Fuel Mileage Question

[mikeysentra]

I have a question on fuel Mileage with my 97 XE 4x4. I have been getting 14.44 MPG with each fill up for about a month. When I first got the Pathy in may I was getting 15.9. I am just wondering what you guys/gals are getting for mileage and what I can do to improve mine. My 97 has 132,000 miles on it and every record from the dealer from new. it has been well maintained and I am the second owner. I have replaced the Fuel Filter, all fluids installed a K&N air filter, cleaned the maf 3 times With every oil change with CRC MAF Cleaner, replaced the Cap Rotor and most recently the Plugs and wires with factory replacement NGK parts, and installed a Optima Red Top battery. I have replaced the factory sized tires with 31-10.50-15's on the factory wheels and had it aligned and the front wheel bearings repacked when I installed the Warn manual hubs. is this the mileage I can expect from running 31's or is there somthing I can do to get better MPG out of her?

 

Thanks

[FUELER]

Damn you mentioned just about every MPG maintenance that I could readily think of.

 

As far as the 31" tires, they will impact the MPG, not just because of the extra centrifugal weight, but primarily because your speedometer/odometer will be slightly off. I'd guess that your measured MPG is off by around 10% just because of the incorrect speedometer.

 

Heres a calc, mycomp is too slow so play with it and see what the #'s are

 

http://www.miata.net/garage/tirecalc.html

 

The speedo and odometer get their info from the speed sensor gear in the transmission, which is geared to be correct with 29" or 30" whatever stock is....

 

So with a 10% correction factor, 14.44 off the Nissan odometer is in reality closer to 16 mpg

 

Using GPS to track your mileage rather than the nissan odo, you'd see the real results.

Edited September 26, 2007 by FUELER

[XPLORx4]

The difference in your new mileage vs old mileage is 15.9/14.44 - 1, or about 10%. If your observed mileage has gone down since installing the 31" tires, then that's the expected result. And to be honest, a 10% observed change is actually pretty good! Here's why:

 

31" tires are larger than the originally-installed size of 235/70R15. This will introduce an error in the speedometer and odometer. 235 tires are about 28" in diameter. 31" tires are 31/28, or 10.7% larger. The larger-sized tire will cover more actual distance than your odometer indicates. In other words, your odometer will read 10.7% lower than you've actually traveled.

 

Additionally, 31" tires are heavier and have a little higher rolling resistance, which will tend to reduce fuel economy.

 

Finally, the 4.363:1 final drive ratio (ring/pinion gears in the differentials) in your 97 XE is intended to be used with smaller tires in order to help keep the engine operating in its most efficient RPM at typical highway speeds. The larger tires shift the RPM lower, forcing the engine to work a little harder (i.e. out of its power band) at typical highway speeds.

 

All of these factors combine to work against good fuel economy.

[zonianbrat]

Just tonight I have been checking into this myself. I am in the middle of nowhere but the rural highway has mile markers and I determined tonight that my odo is exactly 1/10 of a mile off or 10%. I am running 31x10.50 BFG At's. And I also have a 97 XE 4x4 with 132,000 miles.

[vengeful]

Hrm....odd you mention tire size correction Dean and Fueler...because when I upsized from my OE 265/70R15s to 32x11.50x15s, my odometer actually started reading accurately for a change. It read significantly fast with the OE tire size, when compared to my GPS unit and mile posts on the highway. Now it reads accurate to within 0.1% error - and I didn't change anything. Granted, I do have the SE, which comes with slightly larger tires from the factory, and the 4.636 gears, but you would think that Nissan would have the foresight to properly calibrate the odometer before shipping out a vehicle.

 

In any event...with my 140,000 miles, 3" Lift, 32s, several DTC codes (EVAP, IACV, EGR, etc) and a bunch of crap on the roof, I have been getting 16.5-17.5mpg mixed lately. I do have the Warn hubs, I've got fairly new (June 2006, 25,000 miles) wheel bearings, and a good alignment. I run my tires at 42psi front and 36psi rear - mostly, I do have a slow leak in my RF tire, so sometimes it runs a little low. I use plain old 87 octane gas, and do 10,000 mile oil changes - which reminds me, I'm due. I run a can of SeaFoam through the brake booster every other oil change (appx 20,000 miles), and one in the gas tank every oil change. I have a leaky exhaust, and an ARB snorkel.

 

After installing the snorkel, I have noticed a fairly significant increase in highway fuel economy, and a significant increase in coasting ability. The "Ram Air" factor of the snorkel seems to force more, cooler, air into the engine allowing it to coast down hills at 65mph instead of powering down them. I also noticed that I am able to cruise right up hills at 65mph without downshifting or unlocking the torque converter now that I could not before. Strictly highway driving, I am seeing anywhere from 17.5-19mpg. Driving habit has an extremely significant impact on fuel economy. At 60-65mph, I see 17.5-19mpg, depending on terrain and traffic. With more traffic and flatter terrain, I can see the sunny side of 19mpg. At 75 mph, my fuel consumption increases dramatically, and the economy drops to about 15-15.5mpg. I do not have fuel economy data for above 75 mph, because I do not hold speeds higher than that for extended periods.

[adamzan]

Yeah I try not to allow the engine to go over 3000 rpm, which means I can only go up to about 110 km/h which is roughly 68 miles an hour. Although i'm not proud of this, I've had it up to about 100mph or 160kmh. Boy did that ever chew up 1/4 tank fast.

 

If you drive so that the rpms are under 3k, you'll probably see an increase. I did.

[mikeysentra]

Thanks for the input. I forgot about the 10%. I add the 10% and I am about 15.8 and I can accept that.

[zonianbrat]

I have been driving in alot of hilly terrain lately and she has been getting around 15-16 mpg. So Dan, you think the snorkel has really helped out your mpg? Well there is a good reason to get one. I was not going to get one since I usually avoid river crossings, lakes, ponds that come up to the door because A I like my interior and do not want it ot stick like poo B I do not want to hydrolock. I might have to do some research and find your how-to and part # on your snorkel install Dan.

[XPLORx4]

zonian- Getting better gas mileage is totally the wrong reason to get a snorkel. There are way simpler ways to achieve the ram-air effect to get cool air through the intake. Take off the lower part of the intake that's behind the bumper, in front of the wheel well. Cut open the plastic underneath the left turn signal and put a wire mesh screen (such as a rain-gutter guard or crawl-space vent screen) over the cut-out. Use a 3" or 4" plastic flexible drain pipe or dryer hose to connect the hole under the turn signal to the bottom of the intake.