Bigger tires, speedometer correct what about odometer

I just put a set of 267/70/17's on my 05 Tacoma it had (265/65/17) The speedometer was always off by about 4MPH and now it is spot on. Dose that mean my odometer is correct now? I don't see why not , but I saw that these come from the factory with the speedometer off? That doesn't make sense to me I don't see how it can be incorrectly calculating the speed and not the distance? Anyone know? I wouldn't really care, but I use my odometer for my tax right off.

 

If your odometer was correct before, and speedometer was off - and your speedometer is now correct - logic says your odometer will now be off.

Test it - do at least 30-50 miles by mile markers. I'm not convinced GPS is as accurate on distance, it is on speed.

Think about it, speed sensor only knows how many revolutions the wheels made, but it doesn't know the diameter of the tire, which varies as the tire wears, or in your case that a possibly different size/diameter tire is now on the vehicle. Software does the rest, which also accounts for the original odometer/speedometer difference - different section of code calculates each.

 

Not saying the odometer was correct before, I can only assume that if the speedometer was incorrectly calculating the speed that it had to be incorrectly calculating the distance traveled.

 

Odometers are correct from the factory (speedo's are usually not) but will be off now that you changed tire size. Take your new tire diameter (in my case, 31.6") and divide it by your old (in my case, 30.6"). Take that number and multiply it by your mileage and you will have your correct number.

 

Your mph shouldn't change that much with the size you are at now.
I have had 265,285,305,315 size tires. And with 35 inch tires i was about 4 mph slower than what my speedometer was saying.

 

Do you have a GPS?? If not, find one or borrow one and check both speed & distance.

Yes, the speedo & odo can be 'different'

My truck is stock. Stock tires, etc. My speedo is 2mph slow. I have verified the actual speed with a couple different GPS's. I have also verified the actual distance with GPS & actual mile markers on the highway. My ODO is actual - I've verified the ECU readings from the scangauge.

I'm guessing, there's a calibration between the speedo needle and the numbers which has renders it 2mph slow. Not a big deal.

However - anytime you change tires, small or large...DO NOT assume speedo & odo are accurate. Always have both speedo & odo checked to see exactly where your actual numbers are.

 

I also have a ScanGauge. Accuracy changed due to tire wear. Lost an inch in 37,000 miles, replaced tires and had to recalibrate. Place on checking every 10,000 miles.

 

Are you guys stating that the odometer and the speedometer have different inputs?

 

That is hard to follow.....

I wonder how that could be true and like what was said in the past about the speed being off and the Odo being correct.... Not sure how that works.

most the time the get the input from the trans and one talks to the other so if anything in the gearing changes like tire size it will change both.

Mine was almost 4MPH fast, added 265-75-16 and it now reads correct by several GPS units. The ODO is off a little from what I found.

 

The odometer gathers it's mileage as a direct reflection in accordance with how fast the speedometer states it is traveling.

Hope that helps.

 

FWIW, my previous base model 2wd stock both were off, up sized the tires and both were dead on. Have not checked my current truck.

 

They have the same input - usually a sensor and tone ring in the transmission or T-case - but different calibration. The odometer is accurate with the stock tire size, but the speedometer overstates the speed for legal reasons. With stock 31" tires, my speedometer overstates by about 3 mph when compared to the GPS.

Yes, going with larger-than-stock tires will introduce odometer error, but it works in your favor as the truck will record fewer miles than actually traveled.

 

What legal reasons would they have to not have it correct?

 

Maybe we should say for CYA reasons.

To save themselves from lawsuits - ie people have sued manufacturers claiming the speedometer mph was reading low and that's why they got a speeding ticket.

Accuracy is going to change as tires wear - only thing the vehicle knows is how many tire rotations occur, the rest is done by software. As a tire wears, it's diameter is smaller and you cover less distance for a rotation, hence speedometer/odometer become less accurate over time (until you replace the tires).

The old story on new vehicles, mileage will improve after 10,000 miles (or whatever number) is in part due to tire wear.

 

FMVSS apparently no longer has a requirement for speedo accuracy, but European law ECE-R39 does:

http://www.unece.org/trans/main/wp29/wp29regs/r039r1e.pdf

So the speedo cannot understate the speed, but can overstate by 10% + 4 km/h. Since there will always be some % variation with a vehicle's speed measurement system due to mfg tolerances, carmakers as a precaution set the nominal speedo calibration under the true speed.

Odometer accuracy is not governed by similar regulation, though for warranty and leasing fairness the readings tend to be nominally accurate. i.e. an odometer that understate milages hurts the automaker, while an odometer that overstates hurts the customer, and has been the subject of lawsuits.

 

I am planning on changing gear ratios from 4.10 to 3.91, hopefully to give me better mileage. Don't plan on changing tire size, at least yet.

I really want to have an accurate odometer reading. Can the dealer or anyone else calibrate the odo to reflect the change?

Len