I have a 06 coupe (51k miles) and my car cruises at around 3000 rpm which I think is little high when I am 75 mph. Another car I have is 07 Acura TL type-s and this car cruises at barely around 2200 rpm at the same speed. Both cars have same size engine and same 5 speed AT. Is my G35 normal? If so, I wonder what can possibly make the difference in RPM range on the two cars. Does it have to do with engine compression? It might be a simple question but I'd appreciate if someone could enlighten me on this matter.



Regards,

 

This a gearing thing, not a motor thing. Completely normal, and purposeful to the design.

Every car balances power, and gearing depending on the needs of the design. You can't really compare one car to another that way.

 

The RPM at certain speeds is a static number that depends on your transmission, final drive ratios, and also the overall diameter of your drive-wheel tires.

Here is the gearing for your car at various speeds in each gear:

http://www.car-videos.net/tools/spee...B1=Recalculate

You should be at around 80 MPH in top gear. By the way... the term that you would use to describe driving down the highway is not "idling" it would be cruising. Idling is when the car is sitting still in park/neutral with no additional load on the system and your foot off the gas pedal completely.

 

About where my 06 is at that speed.

 

Thank you for pointing that out. I've changed "idle" to proper words.

 

When engineering a car, there is always a tradeoff between performance and economy in every respect (type of tires used, suspension settings, power output, and gear ratios, just to name a few factors). Our G35's are geared for performance and not cruising, so the gear ratios are lower (numerically higher) and closer together to provide maximum acceleration and performance, while giving up mileage and more wear on the engine as a result of the higher RPM's at cruising speeds. The only way to have the "best of both worlds" is more gears, which many of the newest cars are utilizing (7 speed manuals and 8 and 9 speed automatics are now the rage).

When I bought my car (a 6MT), I drove the Acura TL 6MT also, and remember the same thing - cruising in 6th gear at 75 MPH resulted in about 2,200 RPM, while my G35 cruises at closer to 2,800 RPM at 75 MPH. A quick look at the gear ratios for both cars made the difference apparent - 6th gear in my G35 is an overdrive, but BOTH 5th and 6th gears on the TL are overdrives (allowing for lower RPM's at cruising speeds but giving up some quickness and performance to do so).

My $0.02

 

I've always wished that they gave you factory options for rear end gearing on purchasing a new RWD car. I would have gotten a higher gear ratio (lower number) if given the option since I'm not usually ripping around at full throttle or racing people in my car and would love to save some RPMs, fuel and wear and engine/tranny wear and tear while cruising.