No I use a dealer in Cameron, Cantwell is the worse dealer I've ever been to. You can ask them how they did it to my 08 and see what they say.

 

final drive ratio is computed as such:

(new tire diameter / old tire diameter) * axle gear ratio = final drive ratio

so:

(34.9/31.6) * 3.42 = 3.78 <--final drive ratio of 325/70R17 tires from 265/70R17 tires with 3.42 ratio

(31.6/31.6) * 3.73 = 3.73 <--final drive ratio of 265/70R17 tires (no change in size) with 3.73 ratio

If my thinking is way off base here please feel free to show me calculations stating otherwise. I am certainly not trying to get into a "i'm right, you're wrong" kind of battle. I just want my speedometer to be accurate!

 

Thank you for showing how its done. That looks correct to me.

 

I thought all these trucks already came with 3.73 gears in them. Am I wrong? If so, telling the computer you have 3.73s will not change anything?

 

The 6-Speed Manual variants have the option from the factory of being equipped with 3.42 or 3.73 gears. The 6-Speed Automatics can have 3.42, 3.73, or 4.10 gears from the factory. I bought mine with 3.42 gears for the decreased highway RPM's thus better fuel mileage on the highway. Hell, these things have enough torque down low for anything i will need it for anyways, so sacrificing a little bottom end pull for better freeway mileage was acceptable to me.

 

final drive ratio is computed as such:

(new tire diameter / old tire diameter) * axle gear ratio = final drive ratio

so:

(34.9/31.6) * 3.42 = 3.78 <--final drive ratio of 325/70R17 tires from 265/70R17 tires with 3.42 ratio

(31.6/31.6) * 3.73 = 3.73 <--final drive ratio of 265/70R17 tires (no change in size) with 3.73 ratio

BroncoHound,

Yellow68 is right, you are going in the Wrong Direction.

Your formula is Backwards, Divide the Old by the New.

31.6 / 34.9 = .9054441. Your Tires are Taller, Turning Slower, not Faster.
.905441 as fast to be exact.

.905441 * 3.42 = 3.09 Final Drive Ratio.

Conversly your speedo is off by 10%, Indicted 60 is 66 actual.

Just trying to help.

Stealth.

 

Stealth,

Looks like you and Yellow68 are correct. I had them reflash for 3.73's this afternoon and it made the speedometer ~10mph off now. Going the wrong direction. Annnnd, since there is no lower gear ratio offered by Dodge for these trucks than 3.42, looks like i am without a paddle.

At least we know now that it CAN be done by the factory WITHOUT any updates for the '08s. Dont let your service department tell you otherwise, guys. For those of you with 3.73 gears on your truck, reflashing the computer to read a 3.42 gear (the opposite of where I need to go) will correct your speedometer for 315 or 325 tires.

So, this brings me to another question. What aftermarket programmers are on the market currently for the 6.7L that allow speedometer adjustments? I know the Power Pup does, are there any others? Does the Power Pup (or any others) leave a footprint that could cause warranty issues down the road should anything need fixing? Forgive my ignorance but I am not too familiar with any of the modern power programmers and there is just too much about it on these threads to sift through for a basic informational search. Thanks for all you guys' help thus far, I am certain we can get it figured out soon.

-Bud

 

Shows what I know? The formulaic answer provided and concept was real good though. Should have known that with a larger tire you're traveling farther with each RPM and hence you're going faster that the speedo shows. Telling the computer you all ready have a numerically lower gear ratio (which is the same effect as a larger tire) when you really don't will bring it back into closer calibration.

Sounds like one good reason to have the 3.73's or 4.10's, more choices for correction on larger tires...

 

Bullydog says that as long as you dont go into recovery mode, or "opt out" that it wont leave a finger print. By opt out they mean that the pup and truck have the same software update. So if your truck has AV flash and the pup was manufactured say in april...it will notify you of this and ask if you want to continue. If you "opt" to it will not put your key cycles and other info in the right place in the computer and they can tell its been tampered with. So just update the pup and avoid revovery mode and you are good.

It also does the speedo seperatly from the tune.

 

Yellow68ss--Thanks for the heads up. To clarify, I can use the power pup to correct the speedometer WITHOUT putting any of it's performance tunes into the truck? So this device can correct my speedometer and I can hold on to it and wait until I feel more comfortable messing with the tuning before adding any of those. And as long as I ensure that the power pup is loaded with all the latest updates (which can be downloaded from Bully Dog's website i assume?) then I shouldn't have a problem leaving a digital footprint?

I'm going to have a nice little heart-to-heart with the service manager at Love Dodge tomorrow. He is a really nice guy who invited me to check out the StarScan tool while he was putting the 3.73 programming in and we talked a few minutes about any upcoming updates for it. One other concern I had was the TPMS on the truck going off when the tires were cold due to my Nitto's being a load range D tire and only being inflated to 50psi as opposed to the 65-70psi of the factory Michelin load range E tires. He said that on the vehicles that have had TPMS for a while he can adjust the psi with which the TPMS triggers a low tire pressure warning but it is not yet available for the trucks since '08 is the first year of TPMS in these rigs. Once the update comes down the pipeline from Dodge he is going to let me know and we will adjust my TPMS down to 45psi or so, that way it won't trigger unless I really do have a low tire. As it stands now the light only comes on in the morning when the truck and tires are cold. After driving for 15-20 minutes and the air in the tires warms up and expands the light goes off so it isn't that big a deal. I just don't like idiot lights on my dash...lol

While I am there tomorrow, I am going to ask if there are any gear ratio's he can put in the truck lower than a 3.42. I know there aren't any other factory options, but if there is any way to get it down to a 3.08 or so I will be elated. If not I am going to talk to him about his views of aftermarket programmers such as the Power Pup. He seemed interested in aftermarket solutions for these new trucks when I talked to him and, being the service manager, it will be his call whether or not something is warranty-voiding or not so I am going to test the waters and see what he says. I hope to develop a good relationship with this guy as he seems genuinely interested in doing things above the bare minimum and, down here, that is a rare commodity. I will post how the conversation goes tomorrow and go from there.

 

One other thing: after doing some further reading on the Power Pup, I seem to be seeing different claims from different people. Some claim that the Power Pup cannot adjust the speedometer above a 33" tire (assuming a 285/70) on the new trucks (07.5-08), others say it will give up to a 45" tire but only on 03-07 models, and still others claim up to 37" tires on all year models from 03-present.

Yellow68ss--What size tires are you running on your rig? Are you sure the Power Pup will allow you to program beyond 33" tire sizes with the 6.7L? If this thing will only go a 285 for my application, it serves no purpose as my dealer will put it at 285's for free.

 

SUCCESS!!!

I went back to the dealer today and the service manager let me play with the star scan tool for a bit. I plugged in a 285/70 tire size and then went to play with the gear ratios. What do i see, but an option for a 3.21 gear!!! So, based on my quick calculations, it looked like this combination would get me pretty close to accurate. I went for a test drive with my other truck as the baseline and it was just about spot on. Came home elated did some longhand figuring. Here is what I came up with:

265/70R17 tire diameter: 31.6"
285/70R17 tire diameter: 32.7"
325/70R17 tire diameter: 34.9"

(31.6/34.9) * 3.42 = 3.09 <--Final drive ratio of my truck with 325/70R17 tires and 3.42 gears
(31.6/32.7) * 3.21 = 3.10 <--Final drive ratio of the computer programming for 285/70R17 tires and 3.21 gears

I am supremely happy with the fact that I have the truck pretty darn close to accurate without having to worry about adding any aftermarket programmers. A buddy of mine has a Garmin w/GPS and we are having dinner with him tonight so I am going to plug his Garmin in and see how close the speedometer is based on the GPS readout. I will post my results this evening or tomorrow.

So, here is the cliffnotes of my results based on my research, for all others who are curious about correcting the speedometer through the factory for ~35" tires:

If you have 3.73 gears and put on 35" tires (315/70R17 or 325/70R17; the difference in height is negligible), program your computer with the StarScan tool to the stock tire size (265/70R17) and 3.42 gears. Here is the mathematical proof:

315/70R17 tire diameter: 34.4"
325/70R17 tire diameter: 34.9"
265/70R17 tire diameter: 31.6"

(31.6/34.4) * 3.73 = 3.43 <--Final drive ratio (actual with 315/70R17 tires)
(31.6/34.9) * 3.73 = 3.38 <--Final drive ratio (actual with 325/70R17 tires)
(31.6/31.6) * 3.42 = 3.42 <--Final drive ratio (programmed)

Now, if you have 4.10 gears in your truck and go with a 35" tire, you will want to program your gears to 3.73 with the StarScan tool to compensate. Here is the mathematical proof for this:

(31.6/34.4) * 4.10 = 3.77 <--Final drive ratio (actual with 315/70R17 tires)
(31.6/34.9) * 4.10 = 3.71 <--Final drive ratio (actual with 325/70R17 tires)
(31.6/31.6) * 3.73 = 3.73 <--Final drive ratio (programmed)

Finally, if you are like me and have 3.42 gears and want a 35" tire you have to be a bit more creative with your choices. You need to program with the StarScan tool a 285/70R17 tire (factory option on the Power Wagon) and 3.21 gears (not sure what it came on from the factory but it was available with the 285 series tire being selected on the StarScan tool). The mathematical evidence is listed above.

Hope this helps anyone else in the future who is looking to correct their speedometer/odometer relatively close to OEM specifications after a tire size increase and doesn't want to invest in an aftermarket programmer. Thanks a ton for all you guys' help in the past couple of days. I've learned a bunch and hope others have too.

 

Be curious to see how close you really are after you check it with a GPS.

 

Checked with Mizz Garmin today. She told me that the truck was 3.8mph off at 70mph. By this I mean that when my speedometer on the truck registered 70mph, Mizz Garmin told me that I was traveling 73.8mph. When Mizz Garmin said I was doing 70mph, my speedometer registered 67mph. Not as close as I had hoped, but factory speedometers are 1-2mph off from the factory so I still think it got me awfully close. Close enough to the point where I can rest easy and not think about buying a programmer right now. Maybe in a year or two when there are more options available for the 6.7L powertrain and these options are proven to be reliable I will consider something that corrects my speedometer and gives me a bit more power. Until then, I am perfectly content exactly the way she is. This new truck is everything I had ever hoped it would be thus far.

 

That's about where I'm at. I don't like it but there doesn't seem to be any way to dial these it perfect yet without voiding my warranty.