In the "old days" (late 80's early 90's) when we jacked up an old truck  and slapped some 40" boggers on the 15" Cragars (LOL) we would have to change out a gear where the speedo cable went into the transmission if we wanted to have an accurate odometer. That was the only way to do it.

I am trying to determine if the new, more modern vehicles are still relying on such antiquated means to determine distance traveled.

I have a 2008 FJ Cruiser. I replaced the factory DunFlops with a set of BFG KM2's, size is 255-85-16. This gave me what I wanted, a ~33" tall tire, but were narrow enough to allow me to avoid lifting, which I wanted to avoid at all costs.

Here is my question. I am wanting to know if the FJ is electronically compensating for the different overall diameter of the new, bigger tires and is giving me an accurate reading on my odometer. Or, is the odometer actually slightly inaccurate?

The reason I am asking all of this is because I have recently started using an app on my Droid Incredible that allows me to track everything that I do to my vehicles, from all service work, to MPG etc. and it charts it ALL, giving you all kinds of awesome info. Letting you compare MPG with 87 octane vs high test and tons of info in between.

So, bottom line. Now that I am running 255-85-16's as opposed to the factory tires that come with the off road pkg (about 1" smaller in overall diameter, IIRC), is my odometer being corrected electronically (thus giving me an accurate representation of distance traveled), or am I going to have to manually figure actual miles traveled per tank? If so, does anyone already have the formula figured out or will I have to trust a GPS to illustrate the difference so I can make sure the data I enter into my app is accurate? Thanks

I believe a treadmill is involved, those show speed don't they?

when I changed rear end gears in my 02 formula, I had to get it reprogrammed to make the mph work.  some of these programmers you see on tv will do it for the new chevy trucks, in my experience.

Magnets on the drive shaft send signal to the computer



But how do magnets work??

Your speedo my be electronic but unless it's gps then your speedo and odometer will be off...your speedo will be reading slower then you are actually going.  

I believe they can be calibrated or you'll still have to change the gear on the sending unit down in the transmission tail housing.

You see, there's this really long spool of thread and...



ETA: why are you asking this in GD?

MPH and odometer readings are still based on the speedometer cable.

Rotations of the gear you're currently in.

Good choice on the pizza cutters.

They no longer use a gear, but it works the exact same way - revolutions of the axle * whatever stock diameter, so your speedo and odo are no longer reading the same as stock.

I don't know about the FJ, but I put 1" larger than normal TRD off road tires on my Taco (31.something actual to 32.something actual), and it made the speedo and odometer exactly accurate, where as before with the stock size tires the speedo and odo read high. This was confirmed with a couple of GPSs, timed mile markers, radar speed signs, and is a generally known issue in most Toyota forums.

Minus the cable portion these days;-).  Should be a little box bolted to the trans with a a few wires.

That is what I figured. I kind of figured I would be a few percent off.

Prior to the tire change (when odometer was pretty much dead on, I routinely would top 18 mpg. After the tire swap, it lost apx 1 mpg (based on a now less accurate odometer).

I will do some GPS measurements, along with a couple other things, to determine the percentage off that the odometer is, and then adjust accordingly.

The one reason that I thought (and still wonder a bit, to be honest) that maybe there was something electronic compensating for the tire diameter difference is this. When I check my speed using various GPS devices, I find that it is almost dead on with the trucks speedometer. I mean like within 1 mph, so I assumed that the new larger diameter tire was being compensated for. I mean, if the speedo is that close to the GPS indicated speed, then it cant be off very much.

I will do some more thorough testing and report back to you guys. Thanks for your input.


ETA- I routinely risk GD. I am a big boy, and have been around here for awhile. The BS is usually tolerable given you get a pretty decent return on traffic.