In October I upgraded to 305/65/18 BFG AT/TAs with a Rancho 2.5" leveling shock. Prior to the swap, I'd consistently get 17-20 MPGs highway averaging 65-70 MPH. After the swap, I get 15.4 MPGs highway no matter what (according to the stock gauge). In addition to the heavier tires, would the higher front end affect mileage that much, and would a tune/intake/exhaust recover some of the MPGs? I know I drive a truck, yadda yadda yadda, but is it possible? Thanks.

 

theoretically it will, but you will have so much fun punching it that it will nullify the gains

 

Yes. Take a look at getting a custom tuned Xcal3 from a reputable tuner like VMP Tuning.

 

First off, from what I've seen, the stock MPG monitors reads "optimistically," so you may never have quite been getting the MPG you thought.

Second, is your odometer corrected after the tire swap? Or have you figured a correction factor for the error? If not, your mpg (as well as speedo/odometer) will be reading incorrectly and your mileage calcs (whether the old fashioned way or by the factory meter) will be off. Going to big tires makes your truck read fewer miles than actually travelled, so your actual MPGs may be higher than indicated. Step one is to get a tuner or correction device that allows you to recalibrate the speedo and odo correctly. Or have the dealer do it (if they will). That will reset everything to read consistently and correctly. In the meantime, you can do some checks to determine how far you are off.

The quick check is to run your truck over several measured miles at 60 mph. Do it on a straight, level road. At 60 mph, you should cover exactly one mile in exactly 60 seconds and your odo should read one mile. If the odo reads 0.9 miles, you know that the odo (and likely speedo, the errors can be different on each, but not usually) are 10 percent off. Using a stopwatch, time your measured miles (do several until they are consistent) at 60 mph indicated. If you take 66 seconds to go 1 mile, divide 66 by 60 (66/60=1.1). 1.1 then becomes your correction factor for every speed or distance (reading .9 on the odo would also give you a 1.1 correction factor, which is 10 percent). 60 indicated mph is 66 actual. If you start checking your mpg the old fashioned way, miles indicated divided by gallons used, you can just multiply your indicated miles by the 10% correction (if your calculator has a percentage key) and add it (250 Mi indicated x 10%= 25+100=275 mi actually travelled), or multiply by 1.1. Therefore 250 miles indicated is (250 x 1.1)= 275 miles actually travelled. If you know your percentage of error, you can take the MPG readout and multiply by your correction to get a closer reading, bearing in mind it was a little optimistic to begin with). If you were indicating 15, then your actual would be 16.5 (15x1.1=16.5)

In my own MPG testing, the lift alone ( I have the same one) didn't cost MPG, but the tires may cost 1-2 mpg due to the increased rolling resistance (depends on the tire). The extra height from the lift AND the tire together may cost some MPG at high speeds due to loss of aerodynamics... perhaps another 1 mpg at higher speeds (faster equals more loss)... but it's the wider tire and aggressive tread that really increases rolling resistance.

IMO, your best bet may come from a bed cover, which by many reports can get back 1-2 mpg at high speeds.

As to tuners exhaust and intakes: My EVO later Gryphon tuner didn't do a whole lot in the mpg area... less than 1 mpg. The only real advantage is the extra timing advance, but you gotta have good fuel to go with it or the engine can't use it. Not premium, necessarily, but "honest" 87 octane. I've been monitoring the timing advance (on the Gryphon) and can tell when I got poor fuel because it backs off a few degrees from what's "normal". Though I haven't haven't 100% verified it myself yet, many people report better mileage using a premium fuel tune and premium fuel, but the extra cost of the fuel negates the slight gain in mpg.

I did see some gains with a cat back exhaust (Dynomax 3-inch single), but again, they were small. Air intake... no change. Maybe a slight uptick but not outside a margin for error.

IMO, all the devices I added barely made up for the loss from the tires and I went farther negative with a big winch and bumper (a solid 1 mpg loss at high speed).

Overall, all the stuff we wanna do to make our truck look good and work better off-road costs MPG. You can spend a lot of money trying to get it back. You might succeed but it will take years to pay off those improvements in the small fuel savings.

 

I think I remember watching an episode of "Mythbusters" where they proved this theory a myth.

I got my Gryphon a couple of weeks before my tires. On a stock truck it improved my mileage by about 2mpg and really improved the power, shifting and throttle response, the latter two making the tuner well worth the money. After putting 325/60/20's on my mileage dropped back to stock or possible just slightly worse. The power remained good though.

You can really feel the increased rolling resistance of the larger tires, I travel the same work commute every day and before after cresting a hill I could just let off the gas and coast, now I've got to keep going longer to keep momentum.

One of the handiest features of the Gryphon I've found is the readout of instantaneous mpg. It really allows you to find the optimum speed for the best mpg. It really helps you keep a light right foot too. I was pretty surprised at the difference on the highway between 60mph and 70mph. It was almost 5mpg between the two.

I'll be getting my custom Gryphon tune soon, hopefully that will improve things even more.

The bottom line for me would be, no matter if your truck is bone stock or modified, a tuner will make a huge improvement in the driveability and fun factor of your truck. I doubt there's a better way to improve performance for only $400

 

Thanks for the replies everyone. My correction factor is 1.111111 being that my measured commute to work with the stock tires was 40 miles and is now 36 with the new tires. Looks like I'll be getting myself a post Xmas gift.

 

IIRC, Mythbusters talking about tailgates up or down. The show is a lotsa fun, but if there is better, more scientific testing available that contradicts, I will discount them. Case in point:

The best place to look is the SAE. Technical Paper 2004-01-1146 (www.sae.org) is a tech paper that was based on an 2003 study.The SAE study is one you have to pay for and I can't post it here without copyright infringements. It showed by far the least drag with a full tonneau, except that on 8-foot beds, a half tonneau (rear half) did slightly better. Aero drag decreased translates into some level of MPG increase. How effective it was somewhat depended on the type of truck (ie crewcab shortbed, reg cab longbed, etc)

Also, there is a SEMA study done is '07 that showed a decrease in aero drag with a tonneau also. It's all over the web, so you can google for it. I don't have a link for you.

 

In tests, a tonneau resulted in a slight increase in mpg, tailgate down did not.

The only way to really know your mpg is mathimatically checking miles driven divieded by number of gallons used.

To the OP, your mileage, if accurate, is actually very good for that truck. Larger wheels and tires really eat into mpg. Also, did you recalibrate the speed after the taller tires? A tuner will help you get it accurate and make it more fun to drive, but additional mpg will probably be minimal.

 

Looks to me like you've got the tire size AND speedometer corrections all figured out. Just multiply your odometer reading by your correction factor at fillup and you get an accurate distance for gas mileage, and multiply your speedometer reading by that same factor to see what you are actually doing speedwise, if they're strict on speeding in Florida. (You're actually going about 6.67 mph faster than you show at 60 mph).

By the way, I can tell from your post that YOU understand this, but some other members may not. You've hit on a good way to "adjust" for the bigger tires until you can correct the speedometer, and it's a technique other members could use.

- Jack

 

Just remember. No matter how cool that gryphon/edge/xcal/etc looks on the dash, it doesn't change the ammount of energy required to get your truck to and from the wal-mart.

Slowing down will. Going back to stock tires will. Pumping the tires up will.

 

if you want a tuner i have a xcal 2 that has been tuned for my truck by troyer performance, its for sale.