I’ve seen a few questions on tire size vs. speedo calibration. Just thought I’d share a bit to help with tire size decisions. There are tons of threads on the mods needed to fit larger tires, so I'm not touching that.

So, if you’re going to do a lift and want to install bigger tires and want your speedo to read correctly here’s what you can do since the factory programming only allows for combinations of gears 3.55 and 3.92 or tire sizes of 235/75/15 (28.9”), 31/10.5/15 (30.6” same as 265/70/16), or equivalent diameters.

If you have 265’s and 3.55 gears (like me) you’re stuck there without buying new gears.

If you have 3.92 gears, you can have the comp. programmed for 3.55s w/ 265’s, run a 33.7” tire (285/70/17) or equivalent diameter and the speedo will read the correct speed.

From here you would want lower gears maybe even an axle swap anyway. So with the same 3.55 w/265’s setting you can throw in some 4.11 gears and run a 35.4” tire or 36/13.5/18 e.g.

With 4.56’s the target tire size will be 39.2” or a 40/14.5/20 e.g.

 

Or you could save the money and get a PP and adjust up to 40".

 

LOL....Whatever!


or you can do the right thing:

Get the tire size you want, and gear it the way you want, for what you need the truck to accomplish, and for about $50 bucks go to the dealer get the tire size programed "what you have chosen that best suites you" into the PCM and not only will your speedometer be on you will have the very best truck with the power where you need it all for the price it would have cost you to do it correctly the first time. There isn't just a "one size only fits" so you have to decide what you want, it's programed in numerous sizes by the dealer.

The other thing you can do is buy a programer and do this yourself, plus you get the advanced torque and horse power from the PCM update. You don't just have to settle for what the dealer offers and correspond that into gearing and tire size just to get around the actual issue.

Now you can sit there and figure it all out on paper like ZCR1 has done, but I'm just saying there are a lot better options out there.

 

I've been told by numerous sources including dealerships that you can't program the pcm with any tire size and gearing combo you want like the chevy's, but only combos that were offered from the factory. If this is not correct, then why doesn't Hypertech, Superchips etc. offer this option on their tuners for the gen 1 D's like they do on other makes?

 

Hypertech, and Superchips do offer the tuners with gear and tire size combination attributes. Where do you see they don't? In fact they offer many different combinations. Just pick up a phone and call em.

 

Correct, anything can be done with the right computer. They have to allow for more than factory tire size and r&p ratio, it wouldn't make sense if they didn't...........

 

I have a Hypertech for my 2001 now and had one on my old '98 D, neither offer tire size/ gearing options. I have even had my current one updated a little over a year ago. Yes they advertise this option on their overall ad, but it's not available for the dodge tuner (at least for gen one D). Yes I did talk to a Hypertech rep hence one of my sources. I hear every one saying that it can be done, but has anyone? And if they had it done did they verify the tuning wasn't just done with the factory options that just more closely matched their setup?

 

This makes perfect sense in the interests of Chrysler to protect their product. If they were to have an open source program then everyone would be fiddling/screwing up their trucks, then posting on forums and filing complaints on what pieces of crap the trucks are because they don't want to admit they screwed up.

 

Well I have heard of people actually doing it?

I will say this, in my Ram which is a 1999, the speedometer is controlled by the ABS module (the CAB) and the only way for me to correct my speedo is to take it into a dealer and for about $50 they will update my ratios. I often wondered about that very thing, in my eyes I ask why the Ram would have been that different then the Durango even sharing the same basic PCM?

 

Well why couldn't they? There is a single algebraic (I think it is actually calculus based) equation that uses the 2 constants of r&p and tire size and the variable of how fast the speed sensor is reading.They can change the constants so they can use it for every vehicle, the abs module computes just like your calculator to spit out an answer and it is displayed on your dash. If they can do it somebody good with computer could......also when I replaced my abs module in the dealer said they can reflash any ratio into it.