new alternator changes mileage??

So about a month or two ago, I put on a HO ebay alt. It's supposedly just a stock unit "rewound" for more amps. Whether or not I'm getting that, who knows.

The point is that in the same time period, I've lost around 1 mpg. I know thats not much to folks on here but I'm about to regear to 4.10s I would like my mileage to be at its tried and true point so that I have some figure for comparison.

Stats: 96 xj 4.0L AW4 30" revos 2wd. Currently with 3.55s. Before the alt swap(if thats even the culprit) I was getting around 16-17 mpg. Now, I'm lucky to get 15. The only other thing I did in this time period was run a seafoam treatment which I do periodically with no negative effects.

Any thoughts?
will

Since the engine turns the alternator, it's possible, if the pully is extremely hard to turn, for your mileage to go down.
Billy

I vote "no" on the alternator. (just because, LOL)

Have you changed gas stations? Some of those weird formulations like "oxygenated", or gasohol/alcohol affect mileage.

EPA restrictions have been temporarily lessened due to Katrina. Maybe you got a tank with more alcohol content than your area is used to.

...or, you are just crazy. Ya' never know.

Just be happy, I'm getting 8.5 mpg w/ my 96' !!!!

Maybe your fan belt is too tight? That might cause you to lose some MPG.

mgfabpd... 8.5 MPG, that's horrible. My 91 XJ with 180K 4 inch lift and 31 inch Wrangler MTR's gets 20-22 highway. You've got some problems...

I vote "Yes" to the alternator. Rewound for more amps also means it is harder to turn... remember back in your old RC-10 days with the electric motors? Well, if you got an upgraded motor (those things hauled @ss!) you would notice that it is harder to crank by hand. Same principle here. Nothing is free...

TheNerd

OK, lets think about this for a minute. How many HP does it take to run down the road? Lets say around 75hp. If you drop your mileage by 10% then you are expending an extra 7.5hp. at 746 watts per horsepower, that is about 5k watts At 12v output that is around 400 amps. Given a 125 amp alternator, even at full load, you only use around 1.5hp.

Now to the big question. Given it is the alternator, either the alternator is getting VERY hot due to a short or a bad bearing to use that much power, otherwise, where is all the power going? Is the battery boiling? At 125 amps, it will boil in about a minute or two. If it is running down any other wire, it damn well better be the size of a battery cable or it will melt.

Harder to turn more windings in higer-amp alts. Mine went down a wee bit as well. Just more magnetism to overcome in order to produce more electrical flow.

Old wives tails like that keep me in business. Yes it takes more energy to turn a magnet in a field when there are more windings, but only when those windings are loaded or shorted. The voltage regulator controls the field windings and thus the power out of the alternator. It only generates power when the voltage regulator senses a drop in voltage (aka a load), but once the load is overcome (battery charged) the voltage regulator in effect shuts off the alternator.

Just for reference, I am an electical engineer.

Not trying to bust anybody's chops, but just trying to kill some old wives tails.

As for mileage, 1mpg is very little and is under the tolerance for error in the measurement. How do you know that you are filling the tank to exactly the same point each time. In order to be able to get an accurate measurement, you would need to average over many tankfulls and make sure you drive exactly the same and over the same terrain. A change in the battery voltage may actually effect the computer, making it adjust the operating parameters slightly different. It takes quite a few start/stop cycles for the computer to start changing the operating curves. A computer reset might get it to adjust for optimal operation sooner, but you never know. The Renix computer actually has an internal regulator for the voltage going to the sensors. Ever wonder why there is 5v on the sensors instead of 12v?

I've been using the same station for my fillups for over a year now. I always fill at the same point and it always takes right around 15 gallons. The only different factor is the fact that I'm barely getting to 225 miles on that one tank. I know its miniscule, but you still notice these things.