I get about 14 - 15 mpg on LIGHT driving... at least I think I drive light

 

Nope no aftermarket wheels for me yet, just the stock 16's the car came with, actually the tires I bought were just a little smaller than factory size, instead of the 225/55/16 i'm running 225/50/16 ... So this would mean the diameter is slightly smaller meaning the car is actually saying I'm getting better mileage than I'm really getting??? That would suck

And yea my system is drawing about 1500 watts I believe, you think this would affect my gas mileage? I thought only the weight of the system could affect this?

 

well with enough of a current draw and no capacitor technically it could be taking power from your ignition system, causing an inefficient burn because your spark plugs would be weaker, among other electronics. the diameter being smaller would mean you're covering a shorter distance per rotation, therefore your speedometer would be off, saying you're going faster than you actually are (general rule of thumb is for every +2 inches in wheel diameter you can expect your speedo to be +4mph compared to actual speed, and the reverse if the diameter is smaller).

 

Your tire combo conclusion is correct, but I don't know how much shorter the 50 aspect tire is than a 55. I don't think it would be very much, maybe 1/4 inch ?

And about your current draw......when your alternator kicks into full amperage mode I would bet the drag could be equal to the same drag an A/C compressor would create, along with any negative impact the amperage draw would have on your ignition system not getting enough voltage/current to fire properly/completely.

 

i was always laughed at and told that amps and subs don't actually take away from your mpg whenever i brought this up...! but i always thought that it would have had a negative impact on mpg...

also, i had the stock 16's and went with 20" i think the actual size difference all in all is about 2 inches taller, and whenever i fill up with gas i just divide the mileage on my trip odometer by how many gallons i put in the tank and use that to gauge my mpg for example yesterday i filled up 16.75 gallons and had traveled approx 364 miles, which was like 21.75 mpg or something. so are you guys saying that since i have about 2 inches more diameter now that i am actually traveling farther than what my trip odometer is telling me that i traveled?????? this means that i could have traveled like 380 miles instead of 365 miles like the odo read??? thus making my mpg actually better...

 

Would upgrading to a higher amperage alternator help with this current draw issue?

 

Or getting a capacitor?

 

getting a capacitor works wonders. Your lights will not dim anymore and your charging system will be under much less stress.

general rule is 1 farad of cap for every 1k watts you're running.

 

Well my lights don't dim as it is now, neither does anything else in my car. I have a brand new battery which may be the reason but I've always heard that caps are a waste of time and money, anyone on here using one with good results?

 

I think it may have something to do with some others have mentioned on page 2. Has the 10% ethanol mix hit your area yet. Here in NY it is the ONLY gas (gasahol) that you can get and it is PROVEN that you will lose around 2mpg and supposedly it is more expensive to process!

I remember last year I was really complaining about my bad gas mileage for the first time and many guys here were saying that their GS4XX's were getting as much as 19mpg CITY!!! I couldn't believe it...I was and still am getting 14-15 city regardless of how hard or soft I drive...but I also have mods. I later found out that some of these members didn't have the 10% ethanol in their area and were filling up on straight gas. Later on some of these people started complaining about the "new gas on the block"; I think there is even a thread on here about this. Again, in NY we have had this for at least 7+ years and it sucks BIG TIME!

 

I have a 1.2 farad.... I don't know what's up.

Well the way gas is going.... Looks like I've finally been hit with this bull****.

 

I skimmed over these kind of fast but does anyone recommend BG 44k? I've used it on other cars but I'm wondering how the GS will respond to it

 

oh christ, 10% ethanol has been in the northeast for at least a year or two now. I'm still getting about 21mpg in my 01 GS3, 131k on the clock and i certainly dont baby her....

 

Yea we've had the 10% ethanol down here for a while too, but for some reason (even before the mods) I've never gotten good gas mileage ...

 

You've got it !! Your odometer readings will be off by about 5-10% based on how much larger your tire/wheel combo is over stock. You will have actually traveled FARTHER than the odometer says when you have a larger diameter tire.

And people always seem to forget that the alternator puts a load on the engine at all times. If the amperage requirements are high, the alternator can put a significant load on the engine, possibly equal to an A/C compressor.