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Old 11-13-2014, 08:16 AM
 
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Hi,
I have an Infiniti I35 2003 . It spits 240 hp and it effortlessly passes any vehicle. But when I drive in freeways, the engine RPM goes as high as 3.5 to 4. And even within city for 35 mph, the RPM is around 2-2.5.

I remember getting the same speeds with 1.5 rpm with corolla, elantra etc and they were only around 140 hp..

Does this impact the mpg, as I get only around 18 on an avg..
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Old 11-13-2014, 08:22 AM
 
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Depends on many factors. Engine load, gearing, weight of the car, etc etc. RPM alone doesn't affect MPG. Chugging a car at 1.5K RPM under load could be worse for MPG than if the car was crusing at 3K RPM under little load.

I have an Infiniti G35, and at 80MPH it's turning 3200 RPM, but I still get 25MPG or so at this RPM. The Nissan 3.5L isn't exactly a fuel efficient V6 either. I'm lucky to get 20MPG mixed with it
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Old 11-13-2014, 08:23 AM
 
Location: Broomfield, Colorado
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Every engine has a "sweet spot" where it's intended to be cruised at. For that particular engine, I don't know - diesel is my trade. But, to give an example, the Paccar MX13 engine is cruised to run optimally at 1100 RPMs, whereas the newer Cummins engines have their "sweet spot" at 1400 RPMs, which Cat highway diesels traditionally have theirs around 1350. A gas engine, I'm sure will be higher.
Excessive RPMs simply waste fuel. But you can't compare an Infiniti to a Corolla or Elantra... very different vehicles, different weights, different drag coefficients, etc.
If you're worried about fuel mileage, start by getting a service and tune up done on it. Make sure your plugs aren't fouled, make sure your timing is right on, make sure you don't have any restriction in the air intake, since the stochiometric ratio (ratio of fuel/air mixture which goes into the injectors) is very strict on a gasoline engine vehicle (diesel has a little breathing room with this), make sure your lubricants (gear oil in the differential, transmission fluid, engine oil) aren't burnt out, etc.
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Old 11-13-2014, 08:23 AM
 
Location: Prosper
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Engine RPM is determined by your gear ratios at certain speeds.

If you're measuring the same speed as you did with your old Corolla, that means you were probably in the highest gear, or an overdrive gear, while the Infiniti is not. It has nothing to do with the amount of horsepower the car makes.

Corvettes for example use a very tall 6th gear. It's common to see the RPM at 1300-1500 while cruising at 60mph.
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Old 11-13-2014, 08:27 AM
 
Location: Broomfield, Colorado
656 posts, read 927,348 times
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Your RPMs are speed are determined by a number of things, not just your differential gear ratio. Gear ratio of the transmission gears, tire size (that also includes increased revolutions per mile due to tire wear, underinflation, etc).
Below 55 MPH, your primary drain on fuel mileage is going to be rolling resistance... when your tires are underinflated, the rolling resistance increases. Between around 55 and 60 MPH, half of your fuel spent goes to propelling the vehicle, while the other half goes towards overcoming wind resistance.. as you go faster than that, the amount of wind resistance increases, as does the amount of fuel needed to overcome it. So if you're driving along at 80 MPH with underinflated tires, no, you're not going to get what the EPA estimated your car to be at, fuel mileage wise.
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Old 11-13-2014, 10:04 AM
 
Location: Denver
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If you are doing 70 mph and the tachometer is showing 3500 to 4000 rpm the transmission might not be going into overdrive. Count the shifts as you accelerate and make sure it is using all the gears.
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Old 11-13-2014, 10:15 AM
 
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The 2003 Infiniti I35 is rated at 255HP using the same VQ35DE as the 350Z and G35 and has a 4-spd automatic. It's FWD.



my G35's 5-spd auto turns 3200RPM at 80MPH. I've seen similar RPM numbers at that speed from 350Z's and M35's and other nissans using the 3.5L.


Barring a transmission issue and the trans not using all 4 gears, I would say it's normal.
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Old 11-13-2014, 05:37 PM
 
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Thanks for the inputs. And what is the 1-2-3d on the panel? I only used 3 and never used the other 2. I heard the lower gears can be used for uphill roads?
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Old 11-13-2014, 08:20 PM
 
Location: Prosper
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Uh, that's your problem right there. You should be in "D" not 3.
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Old 11-14-2014, 07:54 AM
 
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Quote:
Originally Posted by MckinneyOwnr View Post
Uh, that's your problem right there. You should be in "D" not 3.
The I35 shifter is gated and has a slot for "3D"



So the OP is in the right gear. I do not know if there is an overdrive off button though.
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