I finally ridded my truck of the skateboard tires and put on some Nitto Tera Grapplers 265x75E. They look good, ride good, price was right, etc...


BUT, my mileage before the tires was pretty consistently 20MPG, now it's 17MPG! That's bad. I did the speedo correction w/ a buddy's GPS and the right tire size selection appears to be 30.75" to have it dead on at 80MPH.


I don't understand why they would cause a 15% decrease in mileage.


Thanks,


Tim

The only thing I could think of would be the weight of the tires. Nitto's weigh more than stock, but not sure that would cause that much of a difference. May want to do some research and see what you come up with. Maybe time for a programmer or box. http://www.dieselplace.com/forum/smileys/Big Smile.gif

Yep, it doesn't make a lot of sense really. As for a programmer, already have one, the Quad 135. On the stock tires, I went from around 18 to around 20 when I started using it. When I put the new tires on, bam, around 20 to around 17.


At nearly the same time I finally installed my 4" exhaust system, perhaps that makes a difference? I am thinking of going back to the stock system as soon as I get the chance since the 4" system is much too loud for my taste not to mention the drone at 65MPH. :-(


Thanks,


Tim

luvthesmell-


If you don't mind me asking, what exhaust system did you install?? I just put new tires (265's) and rims on my truck, but haven't burned a tank yet so I couldn't help you with the mileage thing yet..

Bob

Just a thought.


Check your odometer with the gps. I think it is separate from the speedometer. I have about 12% difference on the odometer with the 315's and a 8% diff. on the speedometer. The computer was reprogrammed for 265's by the dealer. i bet you will see a little difference and gain some of that fuel milage back.

luvthesmell,


If you do more city than highway driving moving to a bigger tire will drop your milage. If you do the reverse, it should increase your milage. Truck will need to produce more power on take off, requiring more fuel when tire size increases.


LA DMAX

The odometer is separate from the speedo? Really? I'll have to check that out. That would be strange...


I do have a theory, I suspect the truck came with the speedo (and odo) over compensated for larger tires then what come stock. This would mean since I have had my truck, the speed (and odo) would have read high which means I might not have actually been getting 20MPG and not going 80MPH when it says I am.


For the auto manufacturers this is a big plus, because your warranty eats up faster, you need maintenance more often (by the schedules), and you can trade your car in earlier for a new one all based on mileage that's not accurate. How bout that for a theory? After all, how would you ever know unless you have a GPS or check it with the rader road-construction speed signs.


I still have my original spare, I plan to measure that and play around with the numbers and the box to see if that is/was the case. Not to mention that as the tires wear, you lose probably 1/2" or more in diameter which makes the speedo run that much faster as the tires wear out.


As for my exhaust, it is the DPP 4" T409 stainless system. I still have the original box. I think I'll put it on eBay once I take it back off.


Thanks,


Tim

With my '02, the only speedo calibration was for 245s, and the reflash I got was just for anti-skid.


It is possible you got a tank of winter fuel. We don't get a lot of it here in South Texas, but I notice that mid winter, my mileage drops 2 mpg for no apparent reason. The station I buy from doesn't sell an incredible amount of diesel, so I tend to get winter fuel mid winter to early spring.

I don't have a diesel, but I did not loose any mileage with my 265 Nitto's. I installed a new Wester's tune which had the updates for the larger tires and new shift points. I would definitly make sure yor odometer reading are correct....

Odometer is dead on for ~30 mile test this morning. I don't think I got any winter fuel, I use a high volume station, been getting 20MPG right along all winter. Even went into the hills (of NM) and out to West Texas a few times over the winter, still got 20MPG pretty consistently.


Tim

Bigger exhaust wouldnt hurt on a diesel. Maybe try switching the muffler? I have a kennedy 4"-5" exhaust with the muffler removed and i dont have any drone if I'm not on it. Its barely noticable with the radio off going down the highway. Actually it seems quiter without the muffler inside at a steady cruise than with the kennedy muff installed. Getting on it or outside noise is a totaly different story thoughhttp://www.dieselplace.com/forum/smileys/Evil Smile.gif


I've had 265's since new(well a few hundered miles) and I average 15 city/highway mix per tank with a heavy right foot. Dont know what I woulda got with the factory skins.....I never got reflashed, thats a few extra warantee miles right?http://www.dieselplace.com/forum/smileys/Evil Smile.gif

Tim unless you had your speedometer updated for your 265's, you will have to add 4% to your fuel mileage numbers.

I lost 2-3 mpg when I switched from the 245's to the 265 General AT's. That is with me adjusting the odometer reading by 4%. I run level 2 juice.

Going to work I used to get 18.5 mpg. Now I get 15.5-16.0 mpg.http://www.dieselplace.com/forum/smileys/Cry.gif

I used my Quad135 to adjust the speedo (and odo) using a GPS. The 265 Nittos required the 30.75" setting to be accurate (assuming the GPS is accurate). It's nearly dead on (now).


I have a feeling that it was not set properly before (at stock). I still have not had a chance to verify that.


Running on my third tank now. I think I might be getting better mileage on this tank, not sure why (yet).


Thanks,


Tim

remember; you are going further than your odomoter indicates. I threw a fit when i saw my milage drop with 33'' tires on my 1500. Then I realized i was going further than the speedo indicated per rev. Put some low profile tires on there and you'll be at 30:)