly 11 extra miles not accounted for.

The handling with the higher pressure is about the same as max sidewall. The ride, however, is MUCH stiffer. No worries, I'm young and don't care much about comfort V FE yet. I'll ride out the bumpy ride for some extra MPG's.



Paulhale-
Yes, checking tire pressure often is critical. I check mine once a week or so. Unfortunately, gauges are not even close to accurate. I just picked one gauge and stick with it. If i use multiple gauges, I have to constantly raise and lower the pressure because one gauge says high and one says low. Not to mention making sure to fill "cold" (not having driven on the tires that day yet/not in direct sun, etc). It's kind of a crap shoot.

Do you feel in the pedal a hesitation on the highway? It should be sort of obvious that the lean burn is on or off. It will all of a sudden have more power at the pedal when accelerating from a steady speed. If not, check to make sure you have a VX ecu.
B

[theholycow]

Good points about tire gauges.

I've found cheap digital gauges to be consistent with each other and analog gauges to each be unique, so I avoid analog gauges entirely.

[paulhale]

Originally Posted by benfrogg

I'm trying to burn those off so I can put on the new sumitomo HTRT4's that are sitting in the garage ready to go. I can't see putting on new tires when there's 10k left in the old ones.
B

The Sumitomos are what I have on now.

Originally Posted by benfrogg

Do you feel in the pedal a hesitation on the highway? It should be sort of obvious that the lean burn is on or off. It will all of a sudden have more power at the pedal when accelerating from a steady speed. If not, check to make sure you have a VX ecu.
B

Yes definitely. The car definitely has the stock VX ECU, motor, and tranny.

[benfrogg]

I guess I'd offer a few more possible solutions. One is fuel leak. My tank was leaky for a month or so. See my fuel log for when it happened. It seemed that although I didn't see a huge increase in FE afterward, it was still significant.

Another thing to check is using a GPS to verify speedo readings. Then you can verify that you are inputing correct data to get FE.

Make sure the idle speed is at spec. Mine was high (never got below 900 rpm at warm). I've since adjusted it below spec to squeeze out a few more drops of fuel.

Injector cleaning helped me a little, at least as far as power was concerned at lower rpms/lower throttle. If they are really bad you can use a lot of fuel to compensate for the under performing injector.

Check to make sure the vx specific aero parts are installed. One is the lower lip under the front of the car, another that I remember is a plastic panel under the drivers rear tire area that keeps air from becoming trapped under the rear bumper. Also, there's small plastic aero deflectors that should be installed at the lowest point of the components that hold the rear wheel on. They direct air in some way that is important.

B

[paulhale]

Originally Posted by benfrogg

Make sure the idle speed is at spec. Mine was high (never got below 900 rpm at warm). I've since adjusted it below spec to squeeze out a few more drops of fuel.
B

My RPMs are around 400-500 at warm idle.

Originally Posted by benfrogg

Check to make sure the vx specific aero parts are installed. One is the lower lip under the front of the car, another that I remember is a plastic panel under the drivers rear tire area that keeps air from becoming trapped under the rear bumper. Also, there's small plastic aero deflectors that should be installed at the lowest point of the components that hold the rear wheel on. They direct air in some way that is important.

B

I checked that too. All of the VX plastic is intact.

[add|ct]

My idle is like your VX paul, warm idle this summer is between 400-500 at times full temp/mid day etc. In fact, I think it might be too low?

I'm running 175 right now on stock 13 alloys and according to another site if you drove 750 miles on that size over the 165...the actual mileage driven is 1,000.

I'm too looking at getting the Sumitomo tires, but not certain if I want to get 165 installed. I just got an alignment performed with outer tie rod replacement on these 175 tires. If I get the 165s will I have to get an alignment again?

[benfrogg]

Add-
I'm not sure the math is quite right on the difference between the two tires; the calculator I found says the 175 will turn 22.22 less revolutions per mile. Not sure if that adds up to 250 less miles per 1000 or not... then again I'm not a math major.
Alignment shouldn't matter to changing tire sizes. Toe and camber are all that are adjusted on the vx, and only on the front end. In fact, if memory serves, camber is non-adjustable anyway. That is largely determined by ride hight.

Paul-
How are your mpg's now that you increased the tire pressure?
I keep a tire pressure gauge in the fuel door to remind me every fillup (about every 700 miles) to check the tires. The gauge has a magnet that sticks to the door.

B

[theholycow]

Originally Posted by benfrogg

I'm not sure the math is quite right on the difference between the two tires; the calculator I found says the 175 will turn 22.22 less revolutions per mile. Not sure if that adds up to 250 less miles per 1000 or not... then again I'm not a math major.

There's more calculators linked in my meta-sig. The popular miata.net calculator will give you the practical upshot in the form of percentage and an example (something like "When your speedometer reads 60mph you'll actually be going 58mph", etc).

250 less miles per 1000 would be an absolutely ridiculous change in tire diameter and is probably impossible without changing to a smaller wheel. benfrogg's post yesterday was on the appropriate scale, he mentioned 1.9 miles error per 100, which equals 19 miles error per 1000.

Anyway, here's a set of hard numbers for you:
165/70/R13 vs. 175/70/R13
2.5% difference
Speedometer at 60mph, actually going 61.5mph (or looking at it as changing from 175 to 165, speedometer at 60mph, actually going 58.5mph).
25 miles error per 1000 miles on odometer

[paulhale]

Originally Posted by benfrogg

Paul-
How are your mpg's now that you increased the tire pressure?
I keep a tire pressure gauge in the fuel door to remind me every fillup (about every 700 miles) to check the tires. The gauge has a magnet that sticks to the door.

B

The magnetic tire gauge is a good idea.

I haven't put enough miles on this tank yet to check it. I will post back when I fill up next. Probably Monday or Tuesday.