what could be causing my low mpg. - PeachParts Mercedes-Benz Forum

lsmalley · #1 07-04-2010, 07:50 AM

I just drove about 110 miles and I had a full tank of gas when I started, but now I have a little more than half a tank of gas. I noticed that when I drive with my ac on my mpg is significantly reduced. without my ac my highway mileage is much better and I would only be maybe somewhere between the halfway mark and the full tank. Air filter changed on 5/22/10
*Transmission filter changed on 5/22/10
*Transmission fluid changed on 5/22/10
*Distributor cap changed on 5/22/10
*Distributor rotor changed on 5/22/10
*Fuel filter changed on 5/22/10
*Oil filter- Bosch Premium part number (3421). Done on 3/13/10 @ 254747
*Oil GTX 20-50W 5.5 qts done on 3/13/10 @ 254747.
*A/c cut off switch replaced 6/9/10
*A/c temperature sensor replaced 6/9/10
*Auxillary fan switch replaced 6/9/10
*Air temperature sensor replaced 6/9/10
*Radiator flushed. 1 gal Zerex G-05 added. Done on 11/09/09 @ 248,946 miles.
*A/C-heater control valve done on 11/5/09
*Check air valve. Done on 10/9/09
*Bosch O2 sensor replaced. Done on 10/7/09 @ 246,939
*Bosch H9DCO Spark plugs 6 total. Done on 10/6/09
*Bosch spark plug wireset. Done on 10/6/09 @ 246,560
*Tires inflated and rotated and balanced regularly.

400Eric · #2 07-04-2010, 08:01 AM

Is the car running well? Is it peppy? Is it smooth?

lsmalley · #3 07-04-2010, 08:08 AM

yes, the car seems to be running fine. initial cold start is a little stalled and I have a minor p/s fluid leak, but other than that its fine. I also forgot to mention that I flushed my transmission out....thoroughly with about ten gallons of fluid......for some reason I could not get the damn pan on right and I lost massive amounts of fluid......all better now though.

400Eric · #4 07-04-2010, 08:25 AM

This is a 2.6 right?
Any warning lights staying lit for too long?
Maybe you should try the "resistor mod". (Some cars love the mod, some don't, mine did.) http://www.peachparts.com/shopforum/showthread.php?t=32224

Stretch · #5 07-04-2010, 08:30 AM

Firstly I'd say it is often difficult to get an accurate mpg figure from the indication on your fuel gauge. If you want a more accurate way of reading mpg then you need to fill your tank and then measure the distance driven until the next time you fill. The filling station measuring system (the petrol pump) is more accurate.

Next you need to make sure that your odometer / speedometer is as good as it says it is. Then you'll get a more accurate assessment of your mpg - although it will probably be an average for many types of situations. In Europe the sides of the motorways are often divided by set distance marker points - for example here in Holland every 100 meters - so you can count these points and compare that number with your odometer... Do you have that in the US too? (I've never noticed this sort of thing in the US before though)

If you still end up getting poor mpg then it will probably be driving style or tyre choice - as it looks like you've got a well serviced car there - sorry!

400Eric · #6 07-04-2010, 08:43 AM

Good advice.
Yes, we have mile markers here.
Speaking of tires, how's the air pressure in them? Maybe over-inflate them a little.
Regards, Eric

lsmalley · #7 07-04-2010, 08:53 AM

I did the resistor mod some time ago...actually I forgot all about it. but I still have the resistor in a plastic case with my owners manual. and yes army, I did do the test which you are talking about, I gauged it at 24 mpg. filled the tank, drove until almost empty then I refilled.....I put in about 12. something gallons divided by 300 and so thats how I got my total. actually I forgot to mention that I used to run on 185/60/R15 but I had my tires swapped to 205/65/R15 for a smoother ride, could that be the issue?

Stretch · #8 07-04-2010, 09:20 AM

According to

http://www.miata.net/cgi-bin/tirescgi

Your newer tyres (sorry US spelling - tires) are larger

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
185/60-15 4.4in 11.9in 23.7in 74.6in 850 0.0%
205/65-15 5.2in 12.7in 25.5in 80.1in 791 7.4%

So you are actually going further than you thought! If the 185/60 R15 size is stock for your car then the odometer will be set for that.

If you haven't had your odometer adjusted for your new tires then you are actually traveling further than you think... Look out for speed traps you'll be going much faster than your speedometer reading too! (Assuming it was good in the first place that is)

lsmalley · #9 07-04-2010, 09:25 AM

how can I adjust the odometer to my new tire size

TheDon · #10 07-04-2010, 09:28 AM

with my E320 I get at least 250 miles to the halt tank mark.. somethings not right apparently with the 190E

Stretch · #11 07-04-2010, 09:33 AM

Quote:

Originally Posted by lsmalley

how can I adjust the odometer to my new tire size

To be honest, for your car, I haven't a clue - may be someone else can help?

BUT I wouldn't worry too much though until you've done this test:-

Get a passenger to count the mile markers on the side of the freeway and compare that with your odometer reading. You'll then get a better idea of the distances you are covering compared with th distances being recorded.

If it is within 10% - which it should be according to the tire info given above - then I wouldn't worry too much as you usually get a correction of 10% or so at speed traps.

You can also use this correction (that you calculate) by adding it to your recorded distance traveled before calculating mpg.

lsmalley · #12 07-04-2010, 09:46 AM

I'll do it on my way home. I will fill up and set my navigation. it gives accurate distance, then I'll reset my mileage counter and compare the two once I finish my trip.

Stretch · #13 07-04-2010, 10:22 AM

Using GPS is a good solution for flat ground but I've heard of people getting in trouble with indicated GPS speeds in hilly countries as it doesn't take into account the ups and downs - just an average height (if you like) across the circumference of the earth...

ps2cho · #14 07-04-2010, 10:41 AM

Everyone loves to use their fuel gauges as a measure of MPG, but it is HIGHLY inaccurate on our models. Mine seems to have about '4-5' 'points' before it jumps to the next one. It is not a smooth progression towards Empty. You must do it correctly via first making sure your odo is counting accurately. Once that is established, reset trip meter, drive, fill up, divide gallons put in by miles driven and voila. Now you can figure out if your fuel economy is low or not.
Best to get a reading over 2-3 tanks-worth to get an accurate average of course.

Since the car seems to be driving 'OK', you may want to get a compression and leakdown done before looking into anything else.

lsmalley · #15 07-04-2010, 11:11 AM

what is a compression and leak down