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Discussion Starter #1
I have a 2014 tundra with 35"12.5 R20, and the speed difference is 10% faster from the factory wheels with out the correction in the computer.
Q#1: With that knowledge, would my MPG be 10% off as well?

Q#2: how do i correct the computer for the tire size, so my odometer and speedometer are correct?

Thanks
 

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You can take it to the dealer for a re calibration or you can buy a programmer that supports re calibrating for tire size.
And yes it affects the MPG shown on the computer the same percentages your odometer and speedometer, they are all connected.


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There is more to it than just plugging in 33" tire on the programmer. it may take SEVERAL tries to get it just right using a phone gps speed app as speed reference.
On my dodge with 35's i took like 5 times but was worth knowing how fast and correct mileage
 

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UP-SIZING OR DOWN-SIZING

Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed. This is because a taller tire will have a larger overall circumference, which will cause it to travel more distance per revolution than the original equipment tire. Down-sizing, or installing a shorter tire, will cause the speedometer to read a faster speed than you are actually driving. This is because the smaller tire will have a smaller circumference, causing the tire to travel less distance per rotation than the original equipment tire.

Changing the tire height can alter the speedometer reading because most vehicles measure speed within the transmission based off of tire revolutions.

https://www.discounttire.com/learn/speedometer-accuracy
 

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Discussion Starter #7


UP-SIZING OR DOWN-SIZING

Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed. This is because a taller tire will have a larger overall circumference, which will cause it to travel more distance per revolution than the original equipment tire. Down-sizing, or installing a shorter tire, will cause the speedometer to read a faster speed than you are actually driving. This is because the smaller tire will have a smaller circumference, causing the tire to travel less distance per rotation than the original equipment tire.

Changing the tire height can alter the speedometer reading because most vehicles measure speed within the transmission based off of tire revolutions.

https://www.discounttire.com/learn/speedometer-accuracy


Thank you for making my point metal monkey.

Now how do I fix it?


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Hypertech speed calibrator is the way to go.

Hypertech

Instructions:
http://www.hypertech-inc.com/ProductManuals/inlinespeedo/730119_2_VSS_Instlltn_Prcdr.pdf


I drove my truck from Utah to Texas with the speedo being way way off. At 60mph indicated, the GPS read 76mph. Once I got home, the very first thing I did was install the Hypertech. It is dead on.

One important note is to NOT go by the size written on the side of the tire but to take a tailor's measuring tape and measure the actual circumference of the tire and enter it. My tires say 37"; however, the actual measurement was 36.5".

Tape:
https://www.amazon.com/ZXUY-Measurement-Also-Centimetre-Reverse/dp/B008U4E7RU
 

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When I put 35's under my truck.. It actually decreased my speed by about 5mph. Meaning if my odometer showed me going 30mph if I checked on gps it would be about 35mph. It was such a small difference it never bothered me.. and technically even though I'm not trying to deceive my odometer it's keeping miles off of it when you add up all those tiny amounts. If you think about one rotation of a tire if you compare a larger to smaller tire the larger tire can roll further with one rotation so if you dont recalibrate your ecu your ecu thinks your going slower then you are.
 
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