interesting gas mileage changed

rnln · 06-06-2006, 10:14 PM

I used to have around 20-21 mpg all the time, same route, same time everyday. I used to use Castro 20-50. Recently, I changed to Vavoline 20-50 and get around 26-27 mpg now.
At first, I thought I made a mistaken when calculate it but this is the third tank and I got the same figure, 26-27 mpg.
No, I have not replaced anything except engine oil change. Actually I replaced fan belt, AC belt, and the steering rack bump steer kit lately. The belts I replaced after the new gas mileage figure changed. I don't think the bump steer kit can make diff on the MPG.
Is this the oil manufacture makes the diff. or is it the temp changed does?
No, I am not crazy. I have not done anything more than just the above, not even pump any more air in tires. And I have not lost any weight.

bigrubberjeep · 06-06-2006, 10:26 PM

I've actually experienced the same thing. Not as drastic but it was a good 5 MPG increase on both highway and street.

Zendalar · 06-06-2006, 10:35 PM

You sure the new belt is spinning the same speed, I read somewhere that you actually can gain horsepower if you make the belt turn slower...

If your fan is turning slower now, there is less stress for the engine -> Better gas mileage.

Same goes with electrical devices. They tested that driving without headlights can actually save gas. 0.5 liter per 100km.

rnln · 06-06-2006, 10:41 PM

umm.. I am not sure but the belts were replaced after I noticed the gas mileage changed. Also, I am not sure how you make your fan turn slower.
One thing I realzed is that the old fan belt was about an inch or two shorter than the new belt. And I leave the new belt a tat more loose compare to the old belt adjustment.

dvkk · 11:06 PM

Quote:

Originally posted by rnln

One thing I realzed is that the old fan belt was about an inch or two shorter than the new belt. And I leave the new belt a tat more loose compare to the old belt adjustment.

There's your answer. A longer belt will ride higher in the top pulley, slowing the fan. Since it takes hp to push air, slowing the fan pushes less air and therefore requires less hp.

Changing oil brands will not give you a 30% gain in mileage. Even if you went from dino 20W-50 to synthetic 5W-20, you'd only get a miniscule improvement that would be barely calculable. If it were that simple, everyone would already know it.

Factors that may have contributed to what you saw:

driving style
city/freeway mix changed
calibration of the gas station's pumps
weather/wind

jevvy · 06-06-2006, 11:02 PM

maybe it was a combination of things, correct tyre pressures also help.

Good news none the less

rnln · 06-06-2006, 11:19 PM

ko so the tension on the belts then. But wow.. it makes that much diff.
Thanks.

dvkk · 06-06-2006, 11:23 PM

No, the belts won't make for a 30% change. Your method of calculating will.

Jim Sims · 06-07-2006, 04:26 AM

"Your method of calculating will". Or the input data; too many of the older Porsche VDO odometers slip/grab intermittently to trust them. Once mine just stopped turning for a half an hour while crossing Wyoming. Makes for very poor apparent fuel economy when no miles are being "racked up". I agree, neither a belt nor lubrication change can give you a 30% gain or loss of efficiency. Also, any change less than 10% of nominal can usually written off as statistical variation unless one tests under very controlled conditions with calibrated instruments.

na2ub · 06-07-2006, 05:53 AM

Could the bump steer spacers have changed the front alignment at all? Certainly any change in alignment would affect mpg.

Doug

rnln · 06-07-2006, 05:53 PM

I know for sure either it is one of the factor below, or all:
1- diff oil manufactures
2- belts tension
3- the steering bump kit
.....................................
4- I messed up (this is the most likely)

Early_S_Man · 06-07-2006, 06:12 PM

#4, combined with the odo behavior Jim described ...

If your route is always the same, determined length [double checked] x no. of trips = known distance /fuel qty Delta to same point in filler neck [shaken to eliminate air bubbles] == fuel mileage.

Hugh R · 06-07-2006, 08:14 PM

Hum

I'm thinking, if you have a two inch shorter belt, doesn't that mean the diameter of the driven pully (fan) is turning faster, not slower. The alternator pulley effective diameter is smaller than with a longer belt (less spacers inside the pulley than outside). Think of a 10 speed bike, the rear wheel turns more revolutions with the smaller gear, not the larger gear in the rear. Something about 2 x Pi x radians if I recall correctly. Not that this has anything to do with your mileage change.

dvkk · 06-08-2006, 10:35 PM

Quote:

Originally posted by Hugh R
Hum

I'm thinking, if you have a two inch shorter belt, doesn't that mean the diameter of the driven pully (fan) is turning faster, not slower. ..

Hugh, you have it backwards. His new belt is longer, not shorter:

Quote:

Originally posted by rnln

One thing I realzed is that the old fan belt was about an inch or two shorter than the new belt. And I leave the new belt a tat more loose compare to the old belt adjustment.