My first issue is my transmission. If i shift into 2nd at 3/4 of WOT to WOT above about 5500 rpm it will grind and kick me out. Shifting into 4th takes about 2-3 seconds, and is impossible until my revs drop to about 2500-3000 rpm or so, and this is with it grinding. Btw this only happens when the transmission is warmed up. I'm guessing the transmission is going so I am thinking about dropping a new one in.

My second issue is my gas mileage. I just calculated that on my last 10 gallons of gas I got roughly 15.5 mpg, with 130 of the 155 miles driven being highway miles averaging about 80 MPH, with about 80% of that time being in 6th gear, plus 60-70 of those were with no air conditioning and windows up. The engine was replaced by mazda about 3500 miles ago (car has 101,000 miles), so I would think the coils and spark plugs would have been replaced around then as well. What could possibly be causing the gas mileage to be so bad? I cleaned the MAF sensor and that helped very slightly, but this gas mileage is what you would expect of a hummer. the 6.2 liter denali XL I had got better gas mileage then this, and I didn't have to use premium. I know I could probably gain 1, maybe 2 mpg if I put the fender liners and undershroud on, but that still leaves me with atrocious gas mileage. I dont expect "good" gas mileage, but getting close to 300 miles on a tank would be nice. Could the transmission have to do with this horrible gas mileage? if anyone has any insight I do appreciate it.

 

not sure about the gas, but the rest sounds like synchro issues... im in the same boat with 2nd gear

 

Have you checked the condition of or adjusted the hydraulic clutch? If you haven't replaced the trans fluid with synthetic yet try that. How many miles on your last tune up....plugs,coils,air filter. How aggressive(gas peddle) a driver are you?

 

How do I adjust or check the condition of the hydraulic clutch? Im going to be having a new transmission put in for $800 with synthetic fluid. I can check the papers to know the miles, but the entire engine was replaced about 3500-4000 miles ago. When I had my 350Z I was an aggressive driver and still managed 24mpg highway, However I have been really calming on that with this car due to the gas mileage.

 

15-17mpg sounds like average.
Coils and plugs DO NOT replace with the motor, all they replace is the motor itself, only.
Transmission would definitely effect gas mileage, higher rpm=burn more gas.

 

Then I will look at the service record and see when those were replaced last. If it was a while ago I will replace those. Im having a new tranny put in in a week or 2. I'm going to make a home made grounding kit tomorrow, when I had the z it added like 1mpg. It might help, it might not but its worth a shot.

 

you should check what others are getting for mileage.
You're not far off the norm...

And yes, make sure you check the service records. My bet is you're running on bad plugs/coils. You could check your eccentric shaft sensor as well, followed by resetting the NVRAM.

Lately, did you follow the proper break in procedure, after the new engine was installed?

 

I bought the car when the engine had about 5-600 miles on it, so from what I read that is already broken in. Ill check the things you mentioned out as well.

 

Try double clutching. It that works then your tranny problems suggest bad synchros.
Your mpg seems low if you only drive on the highway. I'm pure urban and I get 12-14mpg w/ redline a day. On a roadtrip, I averaged 21mpg going 65mph.

 

double clutching is primarily for downshifting though, isnt it? the only downshifting issues I have is are if I try downshifting to 2nd around 50-55. If I shift VERY slow its fine, although 4th always grinds a little when its warmed up. I would be happy with 18-20 on the highway, but 15 highway is the same as a bugatti veyron, and that has a 1000hp quad turbo 8.0L W16 engine...