i never change out the speedo gears. they're not even accurate from the factory, and it's off a few percent with just low tire pressure.

like an old mechanics instructor used to say, "it's an INDICATOR, not a gauge". i.e.: don't trust 'em.

265/70/r17 (factory).....
...265mm width x 70% sidewall height x 2 sidewalls(above and below the rim)= 371mm
.......371mm / 25.4mm per inch = 14.6
.............14.6 + 17" rim = 31.6" factory tire (could be anywhere from 30" to 32 1/4")

now ya gotta calculate the tires that're on there now, and come up with a percentage of increase........35" / 31.6" = 1.1 (10% increase)

so, 60mph on the speedo is actually 66mph. ...and 15mpg is actually 16.5. (about what i'm gettin)

my "35 inch" bfg 315s actually calculate out to an increase of 8%. i just tell the wife to "DO THE SPEED LIMIT" according to the speedo, and ya should be alright.

an easy way to verify all this with little math is to go 60mph on a straight stretch for 5 miles. it "SHOULD" take exactly 5 minutes. if not, do the math. 4 minutes, 30 seconds = 300 seconds / 270 seconds = 1.11 (11%)

cheese, some of you nancy boys need to tuck your skirts in and think a little. y'all "have a shop do it" waaaaay too much.

kevin

 

I hate math....

 

I think I see the root of your problem. If you are getting GAS mileage in your diesel-powered truck, you really screwed up back there at the filling station! Try a little diesel next time and I bet your FUEL mileage gets better!

Seriously, I just cleaned the before-mentioned sensors (it's been a while) because my fuel mileage has been slipping pretty badly lately and instantly picked up nearly 4 MPG on my daily commute (428 miles on 19 gallons = 22.52 mpg). Been struggling to get 19 mpg for the last several tank-fulls. On your 2004, the MAP sensor is located in the intake manifold, close to #4-#5 injector lines. The IAT sensor is in the intake tract between the air filter and turbo. ON the 2004.5 and above Cummins "600", the MAP and IAT sensors are both in the intake manifold.

If you have an air filter that needs oil, that oil can really play hell with the IAT sensor. I just use carb and choke cleaner and hit em with a puff of compressed air.

 

You use carb cleaner on a sensor? Maybe you should try ether too?

Never use anything other than a clean rag and electrical sensor cleaner on these little things. They are not cheap and are critical to the operation of the truck.

 

Any dealer should routinely make the Starscan input to reclalibrate tire size. The one I use around here charges $17 for such service. Call and ask if you need to make an appointment to get it done. I did not even need an appointment to get the high idle enabled (similar type service) and it took a total of 15 minutes from the time I got out of the truck in the service entrance to when they brought it back to the parking lot out front. Just tell them you got bigger tires and want the speedometer adjusted. Pretty simple.

On a side note, this is one aspect of the programmers I hate: Protecting their profits. Maybe to set a full tune. Maybe. How many people do they really think will be running their tuner all over the neighborhood flashing everybody and their brother's ECM? Most if not all the few people who will do this aren't gong to spend the money to buy one anyway. It's like downloading bootleg MP3s or videos. The market impact on corporate profit is insignificant at best. But for the simple stuff like tire size? Why won't Smarty let you do that on any truck you want? That's just greedy, and why the prices are so high for a little plastic box with some software and a cable. The profit margins on those things are insane. Just one of the very few examples where the dealer wins on price by a wide margin.

And just in case any of the BD, Mads or other guys or their vendors wants to come on here and tell me all about how their coding skills are oh so intangibly valuable - don't bother. I work for one of the largest Aerospace corporations in the world, presently in a shop using our own custom software flying imaging satellites in low earth orbit. So I "get it."

Rant off.

 

I hear you.... I almost agree completely too. Along with the R&D costs I think a lot of these guys do take a higher profit than necessary sometimes. I have to wonder with our current 'sue' happy and everything generates a liability type mentality that if they aren't putting money away for a rainy day.... It all ads up to higher costs for us the consumer! That is the part I hate!! The great thing about our country is that if it is just greed then someone can come along and 'do it better' and then all us consumers win!

A suggestion if you are going in to have your speedo re-calibrated: Look up your tire diameter and Revolutions Per Mile from a reliable site (Like the manufactures site!) so you can give them accurate info. Just because a tire is a '35' or what ever doesn't mean it is exactly that size. Different manufactures actually make them different from one another even though they are the 'same size'. As accurate of a figure you can give them will just put your speedo that much closer to being correct!

 

Sorry I will refer to it as "diesel milage" from now on .

Well I did the 60 second mile test and my speedometer is dead on. It took me exactly one minute to travel one mile. I am getting 22+mpg with an oil change and sensor cleaning. Everything is running great. Now I need to pull the water pump to see why its leaking and fix the axle seal and everything will be in running order (finally ).

 

Maybe I will go out and check my sensors . . . . Gently of course!

My '06 still has an IAT sensor in the intake tube just after the filter . . . . Just like the older ones do. You didn't mean that they had been combined, did you? I do believe the MAP in the intake has a temp function also though....

 

Ok Nancy

 

On my 06 the dealer couldn't recalabrate my tire size their only options was factory size tires. On the 05's and older they could change the tire size. I used a smarty.