Matthewd5

Rally stupid question, having a bad math day, if my speedometer is reading slower than I'm actually going, do I set the tire size larger or smaller?

Thanks

Matthew

SH0RTBUS

Not a direct answer, but I have my ProCal set for 37's and the speedo currently reads about 4mph faster than I'm actually traveling. My tires have 40k+ miles on them and are near the need for replacement, but I've never adjusted the ProCal for tire wear. So I need to measure their current height and reconfigure for it, which will be a reduction. So assuming I need to change to a smaller tire size to "slow down" my speedo, I would assume you need to change to a larger size to "speed it up".

Hope that helps.

TheDirtman

you need to up the tire size.

KG6SLC

Let me explain it in what I hope are simple terms as you are not the only one confused by speedometer settings.

First of all keep in mind that your speed is sent to your "speedometer" via speed sensors. In simple terms, these devices sense how many tire rotations are accomplished (over a given time) and your system then calculates what speed should be indicated for this axle rotation rate based on tire size setting.

If you are running LARGER tires you will travel more distance (and at a faster actual speed) per axle rotation.

Likewise, if you are running SMALLER tires you will travel less distance (and at a slower actual speed) per axle rotation.

So, if your speedometer is reading HIGH your axles are rotating at a faster rate than would normally be seen at this indicated speed. which means that your ACTUAL tire size is smaller than what your system is using to calculate your speed.

By the same token, if your speedometer is reading LOW your axles are rotating at a slower rate than would normally be seen at this indicated speed. which means that your ACTUAL tire size is larger than what your system is using to calculate your speed. As TheDirtman said, you need to increase the tire size in your settings to agree with the larger tires you are running.

KG6SLC aka Eugene

Mark Doiron

If you're using a programmer to calibrate your tires, try this trick: Ensure tire is properly inflated. Measure the tire from the ground to the top of the tire where it makes a sharp break to the tread (the "shoulder"). That will likely be quite a bit smaller than what you think of as your tire size. Set that into the programmer. And should get you within a mile or so of accuracy at 60-70 MPH.

14Sport

For me, I just think of it in distance as I find that less confusing.

If you speedo says 60 MPH and your GPS says 70...

then your computer thinks you are travelling 60 miles in 1 hour when you are actually travelling 70 miles in one hour so it thinks your tires cover less distance per rotation so you need to adjust it up.

But I agree with Mark. Instead of a "tweak and see" you can measure. Another way is to make a mark at the bottom of your tire and the ground in the same spot. Roll the vehicle for one tire revolution and mark the spot from the tire again on the ground. Measure between the two lines on the ground (circumference) and divide by pi (3.1416). That's your tire diameter for the computer.

jadmt

The guy who invented the procal Jordan Harriton says measure from ground up on flat surface with a level sitting on top of tire. I live just down the street from aev r&d shop.

14Sport

There's more than one way to skin a cat.

jedg

Yeah, yeah, yeah. There's three way to get to the size. We have a thread all about that topic. I used all three methods and all three are within 1/4 inch of each other.

Matthewd5

Well one year and 3 moves later I finally had time to pull out the AVE Procal and try some of the suggestions...

got it on the first try!

35.75" was the magic number

37" Goodyear MTR/K's on a heavy JK somewhat worn tires

the speedometer is dead on with the gps

Thank You! The replies were very helpful

mattnew