What RPM is your car at speed, say 70 mph, in 6th gear? My car, with a UMW tune, is about 2500 rpm or so at that speed. I think the car could easily use a taller gear and get down to 2000 without any issues at that speed, which would also help fuel mileage. 6th gear is basically useless on these type of sports cars. Top speed is often found in 5th gear, not sure about this specific car, or unless you're heavily modified and trying to go over 200, which only a few ever attempt.

 

Not sure you get what is going on in a motor. Just because you are reving less does not mean you are using less fuel. When I drove to Texas i was get on average 300 miles to 10 gallons. I was doing 80-90mph most of the time with 72lbs injectors. The other car that drove with me was doing even better then that and went faster in the mile, he also was on a stand alone ecu and was adjusting his tune as we drove for fuel efficiency. Load is what burns fuel.

 

I realize engine RPM and feul economany do not have a linear relationship, but all things equal, if I drive around in 5th gear vs 6th gear, I'm going to have better econmy in 6th gear. So, if a particular car with no other changes has taller top gear, it'll achieve greater fuel mileage.

I believe the OP was aksing about a taller 6th gear, not if he was running 72 lb injectors and adjusting his tune to achieve fuel mileage, all of which obviously impact fuel mileage.

 

Read post 13. you think that dropping 500rpm is going to save you gas.

 

Well not totally true, with a modern computerized engine it is dependent on other variables such as vacuum and load. If an engine is not in a happy RPM range; more so unhappy in an almost lugging state the computer has a hard time dealing with those conditions. It starts dumping fuel because that is what it needs to do to develop the power required to maintain the speed or RPM. There are many other variables that go into that too. Air velocity is one of them. At these lower RPM's intake velocity drops and you get many fuel burn issues associated with that; fuel after burn in the exhaust, pollution, excessive exhaust temps, hard on cats.

Basically put low RPM does not equate to lower fuel burn, if that vacuum pressure drops to a certain point the engine thinks it is under greater load like you just matted it even though U didn't. An engine would really fight to accelerate or even maintain rpm in those lower RPM ranges just because combustion is so stinted.


I know that applies to my Porsche, I cannot see yours being greatly different, 6th gear on my car is useless for speeds less that 60mph. It runs so poorly at that RPM / Speed range that it is unbearable. The only way I could see a taller sixth gear being useful on these cars is for Autobahn or Gumball rally cruising speeds. It definitely would not help mileage at regular hi-way speeds; and even if it did you would not pay for the cost of the mod in fuel savings in the entire life of the car.

 

Thanks. Nice logical post.

 

If you run to low of RPMS,thus causing lugging the motor,it will not get better milage >it will get worse mileage because its not in an efficent range..IMO
The MPH with the RPM that your engine likes is going to determine the gearing that is the most efficent for your needs..IMO
My sweet RPM range for mileage is around 2,400 RPM and that reads 71 MPH on my dash but I have larger profile tires so I have a higher MPH then that..