Mileage drop-- ATF or Gear Lube implications?

[dgpretzel]

I just bought a 2003 Chev Astro w/ 29,000 miles.

The first think I did was to change the oil and put in synthetic.

As I was driving around for a few days, I monitored the fuel usage display and noted that, over the course of approximately half of a tank of gas (my first in that vehicle), I had a cummulative gas mileage of 18.5 MPG indicated.

Then I had the ATF and the differential fluids replaced with synthetic (Amsoil). I didn't add any GM limited slip additive in the differential.

I observed the cummulative gas mileage to drop over the nex few days, and when I refilled the tank it showed 17.8 MPG. I think the type of driving was similar over the whole tank, although I was probably driving 5 - 10 MPH slower for the first few days.

There might be many causes for my changing observation. One of them might even be that there has been no change at all, and that my first observations were too "instanteous" to be significant.

Also, I don't want to confuse coincidence with causality.

Still, is there any reason to think that switching to synthetic oils could cause my overall fuel efficiency to decrease 1/2 - 3/4 of a MPG?

Thank you.

Regards,

DG

 

[unDummy]

You need to run at least 3-4 full tanks of gas with MPG calculated(not using dash computers) to draw a conclusion.
So, you didn't gather enough data 'before' to draw an 'after' conclusion.

I would make sure all fluids levels are topped off. And, if you didn't change the fluid yourself, I hope that you watched the Amsoil being poured in. Some techs keep the good stuff for themselves.

Gasoline used, tire pressure, and weather effect the MPG more in my vehicles then then lube chosen. Run a bottle of FI cleaner to rule out a bad tank of gas, check your tire pressure weekly, and wonder if the weather was getting colder.

 

[Pablo]

I agree with the un-guy. Changes in fluid are usually buried in the noise. In a perfect world, maybe you would seen a slight increase in mpg, like 0.3....or something.....but that too would be buried in the noise.

One more thing - it could have been that the old fluids in the car where extremely sheared, and you just put new, albeit higher viscosity fluids in.

 

[dgpretzel]

Thank you both for helpful comments.

Quote:

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Run a bottle of FI cleaner to rule out a bad tank of gas

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Are all FI cleaners "the same"? If not, might you suggest one or two products? Thank you.

DG

 

[bmwtechguy]

I've had good results from several different FI cleaner/maintainers: Redline SI-1 from Advance Auto, Amsoil PI, Pennzoil Regane, and Chevron Techron. Try one or 2 and check your mileage and performance. And no, they are not all the same obviously. Some products seem to be better tha others in areas of cleaning, fuel economy improvements, and performance improvements. I've also used Fuel Power (FP60) from LCD and liked the results, too. I'm expecting to receive a couple gallons of their new FP3000 sometime in Nov. to try out. I would also recommend you deal with one of our excellent site sponsors if shipping to Alaska is not too prohibitive.

 

[unDummy]

I recommend Regane, Redline, Amsoil, Lucas, Techron, Seafoam, VentilSauber, bg44k, 3M, B12, Marvel, Neutra,.....
I make an effort to use 'em 4x a year. Since I believe that they all have pros/cons depending on formula, I rotate among the brands.