speedometer calibration questions

[wangstarr03]

Hey all, I just ordered the speedometer calibration tool from rough country (should be here friday) after going from 275/55r20 (OEM ORG A/T tires) to 295/60r20 "hybrid terrain" tires. Now, everything I've read says that bigger tires should translate to your speedometer reading higher than what you're actual speed (in fact, an online calculator says to expect a ~6% delta) but, in real world testing using a GPS speedometer app, it turns out my actual travel speed is faster than the speedometer is posting. Why would this be?

In addition, I'm hoping calibrating the speedometer will help w/correcting transmission shift points - I took a trip from DC > NYC this past weekend and among some other things I noticed, this truck could not decide whether it wanted to be in 7th or 8th which got a bit irritating. Not to mention my fuel economy was abysmal: about 13-14 MPG cumulatively across all the highway driving (no, I didn't disable MDS). Hoping someone can provide some insight, thanks!

 

[clazer]

Think about it this way:
Your truck had tires on it that were a certain size around. The speedo was calibrated so that each of those turns is certain distance. You put on bigger tires, so now you are physically going further with each turn. The computer doesn’t know that, so it’s still calculating off your old smaller tire size.

Bigger tires=bigger around=faster actual speed than your speedometer

In my case, the truck is going 12% further and faster than it realizes. That means that a displayed 15mpg is actually 15*1.12= 16.8mpg actual

 

[Yellotsacul]

Your speedometer reads slower than actual because the bigger tires take longer to make a single rotation than their smaller counterparts, therefore making the computer think you are traveling slower than you actually are.

For the 8th/7th gear issue, your bigger tires also weigh significantly more, making the transmission work harder to keep your truck at a set highway speed. This would be more prevalent if you have the 3.21 gears vs the 3.92. A lower gear ratio would allow the engine and transmission to work less to turn the bigger tires, and in turn improving your fuel mileage while also improving your acceleration.


Sent from my iPhone using Tapatalk

 

[WR986]

Hey all, I just ordered the speedometer calibration tool from rough country (should be here friday) after going from 275/55r20 (OEM ORG A/T tires) to 295/60r20 "hybrid terrain" tires. Now, everything I've read says that bigger tires should translate to your speedometer reading higher than what you're actual speed (in fact, an online calculator says to expect a ~6% delta) but, in real world testing using a GPS speedometer app, it turns out my actual travel speed is faster than the speedometer is posting. Why would this be?

In addition, I'm hoping calibrating the speedometer will help w/correcting transmission shift points - I took a trip from DC > NYC this past weekend and among some other things I noticed, this truck could not decide whether it wanted to be in 7th or 8th which got a bit irritating. Not to mention my fuel economy was abysmal: about 13-14 MPG cumulatively across all the highway driving (no, I didn't disable MDS). Hoping someone can provide some insight, thanks!

The responses about tire size are absolutely correct. It takes longer to make a full rotation.
I have the same tool installed in my truck after a lift and tire/wheel change.
It corrected my speedo. Its still slightly off but its close and in my favor. 80 on the speedo is now actually 76 which keep me out of mischief. I may mess with it to get closer but at lower speeds its very close.
It did correct and improve my shift points and the truck is much more responsive. It also tells the electronic ride control how to adjust for the new size.

My experience you will notice the trans and handling change immediately and be very happy. Speedo will improve if not dead on will be close. I'm very happy with mine. Looking forward to hearing how you like it.

 

[Wheeler]

Look into Tazer Dt little expensive than odo calibration tool but more features.

 

[wangstarr03]

Thanks, for all the replies everyone. I think I misinterpreted the information when I first read it but after reading your responses everything is starting to make sense, now. Another question: I've read the tool has pre-programmed sizes (i.e. 33", 35" etc) but what if you have more of a "non-traditional" size? I plan on physically measuring but my 295's are ~34" according to manufacturer spec.

 

[clazer]

Thanks, for all the replies everyone. I think I misinterpreted the information when I first read it but after reading your responses everything is starting to make sense, now. Another question: I've read the tool has pre-programmed sizes (i.e. 33", 35" etc) but what if you have more of a "non-traditional" size? I plan on physically measuring but my 295's are ~34" according to manufacturer spec.

The best way to figure out the real correction is to get on an interstate and see how far you go according to mile markers compared to 100 miles on your odometer. That’s the actual.
Now, I went from 33’s to 37’s, which is 12% bigger. The mile marker test gave me..... 12%.
Moral of the story, you’ll be close enough.

 

[Wheeler]

Wide range of adjustment and up to quarter inch

 

[wangstarr03]

Thanks, all - should’ve mentioned I have 3.21...will report back once the tool arrives; should be Friday.

 

[Ram1992]

Thanks, all - should’ve mentioned I have 3.21...will report back once the tool arrives; should be Friday.

Did you get the RC calibrator installed? If so how do you like it ?

 

[ptacular06]

Look into Tazer Dt little expensive than odo calibration tool but more features.

Wheeler, do you have any issues with gear hunting and the 3.21s on 35s? Did the tazer help? As I also have 3.21s, I'm trying to decide on whether 35s are too much with the help of the tazer or pulsar and pedal commander. Thinking maybe just running 34s instead. What's your thought?