bigger alternator better for fuel milage?
 

the alternator on my f150 is kinda on the weak side while driving with no accessories on its only putting out 12 volts. now i was wondering if i were to get a higher rated alt (current is 65 amps and i have a stock replacement off my parts truck and 100 amp one out of my old diesel that will fit) would the higher amps make it so the alt wouldn't be used as much or am i not thinking clearly?

If there is any difference you probably wouldn't notice it unless you ramp up the accessories (big sound system, flood lights etc.).

Assuming efficiency is the same, power drain should be directly proportional to electrical usage, so there should be no change.

Get the battery checked first though, it may be a weak battery not the alternator, if you can borrow a good battery swap it in and check voltages, if better, then battery is suspect.

Instead of going for a larger alternator, why not replace all your interior and running lights with LEDs?

Don't increase production, reduce consumption! :)

Alternators are more efficient at higher field current levels, but I don't think switching from a 65A to 100A alternator is going to make much of a difference.

That seems to make more sense. BTW probably a higher-amperage alternator would probably be not so good for mileage improvements, unless it achieved the higher amperage from a more efficient design.

Hmm all very good points! but thier mostly in the con side so Ill try to go pro.
If the battery is capable of absorbing the extra amps than yes It should because it wont be regulated on for as long.
If it is same sized I will bet it is just the regulator that was changed, but if you add an underdrive pulleyset up, It will assist you but ever so slightly. It is free to test with exception to your time and labour so go for it.
Would be good if you also weighed them to ensure it was not more mass to drive etc.
I have asked this very question before is there a better energy saving alternator.
Historically the alternator is the energy saver, over the generators of old but then what?? This is 50 years later?????
I myself found an 85A alternator from a Toyota camry to replace the 55 that was in my Kia. Then swapped in a much bigger pulley on it, I would like to think it did something good for my efforts other then break the cycle of replacing the new alternator every year issue my KIA had. Yes notoriously JUNK!

A larger alternator is going to have more rotating weight so it's going to take more energy to turn it.
There is no way your truck in it's stock state is drawing 65 amps, if you are seeing low voltage then my first guess would be the voltage regulator on the alternator is going bad, you might check for an alternator rebuild kit.
Also check all of your connections, make sure the electrical connections are clean and tight and that your ground straps are in good shape.

To properly test alternator output, first lightly discharge the battery with the engine off and with your headlights on for about two minutes. Start it up and immediately apply all the electrical load you can i.e. high beams, high fan blower speed with an engine speed of 2000 rpm. Look for 13.2 - 13.4 volts and initial amperage near the rated output of the alternator. The amperage should slowly come down as it recharges the battery. If the amperage drops quickly and stabilizes to the remaining load, it could be a sign of a weak battery. If you have a battery with caps, hydrometer testing will give you a decent idea of the batteries condition. Surface charge it first and leave it rest 20 minutes before testing.

You can decrease load for driving under low RPM conditions by increasing the diameter of the alternator pulley but you will need to make up the charge either by higher RPM driving which would be a wash from the economy standpoint, or by externally charging later (solar would be the best IMHO).

Sounds like a bad alternator.

With nothing on your truck cant be drawing any where near with the alt can put out.

I say a free 100a alt is better than buying a new 65amp unit.