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Hey guys, I recently replaced by worn out factory Michelins with a set of Goodyear all-terrains. The Michelins were 245/75-16 and I averaged around 15 MPG. With the new 265/75-16 Goodyears I average around 13 MPG. I haven't changed anything else on the truck and I still drive the same routes everyday. Should the change in tire size affect me that much? Please help. Thanks.
 

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it's not actually affecting you, it's just throwing off your calculations. you went up a couple of inches in tire so you odometer is a little off as well as the speedo. your gas mileage is probably about the same as it was but the larger size tire throws the numbers off a little
 

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99redram, cal your local dealer, parts, and tell them you changed your tire size. Your looking for a new speedometer pinion gear.
You'll need your metric tire size (265) and your trucks gear ratio (3:55, 3:92, ect...
The gear ratio can be found on a label under the hood. To change the speedometer gear, it's real easy. 2 bolts and it's out, It's located on the drivers side, on the transfer case. Near the back. One bolt holds the unit in place on the T/C the other bolt holds the speed sensor to the unit.
There are numbers on the gear unit, these numbers corespond to the "number of teeth" on the speedo pinion. Befor remove the unit, take note of what/where the numbers are located.
When you get the new speedo gear (for your tire size) the tooth count should be different than what came out. Rotate the unit until you see the range of numbers for the gear your installing. Tighten everyting up, (in/lbs NOT ft/lbs)
Oh yeah, have some tranny fluid on hand to use as a lubricant. You want the new gear lubed befor you install it.
Your new gear should run around $15.00
I know this because I just did it to my truck last week. I removed a 31 tooth gear, and installed a 38 tooth gear. After installing 4:10's my speedo was 10 mph off, ad the odometer was regestring 1 mile every 1/2 mile! Now, after the speedo gear swap, my speed is correct according to my GPS!
Troy
 

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Well 97 was the last year they used "Speedo gears". 98+ uses the VSS/ABS system to calculate speed so ou need something like the SuperLift Tru-Speed. Wires inline with the sensor and adjsuts the sensor output to the correct settings.
 

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Luckily, he went up to 265/75R16, which is the largest size that the dealers can "flash" into the computer. EPA lets them flash in any "stock" tire size, and that's the stock size for the Sport models.

Probably cheaper than the true-speed, I'd bet.
 

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Discussion Starter #7
Hey woody, what do you mean "flash" into the computer. Can any dealer do it and how much does it cost.
 

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Any can do it, and they'll probably hit you for 30 minutes to an hour of labor.

Basically, they hook up one of their "special" scan-tools that can also write to the memory in the PCM, and they tell it how many tire revs per mile. HOWEVER, they can only enter the numbers for a tire that comes on a truck from the factory. So, the 245/75, 265/75, 265/75R17, or whatever the Off-Roads run, etc. Maybe 255/85R16 from the 2500s...

If it's a good dealer, they will do it, if not, then don't bother.

Or, if you ever plan on upgrading tires over 265/75R16 in the future, you either need a custom PCM ($400), or a true-speed ($150-250 depending on where you buy it).

scroll down to the KRC Gear Box:
http://www.krcperformance.com/krc_performance_pcm.htm
 

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I heard when they "FLASH" your computer you lose some power out of your motor.
true or false???
 

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Hmmmm. I don't think that you would lose power just for uploading something into the computer.

Whoever told you that probably heard from somebody else but the only way you would lose power is if they uploaded the wrong figures into your computer and things got out of sync.
 

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Probably heard that from a Jeep owner. There was a flash that the dealers were doing to V8 WJ's that were for emissions, I think. Afterwards, alot of Jeep owners were complaining thier Jeeps were neutered.
 

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Rawkon is right, on the dodgetrucks.org web page there is about 20 posts in there talking about the flash issue. Basically people with pinging problems take the truck into the dealer and the dealer flashes the PCM which slows the ignition timing to eliminate the ping which in turn destroys the power you once had. So don't allow your dealer to flash your PCM.
 

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There are updated timing variables for vehicles that are pinging, and this WILL, yes, castrate your truck to some degree.

However, just changing the pinion ratio (I think that's what they call it), to recalibrate the mileage, speedo, and odo shouldn't affect that at all.

IF you have a Performance Computer in your truck (custom or Mopar), NEVER allow a dealer to reprogram the PCM, and state it in writing on the form you fill out when you drop off the truck. Otherwise they can reprogram it back to stock (and you're out $250-500).

However, changing the revs per mile shouldn't touch the main programming at all, it's just a parameter used by the main program.
 

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Jason is right they have to reprogram your computer, which is possible for your size tire, anything larger you would need a calibrator from superlift.
 

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ive had one dealer say they could and would change the pinion factor for 36's and another dealer that said never heard of pinion factor. ive also had 2 4x4 shops in illinois tell me the same thing that the dealers can change the pinion factor for 36's.
(Godmudders in Alsip and Attitude Performance in Elmhurst)
 

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It gets interesting, you see the dealers are CAPABLE of setting the pinion factor up to like 36s, the computer will take up to 44s, but the problem is that the EPA won't let them enter a value that doesn't correspond with a size tire that comes on the truck...
 

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so... have them hook up the PC... have some one page them while its hooked up... walk over.. calibrate it yourself, unplug the PC, when the guy comes back, give him the unit, and say thanks, and drive away... slip him a 20 and he probably wouldnt care :): I might have to try that.. my service advisor is now my best friend (they get that way when you report them for crappy service) he just might do that :): HEHE
 
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