fastbackford351

So the automatic transmission took a big dump last month and had it completely rebuilt by a very trusted local transmission guru.

Ever since then the transmission shifts like it is brand new but my mileage has absolutely fallen off the face of the Earth and I can't figure out why.

Where I use to get 240-250 miles per tank now I'm barely getting 200 miles per tank before I need to refuel.

The first full tank after the rebuild I probably got on it a bit more often than I usually do just because I was tickled to once again have a transmission that shifted properly so I wrote that tank off without giving it much thought. But since then I have gone out of my way to drive with mileage in mind and have proven that the crappy mileage out of the first tank was no fluke and it seems that this is now the norm.

What gives? What could possibly cause this drastic a drop in gas mileage?

sethalot

My bet is that everything in your trans is tight again so you may be experiencing extra friction causing the drop in mileage. I would say after a few thousand miles the fuel consumption will go back down.

Sonic Mustang

That would explain the big mileage gains new cars get after a thousand miles. Wait. No, it doesn't because no, they don't.

In my opinion, the OP's mileage is being determined by his right foot. Additionally, measuring MPG by miles per tank is 100% bull**** unless you fill up and run it to empty - as in out of gas.

If a transmission could be "tight" enough to cause a 20% drop in MPG, it would burn up in a few miles. And, before it self-immolated the additional drag surely be noticeable.

sethalot

It is common knowledge that cars do in fact provide improved mileage after several thousand miles to the tune of a couple mpg. That isn't much but with a nominal loss of 15-20% noted by the OP that equates to a couple mpg lost. Is it unreasonable to believe that a freshly rebuilt transmission will have a period of increased wear as internal parts wear into common tolerances? As for the transmission burning up, it would puke its fluids out onto the road giving ample warning that something is wrong before it ceased to work. I plow snow, been there, done that, we top the fluids off and keep plowing.

Sonic Mustang

Is it unreasonable for a new, "tight" transmission to cause a 20% drop in fuel economy?

Yes. Yes, it is.

As for your logic comparing the slight MPG increase as new cars break in, let me remind you that tiny increase is a result of the entire drive train bedding in and the transmission being responsible for a fraction of that increase. Here, we have the assertion that that a rebuilt transmission is solely responsible for a 20% drop in MPG.

You can't measure MPG by the tank, and even if you calculate correctly it take more than a a few tanks to get a true average. His transmission was rebuilt last month.

sethalot

20% in this case equates to a little less than 3 mpg, improper tire inflation can cause similar numbers. 20% percent sounds a lot more intimidating than 3 mpg. Its like saying you increased the power of a 1.6L eclipse by 100%, that's just shy of an additional 100hp. Compare that to a gain of 50% on a 200hp 5.0L which still equates to an additional 100hp. Percentages are relative.

As far as measuring by the tank if the process is the same each time and the only variable changing is the mileage driven then you can consider the full capacity of the fuel tank as a constant value whether or not that is the actual amount of fuel consumed. Then the claimed mpg simply becomes an output only suitable for comparison on that particular vehicle.

How about you provide a better explanation for the OP's plight than "keep your foot out of it".

fastbackford351

I appreciate the feedback.

I have refueled at least 4 times since the rebuild and the foot is out of it. If my foot was any further out of it, it would be faster to walk.

My "100% bullschit" way of using miles per tank instead of miles per gallon is because that is how I roll. Consistently when the low fuel light comes on I refuel at my earliest possible convenience & I top off the tank with between 13.5-14 gallons of gas, depending. 90% of my driving is around town.

cliffyk

When the tranny was rebuilt was the valving changed--for a given load/throttle position does it still shift at more or less the same engine speeds, it is holding in lower gears longer before upshifting?

Or downshifting sooner--anything that is keeping it generally in lower gears than before it was rebuilt?

Sonic Mustang

Quote of the Year!

You might try rolling around the bull**** instead of over it.

Sonic Mustang

Proper tire inflation is worth 3 MPG on a Mustang? Really? You couldn't spend .0756 seconds Googling before attributing a 20% MPG loss to tire inflation?

According to Goodyear a 10 psi drop in air pressure on commercial vehicles is worth ONE PERCENT LOSS IN MPG. That's on 80,000 lbs. trucks with 18 low tires. To get a 20% MPG loss on a Mustang, you'd have to drive over a spike strip while being chased by a police helicopter, then the wheels would have to fall off.
http://www.goodyear.com/cfmx/web/cor...y.cfm?a_id=861