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help on tires/gearing effect on speedo and odometer

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Old 08-11-2011, 08:24 AM   #1
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Question help on tires/gearing effect on speedo and odometer

question:

how do our trucks tell the speed we're going?

how do our trucks tell the distance we have traveled?

...my thoughts and my confusion

when you move up in tire size, the speedo will read lower than actual speed due to the diameter of the tire being larger than stock causing you to travel a longer distance in the same amount of rotation. so in my case when i went from the stock to a 285/70/17, my speedo would read 60mph while i actually was going ~64mph. that all makes sense..

tire calculator to figure change in speed from the change in tire size
tire calculator

the confusion sets in when you re-gear...

so now i have 4.56 gears with my 285s...

using the formula set forth by WhatThePho? in http://www.tacomaworld.com/forum/per...r-formula.html, my true speed is now .846 of my speedo reading...

so now when my speedo reads 65mph i'm really going 55mph

does this also mean that the mileage on my odometer will read higher than the actual mileage traveled? if it reads 100 miles, does it mean i really only went 84.6 miles?

and this is where the question came up...how exactly do our trucks calculate the speedo and odometer readings?

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Old 08-11-2011, 08:57 AM   #4
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In simple talk: The stock tires (265/70-16) cause the speedo to read faster than you are really driving: When it says 65 mph, you are really doing about 62-63.

With a 1" taller tire (265/75-16), my speedo was right on.
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Old 08-11-2011, 09:01 AM   #5
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I too would like to know the answer to this.
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Old 08-11-2011, 09:41 AM   #6
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Quote:
Originally Posted by Dark Knight View Post
get a speedo calibrator from Hypertech

$165
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Old 08-11-2011, 09:50 AM   #7
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To answer your questions of how they get the readings.....

If you have an automatic trans truck then the reading is sent from both rear ABS sensors to the ECU that then sends it to the cluster.

If you have a 6 spd. trans then the reading is sent from a sensor in the rear output of the t-case to the ECU and then to the cluster.


An easy and cost efficient way to get it reading spot on, Dakota Digital recalibrator. It's not as fancy as the hypertech and requires you to actually cut and splice wires but it works great. I've got it installed on mine and I can go 100 miles on the interstate and watch the odo trip each mile as I past the marker posts.
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Old 08-11-2011, 10:24 AM   #9
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Quote:
Originally Posted by NYCO View Post
if it reads 100 miles, does it mean i really only went 84.6 miles?
no. if the odometer shows that you went 100 miles, then you actually went about 103.5 miles.

the circumference on the 265's is about 99.29 inches
the circumference on the 285's is about 102.76 inches

thats a 3.5% greater distance traveled in a single revolution of the tire.

gearing will not effect the distance that the truck travels in a single revolution of the tire.

however, gearing will effect the rate that the truck travels in a single revolution of the tire.
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Old 08-11-2011, 10:30 AM   #10
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Quote:
Originally Posted by NetMonkey View Post
no. if the odometer shows that you went 100 miles, then you actually went about 103.5 miles.

the circumference on the 265's is about 99.29 inches
the circumference on the 285's is about 102.76 inches

thats a 3.5% greater distance traveled in a single revolution of the tire.

gearing will not effect the distance that the truck travels in a single revolution of the tire.

so the gearing effects the speedo but not the odo?
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Old 08-11-2011, 10:45 AM   #11
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Quote:
Originally Posted by NYCO View Post
so the gearing effects the speedo but not the odo?
from what Fire931 said:
"If you have an automatic trans truck then the reading is sent from both rear ABS sensors to the ECU that then sends it to the cluster."

the distance traveled is based on sensors at the wheel. i would assume that these sensors are reading each time the tire makes a single revolution rather than how long it takes to make a single revolution.

so ya, i would say that gearing effects the speedo and not the odo.
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Old 08-11-2011, 11:10 AM   #12
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Quote:
Originally Posted by Fire931 View Post
To answer your questions of how they get the readings.....

If you have an automatic trans truck then the reading is sent from both rear ABS sensors to the ECU that then sends it to the cluster.

If you have a 6 spd. trans then the reading is sent from a sensor in the rear output of the t-case to the ECU and then to the cluster.


An easy and cost efficient way to get it reading spot on, Dakota Digital recalibrator. It's not as fancy as the hypertech and requires you to actually cut and splice wires but it works great. I've got it installed on mine and I can go 100 miles on the interstate and watch the odo trip each mile as I past the marker posts.

but with the manual, it goes off the t-case output? so it will be off because my t-case output would be higher due to the gears spinning more to push me the same distance...

why would autos read from the wheel and manual read from the t-case?
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Old 08-11-2011, 11:11 AM   #13
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or is speedo from the t-case and the odo is from the wheel?
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Old 08-11-2011, 12:43 PM   #14
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No they both read off the same thing, the difference is the type of transmission you have. Why Toyota did that is unknown. But the speedo and odo should be affected the same being as though they both use the same signal.


Due to my adding the Lefty t-case in my truck and the difference in speedo gears mine was way off. At 70 mph it would read about 45 mph, and the odo was way off as well. I added the Dakota Digital box and all is well with both of them now.
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Old 08-11-2011, 12:45 PM   #15
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Also the nice thing about these boxes is they are always adjustable. Right now mine is spot on for the setup I have (285's and stock gears) however if I change gearing or tire size it will be off again. To correct I simply pull out the box and click a button a few times to dial it back in to the new setup and I'm back in business.
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Old 08-11-2011, 12:47 PM   #16
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is this what you got?

http://www.dakotadigital.com/index.c...rod&prd126.htm
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Old 08-11-2011, 12:50 PM   #17
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That's it. I bought it off ebay from whoever was the cheapest place at the time. I think I got it for like $70-75 shipped. WAY cheaper than any speeding ticket that's for sure!!!


I've also got a write up over on RST on the installation for my truck. It may differ a little with an auto trans however I can help figure out the wiring that needs to be done for whoever is the first to do it.
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Old 08-11-2011, 12:51 PM   #18
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what's RST?
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Old 08-11-2011, 02:08 PM   #19
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so what was the outcome of this?

gears will also effect the odometer readings?

i'll have to find a highway with mileage posted to see for myself...if it is off then i may go the dakota route and get things back in line with each other so i'm not racking up "miles" on the truck that aren't really there...
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Old 08-11-2011, 02:12 PM   #20
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Quote:
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what's RST?
Rock Solid Toys
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