Schwendy

My 79 first gen RX7 seems to have a gas gremlin lurking about. I have new plugs, timing is okay, oil change, running pretty good, but I swear I only drive it about 30 to 50 miles and the gas tank reads half full! New fuel filter too. Don't smell gas or see any leaks anywhere! Where is my gas going? I use premium only. Do old RX7's burn THAT MUCH!

Hellstrom

There might be an issue with your fuel gauge. How many miles do you get out of a full tank?

miasmicmonky

iTrader: (1)

Also unless you have tuned your timing for premium grade gas or are turbo, you will get worse gas mileage. our cars are designed and run best on 87 grade gas and really like no ethanol. i filled it with 93 the first time i put gas in after i bought the car and had around 15 mpg. i went to 87 the next fill up and immediately went to over 20 mpg under the same driving conditions. Also don't forget to red line the car once a day or take it out for a 20 minute drive on the freeway once every couple of weeks to help reduce carbon build up. carbon build up seems to reduce gas mileage and efficiency also.

However hellstorm's question still is pertinent. How far are you getting on a tank of gas before filling up? how many miles are you driving between filling up and how many gallons are you putting in at fill up?

Schwendy

Thanks for the responses! I never let the tank get too low so I'll have to do some sort of calculations to see what mileage I'm getting. Just my initial observation, shock actually, that the gauge drops so fast! It could be a bad gauge but it does go to full when I fill it all the way up and does drop as I drive so seems to be working. Didn't know about the premium gas issues. I thought I was supposed to use premium! I'll try regular next time. Maybe the tank is so small and I'm just used to filling my F150!

miasmicmonky

iTrader: (1)

Some people don't realize n/a rotary engines want a fuel source with quick combustion. the 90+ grade gas burns slower and does not give full efficiency thus causing a loss of power. It is great if you want to show off your backfire. In the 80's, they used to use 85 and sometimes as low as 83 grade gas in rx7's on the race track. If you ever get the chance try to use ethanol free gas. you can find ethanol free gas in your area at pure-gas.org. It costs more but as for fuel efficiency it is usually justified by the price. However the distance to get to a gas station with it may not justify the price. haha.

Evil Aviator

The fuel gauge is not linear.

Do this:

1) Completely fill the tank with fuel.
2) Zero-out your trip meter, or make a note of your car's current odometer reading.
3) Drive the car as you normally would.
4) When the fuel gauge gets down to about 1/8 tank remaining, fill it completely with fuel again. Make a note of how many gallons were used, which is usually printed on the receipt. Also make a note of how many trip miles were driven, or figure out how many miles were driven based on the odometer reading.
5) Calculate the gas mileage by figuring the miles divided by the gallons. For example, 199.7 miles / 11.745 gallons = 17 miles per gallon.

I do this on a regular basis to monitor the health of the car. It doesn't take much effort if you just write the numbers on the fuel receipts.

The octane rating is only an indication of the fuel's anti-knock properties. If your engine does not have a problem with knocking (due to turbocharging, high compression, high loading, wear, hot spots, etc.), then you are just wasting your money on the higher octane fuel. It depends on specific formulation, but it is not uncommon for lower octane fuel to yield slightly better power and gas mileage, although the difference is usually within 1%. I have even seen one case in which the mid-grade fuel was the worst performer of the three grades.

Besides the power and gas mileage myths, the other common misconception about premium fuel is that it burns cleaner. All pump gas in the USA must meet strict government detergent standards, so any grade will work fine. If you want more detergents, then just use any grade of Top Tier gas:
Top Tier Gasoline

LOL, I have the same issue. My 1988 RX-7 gets about the same mileage as my 2000 F150, but it sure seems like the RX-7 spends more time at the gas pump than it does on the road.

The octane rating is based solely on anti-knock properties. The other properties of the fuel will vary depending on which of the many methods a given vendor uses to obtain that octane rating. Therefore, the flame speed is not necessarily related to the octane rating. If you are concerned about flame speed, then you will need to ask the fuel vendor for that specific statistic.

That is really only for racing fuel, which can be radically different to the point that it may even require major modifications to the fuel and/or ignition system. However, at a given gas station, the various grades of pump gas will rarely require retuning on an engine that is not prone to knocking, and they will all yield about the same power and gas mileage unless they vary greatly in alcohol or aromatic content. In most cases, the only significant difference is the anti-knock property.

Yes, ethanol sucks.