315's and poor fuel economy?

[RHinds]

About 2 months ago I mounted the BFG 315's and have since seen my fuel economy actually go down a little from what I was seeing with the stock tires. I have had the speedometer recalibrated to 603 revs/mile, so I know my numbers are accurate.



I was expecting a little less than a 2 mpg increase, but I have actually seen about 1 mpg decrease. Check my signature for my truck configuration. Note, I listed the Buckstops, but they aren't installed yet. Also, I have over 7500 miles on the truck.



Anyone else experiencing anything similar? Or did DC maybe change the ECM fueling setup when they programmed for the larger tires?



Any help is appreciated.



Thanks

 

[XcumminsX]

Do you drive around the city alot?



Nick

 

[VStahl]

fuel mileage up with 315's

I am getting just under 2 miles more per gallon with my 315's installed. I have the 4. 10's though.

 

[RHinds]

Nick :

My driving is about 35 miles one way to work, 2 and 4 lane highway. 15 miles of 55 mph and 20 miles at 65 mph. Last week I traveled to southern Mo. , about 500 miles one way. I got about 18 mpg going down and 19. 2 mpg coming back.



My comparison is to my normal daily driving though, with the stock tires I was seeing about 20. 5 mpg and with the switch to the 315's I am down to about 19. 2 mpg. This is all per the trip computer, but should be accurate for comparison.



I'm tempted to call the dealership and talk to one of the technicians and see if they think that with the 3. 73's that I may be under powered for the tires and gearing.

 

[JARichard]

I am getting about 1 mile less highway and WAYYYYY LESSSSS in Town. Standard Output with 373's.

BUT IT LOOKS AWESOME.

Another excuse to add Edge EZ or Ramifier!!!!

 

[klenger]

Rotating mass and higher rolling resistance because of wider tread !!

 

[XcumminsX]

I am getting rid of my 315's with 4000 miles on them because of this. Yes it looks awesome but i want a smaller tire



Nick

 

[Mathew Clausen]

The 5 rib smooth factory michelins would be tough to beat for rolling resistance. With a wider and more aggressive tire like the BFG, the rolling resistance almost certainly will be higher. The frontal area will be greater as well. Also, as noted by klenger - there is more inertia to accelerate in stop and go situations (although at steady highway speeds that should be insignificant). I would definately not expect a 10% (2mpg) increase in mileage in normal driving.



Maybe this will help to explain. There are three things that take power when a vehicle is moving - air drag, rolling resistance, and friction losses in the engine/drivetrain. At highway speeds a large percentage of the power you are using is to overcome wind resistance - so a slight reduction in engine rpm will not change the fuel consumption much. Air drag goes up with the square of speed, and is proportional to frontal area which increases slightly with larger tires. Rolling resistance is a function of the tire - and typically the more highway looking it is the less resistance. Friction losses are a combination of the engine and drivetrain. The larger tires definatly lowers the engine rpm which lowers the friction losses slightly. But at the same time you increased rolling resistance and frontal area.



Hopefully the above rambling makes sense.



Mathew

 

[RHinds]

Contradictions

Mathew :

So, you're saying that it isn't surprising to see a lower fuel economy?



Why is it that VStahl and my uncle (off line from TDR) with 4. 10's have commented as to achieving 1. 5 to 2 mpg increases?

 

[CLAYTON]

Had my 315's for quite some time now and I am very pleased with them. . my mpg didn't go down at all. I'll usually get 21 -22 mpg on a road trip empty at 75mph (numbers are hand calculated. )

 

[VStahl]

could it be that I put the tires on my truck when I had under 10,000 miles and i am now over 24,000 miles so my engine is broken in more, which is why I am getting better mileage?

 

[DougR]

You guy's are killing me with your 20-22 mpg highway mileage. I never get better than 14 mpg,before or after the 315's. By the way,I have had BFG AT 315's or 35's on my last three trucks. I have had three different ratios. 355,373,410. They all got wose mileage with the bigger tires. The truck performs much better though with 4. 10 gears,whigh is why I ordered them. 373 is Ok if you don't pull a heavy load,and the 355 did not work at all,even empty it struggled.

 

[fencebuilders]

You got it.....

Originally posted by JARichard

I am getting about 1 mile less highway and WAYYYYY LESSSSS in Town. Standard Output with 373's.

BUT IT LOOKS AWESOME.

Another excuse to add Edge EZ or Ramifier!!!!

Couldnt have said it better myself! See sig.

 

[jimnance]

I had the 315's on mine and saw a 1-2 MPG decrease in fuel economy. In theory running lower RPM's might improve fuel economy, but I think the six speed and 3. 73's are just about optimum.



I believe that the extra mass of the tire and greater rolling resistance was the problem.



If you had 4. 10's and weren't pulling anything the bigger tires might help fuel economy very slightly, but it probably isn't worth it unless you are going for style. The 315 BFG's did look very cool.

 

[R.M.THOMPSON]

Don't forget about height/lift , it has a negative effect also !

 

[CLAYTON]

If you don't get your speedo corrected then you over head will show less then what it is actually getting right? that's saying if it was acurate. I haven't had mine corrected but I figured with a hand held gps that I am about 9. 1% off from where it should be. . so if I am going 100mph then I am really going 109. 1mph. So I have to factor that in when I do my hand calculations.

 

[Blakers]

Use a gps for counting miles and calculate your mileage by hand. Takes many times to get a good average.



It seems what kills the mileage the most is running over 2100 rpm on the highway. I believe if you run high speeds all the time, the drop in rpm will have more positive effect than the negative effect due to tire weigh and wind resistance. But. . at stop and go driving, you probably won't see the benefit of the lower rpm.