I have been looking at this site and for a 05-06 6spd sedan for a while now. The main gripe I've seen with this car is gas mileage. I understand that Nissan geared the car for performance, but after driving a few, as many others have noticed, the RPMs are quite high at highway speed. That combined with the fact that 1st is pretty much a granny gear, leads me to believe that there might be some benefit in taller gearing. I've read on here a few people making claims of increased mileage with a taller tire. And since I'm not particularly crazy about the factory wheels, I was thinking that 245/45/19s would be an easy way to effectively lower final drive ratio by about 5%. Only problem with that is speedo calibration. Has anyone here had there speedometer calibrated? And if so, how much did it cost you? Another thing that I noticed is that the AT cars have a slightly taller rear end(about another 5%). I wonder if there is any difference in the rear housings, axles, driveshafts, pinion angles, and what not between the AT and MT(I shouldn't think so, but who knows, seems there is a difference between the coupe and sedan rears)? Of course I wouldn't want to loose the LSD either. So I guess the plan is to see how the slightly taller tire will effect mileage first, since I'd want to replace the rims anyway. If there's no benefit, I could just drop down to 40 series tires. If i did see a benefit, then I might regear the rear, or just replace it with a AT LSD, and see what benefit that might have. Has anyone tried anything like this, and if so, what were the results? Please save all the "if you want mileage, buy a Prius" comments. I have an Eclipse GSX for my high speed gollies. Thanks, Dan

 

If you're 5% out of spec like that, you'll likely run into issues with the VCD. Guess you could drive with it off all the time, but that's dangerous.

 

Hmm.. never tried it but with all the stuff you are doing it might only increase MPG by a little bit. And as for your GSX, shouldnt that car be saving more gas than the G? Given that you dont hit boost too much LOL. its hard though I know. I loved it the last time I was in a GSX.

 

Hmmm. What I can find related to the VDC system seems to indicate problems if there are height differences between front and rear. I'm talking 245/45/19s all the way around. Is there something that you've seen that suggests otherwise Ryan? Not terribly worried about that system anyway. I had a Supra at one time, with about 500hp, no traction control or other gizmos, and managed to keep it out of the trees.

Haha, I'm lucky to get 20 mpg with the DSM. Big turbo plus big injectors equal lots of fun, but not great mileage, lol. It get's mid 20s if I baby it. But it's not a DD type of car for me anymore. I'm in my mid 30s, time to at least appear somewhat responsible.

 

I always thought that if it was more than 3% out of spec you could run into problems. I could be wrong though. Could just be if there's a difference between front and rear.

 

MT does have different suspension. I am not sure if rear struts on AT are a bit taller....

 

You will only run into problems with VDC when the tire circumfrence is different by 3% or greater. It will be fine with a square setup, a number of members run 245/40/19 on all 4 corners.

 

i dont have specific mpg numbers to give you but my 6MT has the LSD from the 5AT which i believe is 3.2. i spend most of my drive time on the highway (64 rep va) and i love the difference simply in rpm. im at about 2750 at 80 and rollin 85 at an easy 3k. that difference definitely helps mpg... just not sure how much.

 

While the gear ratio will be increased with a bigger tire, and thusly increase the distance travelled in relation to revolutions, you need to rememer you're also adding weight, and the further fromt he center of the wheel you add that weight, the harder those wheels are to turn.

For the G, with it's endless torque, this isn't a huge deal. But keep in mind that when your goal is fuel mileage, normally keeping the same size and going with a lighter wheel is more desireable. Unless you granny it around town and do 80% highway driving.

Food for thought of course. It can always be argued that lighter wheels have less inertia and thusly can result in lost mileage on the highway. So it's kind of a give/take scenario in either event depends mainly on your application (city/highway, and how hard you drive the car)

Also bigger tires raise the car, unless you're putting springs on it you're altering the COF and that's going to cut into mileage on the highway also.

IMO, there's a lot more effective ways of increasing your fuel economy such as better flowing intake and exhaust mods and a tune. Maybe a slight drop as well. And low rolling resistance tires.

Then swap gear ratio in the rear end if you desire it. Check the Drivetrain section there's a few guys anyhow that have changed gear sets.

 

the speedo is derived from the wheels speeds sensors, so changing the final drive ratio won't upset the speedo.
Larger diameter tyres will give the same result, but will upset the speedo.

Higher cruise RPM, doesn't necessilary mean reduced fuel economy, so if your only goal is economy, you might end up disapointed.

 

so you want to spend thousands of dollars on gearing and wheels to save a few hundred at the pump?

 

OP, from my research, theoretically you should be getting 1-2 mpg better with 3.3 FD on highway, given all other factors unchanged. It's something but not earth-shattering better. I think it's reasonable for the cost of the 3.3 FD rear differential. If you plan to keep the car stock, just check your intake filter often, and make sure your tires are properly inflated. When it comes to replacing the tires, get something weigh less and with lower rolling resistance, those things should also help.

 

If you're going to put all that work and money into it you might as well just leave it the way it is and put it in the gas tank.

 

My front and rear tires are way different sizes (4.9% difference) and I've never had a single problem with VDC