JimW

If we have a 17 gallon capacity fuel tank, then how low can you go before you start sucking up contaminants and plugging up the fuel filter. I'm just trying to extend my fill ups and monitor MPG with the new "M" flash.

jdl

Just so you don't try to eek out 16 gallons-worth of driving on one fillup, you should know the 2004 RX-8 is fitted with a 15.9 gallon capacity fuel tank

JimW

Whoops, hahaha I guess it's a good thing I only let it get to 13.5 gallons at the most. I knew about the capacity, I was thinking about my former cars capacity, should have thought about it before making and **** of myself. Anyone know a safe margin 14.5 15.5 perhaps?

beachdog

Don't use the odometer to tell when it is time to fill up. Mileage is going to vary based on how you have been driving. I find the gas gauge to be very linear and fairly consistent, the mileage per tank has varied quite a bit. Around town, I 'll go ten or so miles after the light comes on and it will take about 14 gallons. About as low as I want to go.

FirstSpin

I estimated once (and only once because I almost always fill up at around 1/4 tank remaining) that I had around 30 miles (I forget the exact number) left after the light came on. This was calculated using the "I Ran Out Of Gas" threads on here and based on how much gasoline the car would hold.

As for contaminants being worse in the last portion of the gasoline, I'm confounded. The fuel pump pulls from a point near dead-bottom of the tank, so any contaminants that would be there when you've got a gallon left are going to be the same contaminants that are there when you've got a full tank. That is unless you figure the contaminants float. Perhaps they do, but I'm not familiar with floating contaminants. Water, the one I'd worry about the most, will most definitely sink and as stated above, once it's sunk, it doesn't matter if you've got a quart of gasoline above it or 15 gallons, it's still on the bottom.

Sea Ray

My thought would be that if you do run it very low, the sloshing around may stir up or knock loose any crud that may have built up on the bottom? I have never seen the inside of a gas tank so I don't know if build up even happens.

jdl

Another thing to consider -- the fuel pump unit sits within the fuel tank, like most cars these days. The fuel itself provides a pump-cooling functionality during operation. As such, regularly running on very low fuel may reduce the operational lifetime of the fuel pump, so it's probably a good idea to fillup at 1/4 tank. Just a thought.

Cheers,
-jd.

mngpao

I'll bet that's why the manual says to fill up at 1/4 tank.

JimW

Thanks for the help posters. beachdog, you have the best idea. I guess filling it up, till the handle clicks twice and driving 20- 40 miles and then fill up, till it clicks twice at the same pump, is enough to accurately record mpg. And it is the best idea to follow what the manual says about not going below 1/4 tank, but to drive less than 200 miles before fill-ups is a hassle at times. Well I never meant to make a habbit of this, it was just to test the "M" flash for mpg, but I will follow beachdogs advice. Thanks All.

zoom44

i have had the occasion on a long trip to put 15.3 gallons in that tank. closest i ever want to come ever again. especially if the wife is in the car.

mngpao

Unless the 20 - 40 miles is very typical of your normal drives - same number of starts, stop and go, hiway travel, etc. - you may not get a very accruate or meaningful number.

Xystas

I run until the light comes on, and drive for another 5 miles...
Usually my fillups are 14 gallons.

Spf006

Just posted to a competition related thread, but FWIW: another good reason to keep that gauge above 1/4 tank follows...
I recently took my baby out on VIR and almost IMMEDIATELY after the gas gauge dipped below 1/4 I would lose power consistently out of sweeping lefts. Happened in the EXACT same spot on course three laps in a row, then I finally figured out what was going on! (newbie, yes... I think I was still too excited about even being on the track to pay attention to the fuel gauge and make logical deduction)

I can only imagine that this problem may be exacerbated by autox-ing, but physics has been known to prove me wrong before!

FWIW: you probably want to start your run with just above 1/4 tank to avoid inertially induced fuel starvation, and to help balance the car... it was designed with the location of the fuel tank in mind for the f/r balance.

$0.02 from a guy who's never tried autox, but will be playing soon!

JimW

Yes, my driving style is the same. It's 100% city driving. 20 to 40 miles will get me back and forth to work a few times. I read a post where someone put in several different brands of fuel, same octane and was able to net better mpg as well as the car feeling more responsive. I tried this due to the station pumps ( Chevron)quiting after a few gallons. I then took a drive to the Amaco and filled the rest up, this was along with several gallons of Shell I already had. The car felt like it had more power and pull off the line. I don't know if this is something with the ECU or that all the different blends contributed to this, it sure wasn't temperature, because it was in the middle of the day and 85 degrees. I had the car for a month since the "M" flash, it would be strange if it kicked in even more power after this length of time. I don't know, anyone else try this?