Just out of curiosity dose your gas gauge do this. When I drive the car my gauge seems to fluctuate a lot. When I start it it seems to drop half a tick to a full tick right away or with in a couple of minutes. As I drive it the gauge slowly comes back up. Every time I start it it dose the same thing, the gauge is always lower then when I turned it off. It seems even more pronounced at the last quarter of a tank. Anyone else get this?

 

Yes. I get excited by noticing how many miles per tank I have then the excitement is shattered when it drops two or three ticks in a couple seconds.

 

I don't think that is normal. Sounds like your sending unit, probably located in the fuel tank, is acting up.

 

I think everyones probably does this. The system is updated constantly and is always calculating the position the gas gauge should be at according to how your currently driving, i.e. speed, throttle psoistion, etc... I would think its normal. Correct me if Im wrong though.

 

Seeing that the RX has a saddle tank, which straddles the drive shaft there must be a pump that moves fuel from one side to the other. This could explain the wired movement. This is just speculation, anyone no the facts of this?

 

YEA that's normal, quite takin them corners fast. With all that gas sloshin around, it gets confused.

 

Doesnt seem normal to me. Mine does not behave that way. The sweeping action is very steady (and fast!) as gasoline is being sucked outta the tank.

 

I am not 100% sure about the RX-8 tank configuration but I work for a Fuel Pump and Module Manufacturer and know of this same problem for other similar designs (saddle tank) were the gauge has "funny" readings. There is a venturi that transfers the gas from the "minor" side of the saddle tank to the side where the fuel pump is located. When the engine is tured off, some of the gas from the fuel pump side of the saddle tank flows back to the side without the fuel pump. This gas transfer (when the engine is off) causes incorrect fuel level reading for a short time when the engine is restarted. Our customer has installed a check valve in it's newer products to prevent this problem from happening. We don't make the fuel pump and module assembly for the RX-8 so I don't know the specifics of their system.

 

I stand corrected...but it is kinda alarming how quickly the gas gauge needle goes from "full" to "3/4"! For me it's about 20-25 miles.

 

I'd laugh if I weren't crying.

You have no idea how often this happens to me.
"HOLY CRAP! I've driven 100 miles and it's at where I normally drive only 70 miles! This tank is gonna kick ***!"

two minutes later, the needle has dropped 2 ticks.

"Damn."

It happens the other way to me sometimes though. Like according to the gas gauge I've driven like 20 miles less than I normally do at that point, but give it a day or so and I'm back to normal. The gas gauge is not to be trusted.

For me, usually I go to quarter at 50-60. To half at 110-120. To 3/4 at around 200. To light going on at 230.

 

Could be worse.. yanno if your low gas warning light didnt work properly at all.
I notice this happening every so often but as you and others stated its just the gauge adjusting itself. I had a Chevy W/T before this and that thing was yeah horrible, I never knew how much I had in the tank because a friend speculated that the floater or whateva was most likely messed up royaly. I'd fill the tank up, all 27 gallons of it and it would either be at Full or be about 1/8 the way past Full. I'd say keep a eye on it and if it annoys the hell out of you take it to Mazda and ask them to take a look at it.

 

It happens to me also, which is weird because I will be at just above 1/4 take for 40 miles

 

yeah i'm with ya, mine enjoys staying at a 1/4 tank for awhile

 

Yeah Its The Fuel Moving From One Side To The Other

 

^ +1

 

lol! youve just described the exact emotional rollercoaster i ride with every tank of gas i drive on in my rx-8, including mileage umm milestones.

to the poster: i dont think ive noticed anything as drastic as what your describing, just an overall "the needle isnt consistent" sort of thing.

 

my moves around a fair bit, but it's almost always a consistent 70 miles per quarter... and another 50 or so below E...

 

Well at least I'm not going crazy, glad to hear your all in the same boat. I also have felt the crushing pain of what appears to be great mileage disappearing in the final 1/4 thankful. Its like having a good looking girl slowly unbutton her sweater only to find out she is wearing a tee shirt ;-)

 

Mine seems broken, it drops to the bottom like a rock

 

I WISH... I get ~200mi off a gas tank, so it's 50mi at each major tick 'cept for the last 1/4 tank of gas is hillarious. I'm at like, 200 with just under a 1/4 left, then at 210 I'm on the light and at 220 (if I'm lucky) I'm below the red final tick.... it's hillarious how OFF it is on that last 1/4 tank.

 

Also, fuel expands a lot with heat. That will make the float change. Try putting gas in a red plastic gas can and set it in the sun. It will expand a lot. So driving, the exhaust will heat up the tank and the gas will expand.

Mike

 

I mainly notice that the gauge doesn't go down consistently for the readings so much. It's not that far off but doesnt' drop consistently. Starting at full, my has around 80 at the first 1/4 down mark. It has around 150-160 at the half, the next quarter shows up anywhere between 220-250. My light will come on around 290ish.

 

Very good point, I forgot the exhaust pipe runs right next to the gas tank. This would explain why the gas gauge rises when driving in the car for a while and falls again after the car cools off.

So based on the feed back we now know the gas gauge is a little inaccurate and the heat from the exhaust further effects the accuracy by expanding the fuel during use which accounts for my gauge raising during use and falling considerably when the car is restarted the next morning or at the end of work. It would also explain why the readings fluctuate more when your down to a quarter of a tank, the gas expands even more because there is less of it absorbing the heat from the exhaust. Put the saddle tank into the equation and the mystery is solved.

Excellent, thanks for the input.

 

This is not good. If this happens to me tomorrow I may run out of gas before I make it to shell(5 or 6 miles away.) I have about 2 or 3 ticks left but no light. I'm at about 250 miles.(70% highway)