I dont know what's going on with my stuff. I went from a 265 factory tire, to a 305 and the speedo read dead on (I guess that means the odometer was dead on also). I've now gone to a 315 and I know the speedo is off (I can feel the difference). But I don't know about the odometer. I used to get 220 out of half a tank. I'm NOW getting 250-260. Yet when I do the calculations (both of those were with 10% added in), I've lost mileage. Explain that to me.

 

larger tires may give you more MPG on paper, but they require more HP to turn, so your ACTUAL mileage will almost ALWAYS go down. You might see a slight increase with 285 hwy tires, but go to 315 MT/Rs, and it's almost a given that you'll lose mpg.

jmho

JLH

 

I agree with JLH here. I went from 245's on my truck to 285's. My speedo went from reading slightly high to 10% low but my mileage DID INCREASE - by means of both the on-board computer and by hand calculations. I don't know what tire came on the truck to begin with but it sure seems to like the 285's - the truck definitely has enough torque/hp to turn the slightly (relative) larger 285's. Going to 315's, though...that's quite a jump. Between the increase in tire height (315's ~= 35", right?) and the increase in width, the rolling resistance must have jumped quite a bit (I'd hafta dig out my engineering testbooks to calculate just how much of a jump), not to mention how much the effective gearing has changed.

But...315's sure look good on the CTD's......darn good!

 

I have 285's and get a correction factor of 3%. (Actual mpg is 3% more than figures on paper). Your 265's should be less correction. Draging brake will definately explain your low mileage.

I agree with above - think CTD can handle larger tires easily due to strong torque. I lost no mileage going from 265's to 285's, after correcting for fewer rev's per mile.

You don't list if auto or manual. If an Auto, your not that far off, depending on your driving habits. If a manual trans your much less than should be.

 

FWIW mine got 17+ mpg cross country, empty, @ 70-75 mph. Depending on your driving habits and terrain 14.5 isn't way out of the ballpark.

The concern would be YOU used to get more....hmmmm

 

You guys need to take your truck to the dealers and have your speedo corrected. Takes 2 minutes, I've corrected mine at the dealer I work at twice.
Go to your tire manufacture website and find the RPM(rev per MILE) and give that to them.

 

just throwing my 2 cent in i didnt read anywhere how fast happy drives i no that on my dads 99 2500 quad cab 4x4 5spd with 265s (245s where factory) that is his only mod to the truck at 70 mph on road trips will show between 19.5 and 21.5 on the overhead depending on wind and terarran so the 14mpg that happy is getting well sux unless he is runnin 90 everywhere

 

If I'm not mistaken, I believe that changing the tire size throws off the overhead, so, unless you hand calc the mileage, we really don't know what your dad's getting.

My truck gets 15 mpg mixture of town and freeway driving, around 18 mpg freeway only ... that's at a steady 2000 rpm (approx. 65 mph).

 

true that but at hand calulations usealy come out to around 19 to 20 on trips depends on where we go and that is factoring in the diffrenece between tire size heck my 01 get 13 to 14 mpg towing my 9k gooseneck empty runnin 70 to 75

 

Happy, your mileage is right in the ball park, not knowing how hard you are loading it or the terrain you're driving in. I saw a thread somewhere on the forum a day or two ago that had to do with fuel mileage. You might do a search and check it out. In my rig, over a 2,000 mile leg, I average 12- 12 1/2mpg loaded running 65-70 mph. Empty, I'm probably around 17 or so, but I seldom run empty very far.

 

asken how i load it just got back from heavest load ive pulled with it and will probly be the heavest i ever pull with it was over 38,000 gross wt that was truck my gooseneck with 21,519lb of metal decking for a office complex in florida had no pulling issues but had some stoping ones lol but normaly have about 16k of metal but your right i had 8k of hotubs on it going through west viginia and had a killer air drag cause they where standing on end and only got 9 mpg in those mountains

 

When I did my florida run from new york I saw 19-21.5 mpg on the overhead actual mileage (not avg). I saw my best mileage at 80mph which was 21.5 with a 24' enclosed car carrier which was empty. I live on Long island and around here its mostly "around town" type driving. My overhead currently reads 11 My friend has a stock 04 dually and hes seeing only 12 so I guess its the type of driving we do. And as far as additives go I buy Howes from the local peterbilt dealer.