Speedometer error with stock tires?

So when I bought my '99 a couple weeks ago, it had 31" tall tires on it, so I knew my speedometer/odometer would be off, so I just used my GPS for speed. Last week I swapped on some 225/75R15 snow tires that I had from my former truck, and I expected the difference to go away, as that is what the door jam said is the stock size. But it didn't, it actually got worse. I didn't think of it at the time, but with the 31"s the error was the wrong way one would expect. Like if the speedometer said I was doing 70, my GPS said I was doing about 65. Well now that I have the stock tires on there, if the speedo reads 70 I'm really doing a little over 60. The odometer is also off the same 13%ish as the speedo.

I jacked up the truck and spun the tires and it does still have the 4.10 R&P that the doorjam plate says it should have. I also pulled the speedometer sender, and it has the 30 tooth gear on it that from what I could find online should be the right one.

Is there something else I should be checking to see why it is reading off? I mean I don't really want to go to the hassle of installing a dakota digital unit just so everything works right.

 

Toyota calibrates the speedometers so any error is on the 'fast' side (i.e., they read 'fast' but not slow). Factory spec allows for the speedo to read 4 or 5 mph 'fast' at expressway speeds, so your variance was actually 'within OEM spec' with the 31" tires.

If it got worse (and 'way out of spec ...), that would seem to indicate there's already been some sort of adjustment made.

Did you check to see if the previous owner had already installed an electronic speedo calibration unit (like the Dakota Digital one)? That would explain it ....

 

Wait a minute ...

I just checked the factory 1999 spec sheets ....

1999 Tacoma (non-TRD) 4X4's got 4.10 gearing "with optional 31" tires". 4X4's with the 225/75/15's got 3.91.

In other words - it looks like 31's are actually 'stock' on your Taco.

 

Really? so do the doorjam plates always still reference P225/75R15 even on trucks that came with 31"s?(see below) Do they use the same R&P for both the V6 and the 4cyl? because mine is a V6 if that matters. Also are the factory spec sheets available online somewhere, I think it would be interesting to take a look at them.

Oh and there was no sign of anyone tapping into the wiring harness down at the sending unit, haven't torn into the dash or anything to see if someone has messed around in there more than needed for the aftermarket stereo.

Thanks for the help!

 

I'm running 285/75/16s and I figured the speedo would be off by 6% (according to 1010tires calculations) But it was only slightly off, about 1 mph at 60 from the GPS reading. So I assumed the speedo with stock tires was a bit off to start.

 

Except for the vin# and the date of manufacture, that could be the sticker on my door jam.

As EnolaGaia indicated, Toyota sets up the speedo to read high. The law allows them to be off by up to 5% but more important (to them) they cannot be low on the mph readout or there could be federal fines.

According to the GPS, when I pit 32"s on (came stock with 31"s, 265/70-16s) my speedo reads almost exactly right. The ScanGuage seems to be confirming that.

I suspect that your truck came with 31"s from the factory.

 

Okay, well I guess it did come with 31"s then. Guess I need to start planning on adding a Dakota Digital unit to it. Thanks for the help.

Although I still find it odd that they have that much error in them even stock. my other two Toyota's are both within about 1-2 mph across the board.

 

My '05 Tacoma door jams had 2 tire sizes... the 'base' size and then with the TRD Off Road package, the 265/70-16 sticker is added. My '10 only has the one size sticker.

 

Yeah mine is off too, will read 70 when I'm going about 67. All toyota cars I've driven did this same thing. Quite annoying.

 

mine is off too when i went from 245 stock tires to 285 now when i am running 70 my gps says i am running 78