Tires and MPG
 

Hey guys, I am looking to by my first diesel and can i say this site is great! You all have answered alot my questions without me even asking. thanks!

Heres another one though... When changing to oversizes tires on a diesel, have you noticed a drop in MPG like on a gasser or does the extra torque make up for it?

Welcome Wooly,
I copied and modified a Bosch fuel consumption chart for you(because it was written in german).
You can see that rolling resistance has an influence and will increase with bigger(wider) tires.(not directly from the chart below)
Due to fuel effectiveness of a Diesel you won't recognize it that hard like a gasser but it is still there.
You can see that it is speed depedent as long as air resistance takes over. (starting to take over at 35mph)
Please have in mind that you change gearing when you go with bigger tires- that means you can be out of optimum gearing band and that will cause higher fuel consumption, depending on existing gearing.
http://up.picr.de/4594225.jpg

Depending on contact patch, tire thread, the rolling resistance will vary.
Air resistance will incease if you lift your truck. You can expect an increase in wind resistance cw by +5% if increase height by 1", but have in mind that is as much as driving with an rolled down window.
Don't laugh-even wide tires increase your wind resistance 2-4% -same goes for huge rear view mirrows...
those numbers are taken from the bosch automotive handbook.

The short answer is yes you are going to drop some mpg. I dropped 2 mpg when I went form 33s to 35s in town, but stayed about the same or better on the open road. However, to find out what you are really doing, there is some math involved.

First, find the rate of change, new tire/old tire, for me that is 34.5/31.5=1.09524

Next, multiply that times your odometer reading, let's say 285. So 285 miles x 1.095 (rounded)=312.075 ACTUAL miles traveled. (This can be used to find your new gear ratio also ie 3.73x1.09524= 4.08525 so a 4.11 would be REAL close.)

Finally, divide the corrected mileage by your fill up, say 25 gallons: 312.075/25= 12.483 average mpg. If you only do 285/25 you'll get 11.4, NOT the corrected mileage for that tire change.

Thanks karls03
Thats alot better then i have seen on my gassers in the past. But i really didnt have any added horses over stock.

I probably wont do much mudding with it but i do alot of offroading during hunting season and it always feels better to know you got that extra grip if needed. I dont really need to go that big but its nice to know i have that option without losing alot of MPG.

I have an 8 inch lift with 35"X12.5's with Superchips tuner and i get about 16.5mpg when I stay out of the throttle, and it only drops to about 13.5mpg when i'm on it a bit...

That's a simple question that cannot be answered without many specific details: Size difference both diameter and width, weight of the tire, rear gear, transmisison - auto or manual, same wheels or not, tread design, driving pattern - city/hwy.

The trick is to basically get a tire/wheel combo that is only slightly larger diameter, same or less width, same or less aggressive tread and the same or less weight than the original tire/wheel combo if you don't want to lose mpg.

The change is nothing like it is with a gas truck. Its not a bad change on diesel.