Speedometer and Speed Limiter Operation

the head · 08-28-2004, 07:39 AM

I was considering changing to a shorter rear tire for better acceleration. However, I was worried about the speed limiter since with a shorter tire, the speed readings would be higher than actual. In other words, the 115mph speed limiter would "come on" when the truck might be doing 95mph in reality. So I started looking to see if a speedo calibrator box could be installed. Thus far I would have to say no.

After pouring over the downloaded service manual there's a few things I've figured out:
1) The brake control unit sends the speed signal to both the ECU and the speedo unit in the dash.
2) Speed is measured by all four wheels and nothing is mentioned about which one is used by the brake unit to send out a signal.
3) The signal is sent out on a CAN (LAN wire) such that it is not a signal that you can easily change. There is more than one signal on the wire (multiplex).

I'm trying to figure out:
1) Since both the ECU and speedo are receivers of the signal to the brake unit, that makes me wonder if the brake unit has flash memory to send out the signal or if the speedo and ECU have settings specific to the tires used on the truck (or flash memory settings in the case of the ECU).
2) If you run shorter tires in the rear, would that mess with the brake computer reading speed signals from all the tires? In other words, would you have to run smaller tires up front as well?

I guess there is one way to find out... Luckily I have hooked up with the Chevy guys on occassion and some of the trucks out there have an identical bolt pattern to the Titan. I can try to borrow a set of wheels/tires and drive a round the pit area (and possibly make a run) to see if there are any adverse effects.

Soliciting opinions. Anyone better at reading the manual than me or privy to more info?

GETSOMZ28 · 08-28-2004, 09:22 AM

I was actually thinking the same thing, since there are no aftermarket gears available. Though like you eluded to, you would probably have to change all 4 tires or it could wreak havok with the braking system if it were a mismatched pair.

Without reprogramming to take care of the tire change it could also mess with the way the tranny shifts. If it is anything like my Z, both the rpm and speedo criteria have to be met before it shifts.

Sounds like an interesting test!! Keep us posted if you do it.

pciley · 08-28-2004, 10:47 AM

I had to have a previous vehicle recalibrated for a tire/wheel change. I just looked up in the phone book in the automotive section. Any shop that lifts trucks or puts wheels/tires on will know of someone who recalibrates speedos. I'm sure they will be able to do this for you.

JBWilkins · 08-28-2004, 10:52 AM

I'm betting the brake controller send it to the ECU then the ECU sends it to the speedo..... I know most Hypertech Power Programmers for GM and Ford trucks allow for tire size and gear size selections to correct the speedo as well as setting top speed limiters, but GM and Ford are reading their speeds at the drive shaft/transmission (I believe). If we end up getting some kind of programmer it will probably have a feature allowing speedo corrections...

the head · 08-28-2004, 09:31 PM

No, the manual specifically says the brake unit sends it to the ECU and to the speedo via a different route. The ECU is just a receiver of the signal.

Mike H. · 08-28-2004, 09:50 PM

After my tire mishap last week, I called the dealer about recalibration for different sizes. Take this with a grain of salt, but the service rep I spoke with (who also went and asked someone else there) said there is no recalibration available, at least that Nissan can do. When I asked what Nissan does about the different trucks coming with 265's and 285's, he said they are all set the same, and the variation between the two is small. I brought up to him that GM dealers (and aftermarket devices like the Hypertech) can plug into the computer and simply re-input a different tire diameter. He said the Titan computer has no such ability.

What do you guys with larger aftermarket tires do? Just self compensate for the lower reading speedo?

bgh831 · 08-29-2004, 12:14 AM

So what about the traction or BDL control that would be affected too. Doesn't seem to me there is much room to play in that area

Potroast · 08-29-2004, 12:21 AM

Dakota Digital makes a product for this:

http://www.dakotadigital.com/Detail....-7/product.htm

the head · 08-29-2004, 05:34 AM

I don't think it will work since the Titan uses 4 speed sensors. Ganted, it is a square sine wave but you'd have to put 4 boxes in it to work. Perhaps that's the only way to do it. Those boxes work with a single speed sensor. And the signal to the speedo and the ECU isn't the analog variety, it's digital.