Big tires=Bad mpg

[ToddK]

I just wanted to get some feed back from those of you that have put 315's on your ctd. I had 285's on prior to the 315's and I have lost 2-3 mpg. This is on a 2006 3500 MC 4X4 G56.



Pre 315----5" turbo back, sc flashpaq on tow setting.



Post 315---same as above with new additions. 2" coil spacers and rear blocks, cool hose with tagIII.



I went to Texas from Colorado for the NASCAR race and was expecting a mpg gain at highway speeds due to the reduction in rpm's. I was unpleasantly surprised when I checked the first 2 refills. First one was 16. 25 mpg, the second was 15. 5. I have been getting 17-18 around town and up to 19 driving in the mountains on the freeway.



Thanks for any replies.



On a side note, does anyone know if I should shim down the drive shaft due to the 2" blocks in the rear? Thanks

 

[WUnderwood]

what gears do you have? 3. 73 or 4. 10?

 

[betterthanstock]

Have you corrected speedo for tire size?



see here :



Tire Dimensions Made Simple - Discount Tire Direct



Otherwise you would now be traveling even faster, and more miles than you realize.

 

[John - K5AWO]

I would think the larger rubber would be more squirmy (for lack of a better term), have more frontal area (more wind resistance) and most likely dropped your RPMs below the most efficient point (1900-2100) for best fuel economy.

You didn't define "Highway Speeds", but if you are trying for more than 60-65, the power required to push a big bus through the air increases logarithmically with speed. Also, you truck is now taller and catching more air than before.

All of the above are negatives for wanting better mpg.

My dually makes about 65+/- mph (gps) at 2100 RPM with my setup.

 

[ToddK]

G56=3. 73 rear diff



Speedo not corrected, however I used a gps to determine speed which is 8-9% off. So I multiply the shown miles by 1. 09 to get actual mileage.



Highway speeds vary depending on where I travel. I would say the average is 70-80 mph. At 80 rpms are 2250, with stock tires they were 2550 at 80. Even with stock tires turning higher rpms I was getting 18 mpg.

 

[Motorhead]

ToddK, What mileage were you getting with the 285's? Better than stock size or worse?

 

[ToddK]

Motorhead-about 1 mpg better

 

[John - K5AWO]

18 mpg at 80 mph? I would be so happy

 

[Tinman]

I have 315's on the 06. I get about 18 on the highway if I keep it under 70. Lost 2 mpg from stock.

 

[04 Diesel Power]

I have 315's also, slow down and you will get some of that back, I try to hold it to 60 and right now I am getting 19-20.

 

[hearingyourlies]

I keep very detailed MPG records on my truck. Up until 25k miles, I got 16. 98 mpg with pretty much a 50/50 highway/city split. At that time, I put on a Kore Recon kit, 305/70-17 E Rated Nittos running at 40/35 pis. I have gone down to 14. 66 mpg which once adjusted for larger diameter is 15. 68.

 

[firemansdiesel]

I get 16 mpg mix and 18-20 mpg highway. Took a trip down Arizona from Norcal. Cruise set @ 70 mph I got 19 mpg and 20 mpg on two seperate fill ups.

 

[steved]

I got around 23mpg with my 315s... that's corrected and running a conservative 65mph.



steved