I just got my first set of replacement tires for my 2002 Tacoma and
discovered something interesting. The dealer installed 265/70 16's
BFG Rugged Trails the truck came with seem to have been a little too
small. I noticed my speedometer was about 5 MPH off at 70 MPH (checked
with my GPS) I was actually going slower than the speedo measured.

I just replaced them (after 43000 miles) with BFG All Terrain T/A KO's
in 265/75 (these are 1" larger in diameter) and now my speedometer is
spot on. WTF?

The inside door panel shows 265/70's as being the right size and even
the owners manual list 265/70's as a standard size for this truck. So
why then does a larger tire make the speedometer accurate? The NTB
database listed the correct size as 265/75 and they do fit with no rubbing
problems.

I guess I'll just enjoy the extra 1/2" of ground clearance and newly accurate
speedometer. I'm a little bummed about my spare being smaller but I should
be able to get by with it if I ever have any trouble.

I wonder how many extra miles my truck has logged due to the wrong
tire size?

 

Man, my 2000 4runner is EXACTLY the same!! I thought it was only me.
I figured mine was about 4 mph off. When my speedo read 74 I was really going about 70. Now with my 265/75/16's on, after checking it with a gps, my speedo is maybe 1 mph too slow. ...I don't know what the deal is.

 

You will also notice that your gas milage is going to drop as well. I would check and recalibrate your GPS system...

 

built in fudge factor... a speedo is allowed to be too fast, but by federal law, CANNOT under any circumstances be too slow. 3-5mph too fast at 70 is typical

 

Okay, to answer my own question: 5MPH off at 70MPH means that
for every 70 "miles" I traveled I actually only traveled 65 miles.

So 43000 miles later (on the odometer) my truck has really only
43000 - (43000/70)*5 = 39,928 actual miles on it.

 

Those were the correct size tire for 2001-2004.

 

as was posted above, they set up the speedo to read high so because of federal law. but i'm sure the odometer was recording just fine. now with the new tires your speedo will be just right, and now the odometer will be reading low. the same thing is true for my '89. it came stock with 235's which are 28", i have 31" on there now and the speedo is right on when compared with gps, but the odometer reads low.

 

That's weird. I have 31s on my '93 and the speedo and odometer are both off 10%

 

I was a bit sceptical when I read this but my GPS confirmed this behavior
this morning. My odometer is now about 3% off.

It seems strange that they would intentionally make the speedometer
read low. All they would need to do to make it accurate with the
stock tires would be to reprint the graphics on the dial. I guess this is
their way of fooling people into driving slower

 

When I posted on TTORA about going from 31's to 32's and the speedometer accuracy, this is a response I got:

"going from 31 to 32 probably won't make much difference. Several here have done that and their speedo/odo is actually MORE accurate. there is a 5% error factor up or down in the factory speedo/odo and you are only changing by about 3%. Put on the new tires and check your speed with GPS."

By the way what would be a decent GPS to buy that allows you to measure speeds while driving, or do most of them do that? I haven't really looked into them.

 

Guys, about the GPS measuring speed: The GPS could just as easily have that 5% error or whatever. Whenever the GPS is recording many data points, that quickly, the accuracy is going to be a little iffy. I worked on a Geology research project deal this summer and did quite a bit of GPS mapping. In order to get the accuracy needed, I used a differential GPS that took twenty seconds to measure a waypoint. The handheld ones are great, but I wouldn't trust them down to the single digit; I'd say +- 5 mph would be really good accuracy. And don't forget, when the cops tracked Scott Peterson with GPS he was going a speed of about 50,000 mph
-Nathan

 

The exact position a GPS is telling you is a little iffy by design. The speed,
direction, and time seem to be pretty accurate. My GPS reads out speed
to the 1/10 MPH and has matched my other 2 cars (both Honda's) speedometer exactly every time I've checked.

I think the military is being a little too paranoid by forcing the positional accuracy to be off. A terrorist is going to walk/drive their bomb to the
intended target. How many have used a GPS guided missle? :pat:

 

Well, as of two years ago (I think) the GPS system is no longer "locked down" New units you can buy are WAAS (Wide area augmentation system, I think) enabled. The WAAS is just a built in correction factor, previously unavailable to civilians. The very nature of light waves is one of the problems with GPS accuracy. Radio waves bounce off of stuff, getting reflected and refracted. Every year newer, better, faster, more accurate GPS units come out, so eventually the accuracy will be pretty darn close -- if not dead on. The way the super-accurate ones work is with an additional radio tower,broadcasting a correction factor, near by along with a fairly large (and heavy, I found out) computer to process those correction factors. Right now, the hand-held units are fairly accurate, but I still wouldn't trust them for calibrating a speedometer.

 

On my daily commute there are electronic signs next to the freeway that show your speed as you pass by. How accurate do you think those are?

 

I think those are pretty dang close; around here I've seen police sit at them and pull people over that are going fast. I think they're as accurate as what the five-o has.

 

I was referring to SA (selective availability) where they randomize the clocks
in order to introduce error. Only the military units can decode the satalite
clocks correctly when SA is turned on. I had read some time ago that soon
after the president ordered SA to be turned off it was turned back on within
a few weeks due to security interests. The official word seems to be that
SA is now off but they now have a way to introduce error into local areas
if needed. (via local WAAS towers perhaps)

Anyway, this is getting way off topic. The tire size issue seems to be solved
now. The 265/70's are the correct size it is just that Toyota makes their
speedometers under report the speed by about 5% while keeping the
odometer accurate.

I'm glad I went with the 265/75's though. I couldn't find the BFG AT's in
265/70 here locally and I like having the bigger 32" tires.

 

I've wondered about those as well. Seems like when I pass them with the speedo on 70mph, they read more like 64 or 66 (I can't remember).

Gotta double check my tire size (what ever came on when I bought it).

Been wondering about this for a while, glad someone brought it up.

 

sorry about the post hijack but how many miles did ya'll get out of your dealer new tires? I've bought 4 brand new toyotas and they all seemed to have less tread depth than the replacement tires i bought 1.5 years later. is it just me or do the new vehicles come with "half treads"