sea bass

This would be awesome!!!

My guess is 1 & 3 should be pretty much the same and #2 will be a hair taller




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nthinuf

When I bought my ProCal, I couldn't wait to get home and measure. I was running 35's, so plugged in 35". Got up to highway speed, compared to gps, pulled over, dropped the height. Compared, pulled over, dropped height. etc. Turns out my 35's were 33.5" tall. Which matched up to the method aev suggests - a straightedge (2x4 in my case) laid across the top of the tire.

I don't think it matters a bit what method you use. It's a starting point. If the speedo is off, just tweak the height until it is correct. That's one of the reasons you bought a programmer/tuner, so 'you' can adjust it whenever you want...

jedg

Yup, this is what I expect.. that all three will produce measurements that are very close. I'm doing it mostly because I've seen so many folks insist that one method is better than another.

SoK66

Nope, it's dead on per my onboard GPS.

T&ERun

Just checking this thread again. Sorry, what I meant to say was similar to what jtphotoJK said. Measure from the ground to the center of the hub and double multiply by 2. This has worked for me several times also.

zstairlessone

The whole idea is to know how far the tire will roll for each revolution, this with the output rotation speed and the gear ratio will tell you exactly how fast the vehicle moves at any RPM of the shaft. Works the same off of a wheel speed sensor to give you distance for measurable angular (rotational) velocity. Using the wheel height means the Pro-Cal (or what ever flash module used) converts circumference to diameter. The most accurate method would be to use the actual diameter to calculate height - then enter that calculated height into the flash module.

jtphoto JK

There is a difference in tire diameter between running at 60psi and 25psi. This needs to be taken into account if you want to get it right the first time. Theoretically it should be the same but it's not.
Realistically we are talking minor differences here. If you don't mind throwing a number in, checking it with GPS, then re-adjusting, and doing this a few times till you get it right, then do it the way you like.
I run my tires a bit soft 26-28psi so if I recalibrate at the exact diameter its always off by 2-3kph.

jedg

Okay, so here's my pseudo scientific findings...

Spoiler up front..... All three are so close to each other that it doesn't matter.
(EDIT - I took three measurements of each of the three methods then used the average to calculate the diameters)

Measurement 1 - Center to ground - diameter = 33 1/8

Measurement 2 - Total height using a straight edge = 33 1/4

Measurement 3 - Circumference and calculate diameter = 33.223 (so almost 33 1/4)


I used my jeep with 35 inch KM2s that are at 26psi.


Measurement 1 - Measure from center-cap to ground and double (so get radius and double for diameter). This is the second most difficult method I think. Getting the angle that you hold the measuring tape at makes a big difference on the measurement. Also ensuring you are using the center of the wheel. SO, my 33 1/8 is likely off because it's difficult to get an accurate measurment of the radius.

Measurement 2 - I used a level for the straight edge and ensured the bubble was level then measured. Very easy. I think this is the easiest method.

Measurement 3 - Second easiest but I think it's less accurate as you have to eyeball when the mark on the tire is exactly at the bottom. I think I got lucky that my measurement was 104 3/8 inches which worked to 33.223 inches diameter.

SO... there you have it... it's a wash. All three work. For me, I will use method 2 from now on as it's easiest for me.

sea bass

Wait a minute! You have one as the second easiest and one as the second hardest. With only three items - it's the same thing :-)

Just giving you a hard time. Thanks for putting the time in to this! Very interesting findings...

jtphoto JK

So which measurement did you put in your programmer 33.125 or 33.25. I'll bet on 33.00 " being closer with the GPS.