Does Speedo off equal Odometer off?

I'm having a LOT of trouble explaining what I mean....and I have no idea how to explain it.

It has nothing to do with the tacoma in general. It's a simple idea that if you have only ONE data source (input) and that data is calculated and manipulated to multiple data points (output)...... If the original data source is wrong, then ALL the output data is going to be wrong.

Ohhh nevermind...

 

You are dead on the nail, my friend. At least someone understands.

 

I'll beat this dead horse again... (I did base this off what I know and a few assumptions but believe them to be correct. If I'm incorrect, feel free to correct me)

From what I understand the ECU obtains data from the transmission in rotations of the output shaft (I don't think it comes from the wheel sensors, I think that's for ABS). So we all agree there is one point that the distance/speed is derived from. If you install larger tires, you will physically move faster than the output shaft indicates and if you install smaller tires you will be travelling slower than the output shaft indicates, assuming you don't change anything else like the rear end or reprogram the ECU.

So we have a single source which is now the ECU and it converts the pulses from the output shaft into a digital and analog representation. The digital is for the ODO and is highly accurate. Now the analog part is not so accurate. The ECU should create a voltage consistantly throughout the speedo range and is done with a digital to analog converter and that part would be consistant with known values. But the one variable that is not "Digital" and consistent would be the speedometer itself. It's magnetically driven and unless it's calibrated like a voltmeter, then it will not be as accurate. I'm sure there are acceptable tollerances for the needle like it's okay to say you are travelling faster than you physically are. We see that all the time.

Okay, I'm totally done with this dead horse and I think many of us are saying the same thing, just in a different way.

-Joe

 

Putting larger or smaller tires on will change the input which will affect the output - odo and speedometer. They'll both be off and off by an equally marginal amount depending on how large/small the tires you're running (from stock).

 

Yaaaaaaaaahhhhhhh Janster....Holy crap that was simple explaining of a very technical problem

 

Changing the tire sizes have no affect on the "Input" to the ECU since it's taken at the transmission output shaft, well before the tires. But I fully agree the speedo/ODO will both be off in the same direction.

-Joe

 

I think everyone can't explain things worth a shit, but in the end we're all saying the same damn thing....


CAN WE AGREE TO THAT? hahahha..... Honestly, I agree... we all seem to be on the same page.

 

Wrong.

Changing the tire diameter size changes the speed that the wheel spins at a given speed - because a smaller-diameter wheel has to spin more revolutions to cover the same distance as a larger-diameter wheel. As an extreme example, skateboard wheels spin MUCH faster at 60 MPH than say tractor tires at that same speed because one revolution of a skateboard wheel only covers a few inches, whereas a tractor tire covers several feet.

The output shaft in the transmission spins in direct relation to the wheels, so if the wheels spin slower with larger tires, then so does the output shaft. If you put larger-diameter tires on your truck, the wheels spin slower at 60 MPH that they did with smaller-diameter stock tires - so the output shaft in the transmission spins slower at 60 MPH - so the magnetic pickup on the output shaft in the transmission sends a lower frequency of pulses to the ECU - which interprets that as a slower speed - which it sends out to the odometer/speedometer - so your speedometer reads slower than 60 and your odometer records fewer miles than it should.

 

This has reached god damn histyerical beacuse i have been trying to get people to understand how tire size affects the car with mpg and all other kind of nonsense, i am so relieved to see that there are other people who are as intelligent as i am and posess common sense

 

I don't understand the "wrong" statement, I fully agree and that's what I said. The only difference was my constant was the output shaft of the transmission and yours was MPH or distance, just think about what I wrote with that in mind. We said the same thing. I was waiting for you to break out Pi and start talking circumfrence vs. rotational distance or something like that. Oh wait, I just did that, crap.

Yea, time to unsubscribe to this thread. For the most part we all agree but are just saying it differently.

Last note: If you (the reader) believe you are having a problem with your speedometer or odometer, contact your dealership. It's your vehicle and we can only give advice on what we know, or think we know.

-Joe

 

Raven, he's not wrong, he just explained it differently. Do you really think he's stupid enough to truly believe that putting 35s on your truck compared to 30's is going to give you the same readings? No. He just didn't explain it well enough that you understood.

The fact of the matter still stands that we all suck at explaining it. The simple truth is its so damn simple we're all blowing it out of proportion.

 

then he should have said, "Changing the tire sizes WILL have an affect on the "Input" to the ECU since it's taken at the transmission output shaft WHICH ONLY REVOLVES AS FAST AS THE TIRE. THE TIRE IS THE START POINT FOR DATA AND TRRANSMISSION OUTPUT SHAFT IS ONLY WHERE IT IS FIRST COLLECTED.

you cant blame Raven for taking that wrong...as for the caps that was not yelling just the extra rewording. WOW we are beating this to death arent we? hahahaha

 

EXACTLY... thanks.

We may all actually agree, and I apologize if I was too abrupt, but this is an incorrect statement no matter how you slice it: "Changing the tire sizes have no affect on the "Input" to the ECU since it's taken at the transmission output shaft, well before the tires.".

The transmission output shaft spins in DIRECT relation to wheel speed, so if you install larger-diameter tires (causing the wheels to spin slower at any given speed than with stock-size tires), you are directly affecting the input to the ECU.

Now... let's talk about something we can all agree on... like what kind of OIL to use!

 

And I love that dead horse picture!

 

I comprehend and understand that completely, thanks. I also still say you're WRONG if you're telling me that "Changing the tire sizes have no affect on the "Input" to the ECU...". Non-stock diameter tires = incorrect signal going to ECU. Period. End of discussion. How can you possibly say the tire is not the starting point for the data collection when its size is the single most important constant in the formula?

By the way... I went through all this several years ago when I changed the rear-end gear ratio of my 1996 Mustang Cobra from the stock 3:27 to 4:10. There's a cool little device you can buy that splices into the line between the sensor in the transmission and your ECU that converts the signal by whatever ratio you set it for - so that your ECU receives the corrected signal - and your speedometer and odometer function correctly. It's called an Abbott E.R.A (Electronic Ratio Adapter). I installed one in my car and it worked great. Very easy to set up and install. HERE is their website. There may be other devices like this available now... I haven't looked in years. This was pretty much the only game in town back then. I'm sure it would solve you guys' speedo/odo problems when switching to larger tires though.

 

I'm going to add a few minor comments here from Raven65's post and my comments are in red:

We may all actually agree, and I apologize if I was too abrupt, but this is an incorrect statement no matter how you slice it: "Changing the tire sizes have no affect on the "Input" to the ECU since it's taken at the transmission output shaft, well before the tires.". (Not really)

The transmission output shaft spins in DIRECT relation to wheel speed (True but you used the word speed, it's more rotation since speed is calculated over time), so if you install larger-diameter tires (causing the wheels to spin slower at any given speed than with stock-size tires), you are directly affecting the input to the ECU. (I understand what you are getting at but shouldn't it be from the perspective of the vehicle ECU or the Speedometer/ODO if you will. If you're like most people you drive the posted speed limit (we will assume we all never speed, much) by what's on your speedometer. If you place larger tires on the truck and unless you know for sure the difference you will still drive the speed limit based on the speedometer even though you will actually be travelling faster down the road. Do you see my point of view? The transmission still tells you the vehicle is going 55 MPH but you're really going 60 MPH (example) by the cops radar detector. I guess if you drive by GPS speed that could be a different point of view but your ODO will still be incorrect.)

Now... let's talk about something we can all agree on... like what kind of OIL to use! Agreed ! I haven't had my first oil change but it will be a synthetic blend.

-Joe
P.S. Did I say I loved that dead horse today?

 

My brother-in-law only buys Fords and has a sweet collection of Mustangs and some old ones he's fixing up. He's a full blown mechanic, more than I ever wanted to be but it's nice to have those skills but I just don't see how he can afford to buy those things and restore them.

Read that E.R.A., nice, very nice and it seemed simple enought to set up. I'm curious how many pulses the output shaft does emit for a single rotation, is it that simple that it's one pulse or is it 4 (one for each quarter rotation) or more. Well someday I may need to know, or not.

-Joe