Speedometer accurate odometer off

Hey all, my ‘02 double cab 3.4 speedometer is dead nuts perfect/ accurate via gps. As in down to .5 mph. However, the odometer consistently reads 4% less miles than actual. Went on a 110 mile trip today (per gps) and the truck read 105.9 miles.

I thought the odo and Speedo got there info from the same source? Any insight appreciated!

 

It is impossible to get the odometer 100% accurate 100% of the time. As you drive your tires wear down and get smaller. As they wear you need more revolutions of the tire to cover the same distance. The odometer simply counts wheel revolutions and uses math to compute distance traveled.

My son works in electrical construction and commutes to worksites each day. They typically work on a project anywhere from 2-3 months to more than a year. When that project is finished they move to another. He drives 80-120 miles round trip depending on where they are working.

A couple of years ago he asked me a similar question. He noted that for the last few months he had been driving exactly 100 miles round trip every day. Suddenly he was only driving 95 miles each day over the same route. I pointed out that he had recently bought new tires for his car. And this was a 2012 Nissan Sentra. On a truck, especially one with all terrain or mud tires there will be a significant difference in tire diameter of a new tire vs one near the end of its useful life.

 

Thanks for the fantastic info! How can my speedometer be spot on and my odometer 4% off though? I thought they both read off trans/ each other? Thanks!

 

Hmmm. How 'bout this:

1. Get on a highway (not an interstate) and stop next to one of the mileage markers that's on the right side of the road.
2. Note your odo reading or zero your trip odo reading.
   
3. Zero your GPS odometer reading.
4. Drive 50 miles.
5. Compare your odo and GPS odo readings.

Do they match?

 

Without the GPS input you would never know .

If it bothers you that much I am sure looking into High tech Calibration gear you can find something to cure your differences

 

I enjoy this type of stuff as a curiosity. But as a real world concern...it matters zero. Wouldn't worry about it.

 

Thanks for the replies y’all, it’s not a concern just curiosity is all. I will do the mile marker/ trip test though that is a great idea!

 

Somewhere I read on TW recently some gen Tacoma’s use a different sensor/signal for odometer than for speed, not sure if that applies to yours.

 

Davidstacoma that would make a lot of sense if that’s the case with this ‘02

 

for what it's worth, no one asked the important question

what size tires are you running @spencerdiesel ?

from the factory the ODO is pretty spot on where as the speedometer is actually calculating slower (1-4mph depending on how fast). assuming for either safety reasons or just not being able to perfectly dial in the right amount of teeth for the gear in the output shaft of the trans due to manufacturing costs.

I went from stock ~31" tires (30.85) with 4.10 gears to 32" tires (31.75) and the speedometer is dead on (fractionally off but yeah we'll say dead on). now the ODO is off but honestly I don't care anymore and just write my mileage knowing I am getting better than listed.

 

Great question @frizzman! 265/75r16

 

This is worth a read.

Speedometers, Odometers, and Gas Mileage – All Lies!

Spot on.

All 1st gen Tacomas use the same sensor for both ODO and Speedo, so this statement is never true on a 1st gen. In our case, from the factory, the speedo is 3-5mph faster than you're actually travelling @65mph (largely so that Toyota can't be held responsible for, "but officer, my speedo said I was going under the speed limit"), and that means that the ODO is racking up 4.6-7.7% more miles (3/65 = 4.6% - 5/65 = 7.7%) than you're actually travelling.

 

Tech has surely gotten better. Are things accurate now? I'd still say "sort of," despite the sensors in a self-driving vehicle. Several reasons:

• GPS is good for general accuracy, but none of us get access to the military grade stuff.
• Sensors are obviously better, but there aren't tire temp sensors trying to figure out the diameter of your tires (because you can always install aftermarket tires/there are always variances even in a single brand/model).
• All this stuff has to work without internet/GPS connectivity. Think middle of nowhere (internet), tunnels (GPS), etc. You can't have your ODO stop tracking while you're shielded from the GPS constellation, etc.
• Liability - I can't say with 100% certainty, but I'd take the bet that no auto manufacturer releases a vehicle that has a speedo that is reading slower than you're going, when it rolls off the line. They want to be able to say that they aren't the problem, should it ever come up.

Anyway, I agree with your sentiments, and that things are getting better... but perhaps most importantly, I agree with your statement that we are talking about 20 year old trucks here...

 

there ya go, those tires are larger than stock so you are in my boat. we're ~32" so the speedo reads more true and the ODO is off.

the speedo reading faster is true for that reason (at least I assumed so when going through zones with the "speed" signs for construction seeing me going 1-2 mph slower than it read)

 

One problem is GPS accuracy. It's hardly perfect, especially on curvy roads, or if there are lots of trees or buildings. The accuracy of what you see on google maps directions is not your actual position, but an algorithm that places you in the middle of the street it thinks you're on.

GPS doesn't calculate your exact path, but the distance between two points. Generally the points are close enough together, and there's enough of them not to matter. But on a curvy road, it could easily cut some corners.

If you're in an area with lots of buildings, you get a phenomena called multipathing, where the signal bounces of a building or two before getting to your receiver. That can add quite a lot of error. Power lines and other EM radiation also cause interference, again, not usually enough to matter.

I don't think it matters if it reads fast or slow. I think the regulations just state it has to be within some "reasonable" error, like maybe 3-5%. LEOs also have a similar fudge factor because they understand that speedos aren't perfectly accurate, which is why you'd never get a ticket for going 5mph over (unless you're in a 5mpg zone, lol).