1975 300D MPG went from 22 - 26 all of a sudden.

The only thing I changed was where I purchased diesel. Was buying from Thortons, switched to Shell. Wouldn't think it would make that much difference. The car does seem to have less power, so maybe something else is going on. Any ideas would be appreciated, thanks.

Forums

Diesel Engine Specific Aid

Chassis Model Number

W115

Why complain about an

Why complain about an increase in MPG? My 300SDL went from 28 to 15. Car still runs great. Explain that.

Many Reasons for MPG Increase

I can think of many reasons why your MPG increased when you changed fueling stations. Noted below are some of the more significant reasons.

1. Are you calculating your MPG accurately? One of the most accurate methods is to record the mileage and fuel needed to fill up your tank every time you fill up your tank. Then, to get your MPG after you fill up your tank, divide the mileage driven between fill-ups by the gallons used to fill your tank. Doing this at each fill-up will give you an accurate reading of your MPG per tank full.

2. The "octane" measure for auto diesel fuel is called "cetane." All things being equal, typically the higher the cetane of your diesel fuel the higher your MPG. In North American the cetane of auto diesel fuel at fueling stations varies from 40-45 (my source is the internet, so I do not know how accurate the information is). So you may get better MPG from using one fueling station than another.

3. Is the general air temperature the same before and after you started using the Shell fueling station? Air temperature impacts the efficiency of the fuel, and it also impacts the air pressure in your tires. Colder air temperatures decreases your tire air pressure, which decreases your MPG.

4. Did you have any work done on your vehicle immediately after you started using the Shell fuel station? If you had your injectors cleaned, your system "purged", new filters, new tires, etc., that will impact your MPG.

5. Did you use your air conditioning less after you starting using the Shell diesel? Using your car air conditioning has a significant negative impact in your MPG.

6. Are you driving the same route and the same mix of local and freeway mileage before and after you starting using the Shell diesel? Your MPG on local roads is significantly less than on the freeway. So if you changed you route or mix of type of miles that will significantly impact your MPG.

7. Are you carrying less weight (for example: people) in your car now than you did before? Changes in weight may have a significant impact on your MPG.

8. In reference to your decrease in power, it may be a result of a gradual build-up of debris in your tank, filters, etc. Or, perhaps the Shell station had it's storage tanks filled immediately before you filled your tank. When a fueling station has its storage tanks filled it stirs up the debris from the bottom of the storage tank and it takes a while for the debris in the storage tank to settle. Before the storage tank debris settles it is transferred to your tank when you fill your tank.

Your increase in MPG may be a combination of many factors, including some of the factors mentioned above. Your very large increase in MPG is hard to explain by just one factor.

"One of the most accurate

"One of the most accurate methods is to record the mileage and fuel needed to fill up your tank every time you fill up your tank."

This method is okay, if you are positive the odometer is running correctly. My odo was running slow, but I could not tell. As a result, I was getting 13 MPG in my 86 300 SDL. GRRR!

Finally the odo stopped and I had it repaired. Surprise! MPG jumped to 23 city and 27 highway.

Don't rely on odo on older cars. A more accurate way to measure the distance part of the MPG equation is to use a known distance or GPS.