I picked up a JDM 2008 RX-8 at the end of September last year, and I have been right around 8-8.5mpg over a about 6 fill ups of so since then. I serviced it, changed plugs, wires, coils, oil and coolant. It has 88,846 km / 55,206 miles right now.
Setup:
K&N Typhoon FIlter
FEED Rear Exhaust
Stock header
Stock midpipe and cat (I think its original)
Thats it..

The exhaust smell is strong when standing outside the car. No CEL codes being thrown. Drives fine as far as I can tell ( I have never driven another 8 for reference), other that there is sometimes a break up in the high RPM, like above 7.5K rpm maybe. The previous owner did not used to pre-mix. I have now starting pre-mixing. I have not done a compression test, but the car starts hot or cold every time.

What could be causing this bad of a mpg?

 

So, you're getting 100miles per tank?

 

a little more than that. I got about 125 miles from a full tank to about 1/8 a tank left at the last fill up.

 

Actual "mileage" in "non-freedom" units for the last fill up was 201.5 km used 55.97 litres (full tank to refill)

 

Where do you drive to? What are the distances? Are you a mix of city and highway, a lot of city, how much stop and go? A lot of things factor into your actual fuel economy.

The aftermarket short ram hot air intake is going to have some pretty negative effects on your power and fuel economy. Strongly recommend ditching it and getting a factory air intake box back in the car.

Having a significant fuel smell coming from the exhaust is not always a great sign for the condition of the catalytic converter. Have you had it inspected to see if the catalyst media internally to it is still intact? If it is clogged or damaged that can cause significant impacts to the ability for exhaust gases to flow freely. It could not only hurt fuel economy, but it can cause exhaust gas temperatures to be much more elevated and cause damage to the motor as a result. Strongly recommend having your catalyst properly inspected to inspect for any damage and replace if needed.

 

My driving is almost all city, no sustained long rolling runs. 85% stop ad go i most cases. Average daily run is maybe 11-15 miles total.
I have not yet checked my cat. I have been researching / hunting / shopping for a mid pipe to replace the stock one as I suspect it may be on its way out. I don't have emission requirements where I am, but I am also afraid of the heavy smell if it will get worse than this if i go catless. Also i don't need extra noise.. So I have been trying to find the best and most economical way forward.

 

85% city stop and go, yeah your MPG numbers make sense.

 

half to a third of what everybody else is getting?

 

Pre-turbo, I averaged about 130ish miles per tank with probably about 75%-80% city driving. These cars get their best fuel economy on longer drives with minimal stops. Sounds like youre victim of majority city driving.

The fuel expense is the main reason why I ended up buying a second car, the amount of money I was saving on fuel expenses covered the finance cost for my Mazda 6. The only extra expense I took on was the slight increase to my insurance costs, but because of my annual mileage on the Rx8 going way down along with a multi-car discount, the extra insurance cost was only like $20-$25 a month or something like that for full coverage on both cars with roadside assistance (in my case anyways).

 

My other daily is a 5.7 Litre Hemi Ram Truck. You know, surprisingly they are both pretty much the same vehicles... basically same 0--60 and mpg . They just handle a little differently.

 

Last time I calculated my car's fuel economy I was averaging 13mpg since I rarely ever take the highway, and because I redline the engine at least once a day. Not ideal, but could be worse.

 

I would be much happier with 13 mpg.. I generally red line 2-4 per day out.. but at least once

 

Purely city driving in Europe's 2nd worst city for traffic, an 8 will eat up 20L/100km if you really have to do it at the worst times. Even so, to gulp down a tank in just 200km... only my boosted 8 can do that while on a trackday.
You either drive around using only 1st gear or something's up with the car.

 

I am in 1st gear in the bumper to bumper stuff. I normally move in 1st and go up in the gears when there is room to move,
What could be wrong?

 

You can't be doing that for 200 km straight. Might as well walk with a hazmat suit and respirator if that's the case. I really don't see how a well runing 8 can do 30l/100km unless it's being driven on a racetrack. Mine did 29 last time I tracked it.

 

Yeah 8mpg is not healthy in any situation. If you smell rich exhaust even when the car is warmed up and idling, your cat might be dead. Have it inspected before it kills your engine. Trash the K&N thing. Stock, Racing Beat or AEM for intakes, and anyway there's nothing to be gained there.

 

Exactly Loki, that K&N hot air intake definitely takes a bad fuel economy situation and makes it much worse. A dead cat makes things worse too (assuming thats the case).

I grew up in Florida, specifically Miami as far north as Miami Beach/Brickell and south from there into Homestead and throughout the Florida Keys. The traffic from that Miami Beach down through to Homestead definitely took its toll on my fuel economy and I only achieved the low 10's in mpg due to some highway driving I got to enjoy bumping that number up. The hot climate and traffic he could be experiencing could be what is making the MPGs so bad. It can take a very long time to cover even short distances which obviously can significantly contribute to the MPG calculation going further into single digit numbers. Not saying he doesnt possibly have other issues, but the intake and the geolocation are definitely two major factors.

Have you inspected that there are no areas that your fuel tank is leaking from? Our fuel pumps have experienced recalls and are known to leak at or around the point under the driver side rear sear at the top of the tank where the pump housing connect to the fuel line.

 

I am in Jamaica, so its even hotter. I bought a used stock airbox and waiting on delivery. Lets see how that works. Still trying to figure out what to do about the cat. A visual inspection is enough to tell if it is bad?

 

A visual inspection inside the cat yes. You should be able to easily see through it. If it looks clogged, you'll need to get another or straight pipe it.

 

When I did the CAT delete on mine I replaced it with a straight through muffler to help with cabin noise because believe me, an N/A Rotary gets really loud without any mufflers.

 

Inspected the "cat" today...

 

Hmmmmm, do you know if it was deliberately gutted or imploded and the pieces are now blocking your muffler?

 

Resonator side..

 

I don't know much about cats and gutting them, but this look clean. I assume (might be wrong) that if it was fa ailure, you would probably see some fragments still hanging on the side, or something 🤷

 

If your car is a JDM version does it have the 5 speed or 6 speed? Just for reference I had tried the JDM map which was much richer than the USDM fuel map on my 6 speed USDM transmission 4.44 final drive and was about 12-14mpg highway (bumper to bumper traffic) driving

Yeah best to get rid of the k&n typhoon hot air intake. Then do a smoke test on the system. You should try doing before and after the throttlebody to search for vacuum leaks. You can get oil smoke generators fairly cheap or I have even seen people use cigars but obviously that is up to you what you want for residue inside your engine.