When I got my truck my overhead mileage readout was dead on. When I initially put my Edge EZ on it showed an increase of 2 MPG which was nice except for the fact that it truly was down 2 MPG so the readout was actually 4 MPG off. I have since put BFG 315s on and have not had the speedo reset for the bigger tires. I finally checked the MPG by using my GPS and I actually travel about 112 miles for every 100 miles on the odometer. My MPG readout was only 1 tenth of a gallon off! It was on the high side.
I was going to get the speedo re-calibrated but I have figured out the differences in my actual MPH versus what the speedo tells me and I am actually saving miles by not changing it. I just figure that at 25-35 mph, I'm actually going about 3 mph faster. At 40-65 mph it's about 4 mph faster. At 70+ it's about 5 mph faster.
One more thing, I have seen many people post that the mileage goes down with the 315s, which I never believed to be true knowing that for each revolution of the wheel you travel further regardless of friction. My mileage has not dropped, rather it seems to have gone up a couple points....

 

You may want to refigure your MPH with the 315's.

If you actually travel 112 miles for every 100 miles you clock you must multiply you speedo reading by 1.12

Example:

30 MPH x 1.12 = 33.6 MPH

40 MPH x 1.12 = 44.8 MPH

60 MPH x 1.12 = 67.2 MPH

70 MPH x 1.12 = 78.4 MPH

Depending on where Smokey Bear has his radar alert level set the several extra MPH's could get you an expensive invitation to the Policemans Ball.

 

I checked it using my GPS which I'm sure is accurate but I'm not worried about 1 mph.......

 

My overhead monitor is really close for me, too, with my 315s. I checked it by hand (adding the 11% to the trip meter) and the overhead meter was only off by .4 mpg.... 17.9 actual vs. 17.5 metered.

John

 

Here are a couple post I had on another site regarding this topic.

I drove 500 miles this weekend which was the first long trip since I bought the truck. Not towing anything and with three adults inside. I now have just over 1500 miles total.

I filled the tank and reset the MPG Computer. In the first couple miles it jumped to 16MPG quickly and then sat for the next few miles. Then after I made it over the lowest of the passes it began climbing up and up to 19.6MPG and stayed there for a long time. It would fluctuate from 19.2 to 19.6 over hundreds of miles. When I returned home and had to go through the mountains again It started dropping down as I was going up the long grades. When I Arrived at home the computer was at 18.5 MPG

My truck is a 2004 four door and has an automatic and 4:10 axles with a 4" lift and 35" tires. I was surprised that the MPG was this good while so new. From what I have been hearing it should get much better then this as I get through the breakin period. I suppose I can expect to get a bit over 20 mpg when fully broken in. Does that seem like a reasonable expectaion with the Automatic trnasmission?

I drive to Portland and back Wednesday. There are no mountians in the way. Who knows maybe I'll break 20 mpg? I also noticed that when I run over 70mph the mpg drop from 19.6 to 19.2 and if I run 1800rpm which is just under 70mph my mpg goes back up to 19.6 again. Seems like that 2000rpm mark on my truck makes a big difference in fuel use.



Ok I refilled again and really stuffed the tank full. The display said 17.9 and the on paper calculation was 18.5. This is the biggest difference in the last several tanks of fuel. However it was actually better then displayed!

My DTE display has also reached a new high number. When I got the truck it was 480 or so. Then it went to about 550, when I filled the tank today it showed 640. I have had mostly highway driving but there have been a few hidious long traffic jams, one was 45 minutes of hardly moving due to a car on fire. I'm sure that did not help my fuel economy.

The highest I have seen the display read is almost 20mpg, but the average seems to be almost 19mpg. I'm hoping as has been stated here by several guys that the mpg will get better as the truck breaks in. 20mpg highway would be great for me.

What is the gain going to be with break-in?