Speedo calibration walkthrough??

[tording99z28]

So I am just curious guys if there is a good walkthrough for speedo calibration?? I used the tire size calculator and put in my tire size of 275 x 40 x 17 and got a tire height of 25.7. I then opened up the speedometer portion of editor and for wheel size I put the 25.7 and for gear I Put 3.42 since thats what I have and it hit commit and it changed the values for the VSS revolutions and all that. I took the car out and now the speedo is even more off than it was. Before it was about 2-3 mph off at higher speeds like 60+ but now its like 5 mph off at high speeds.

I did a search on here and I saw some things about measuring the rollout to get the accurate wheel size and stuff but I didn't quite get what it was all saying since I am just piecing stuff together from different posts. Does anybody have a good walkthrough or a guide on how to get started and what calculations need to be done and exactly how to do a roll out? Thanks in advance.


Tyler

[jimmyblue]

I recommend you do the "rollout" (that is, drive over
a flat (or expendable) tape measure until you can
wrap it around the rear tire as it sits, measure the
circumference and divide by 3.1416 to get a tire
height number. Manufacturer's real vs marked tire
dimensions are pretty variable.

Check that nobody ever messed with the VSS pulse
to tailshaft revolutions or anything like that, might
go all the way back to a stock speedo settings and
come up clean with the new gears & measured tire
data. Just to sweep away any accumulation of junk.

[tording99z28]

I will give that a shot then. Thanks!!

[Russ K]

The stock F-Body tire dia is 24.9 in the editor.

Russ Kemp

[tording99z28]

The stock F-Body tire dia is 24.9 in the editor.

Russ Kemp

So is this what I should use with my 275 x 40 x 17 wheels or should I do the rollout method and divide that by 3.14 and use that number??

[jackedupcanyon]

Measuring the actual height of the tire will get you closer but the rollout method "should" be a little more accurate (divide by pi - 3.14159265358979323846). Tire circumference divided by pi = tire diameter (height).

[jimmyblue]

Measuring sitting height is not going to be dead on,
the bottom flat and all. But the wheel/tire has to
travel the full circumference, no way around that.
Tire height in the editor is really a roundabout
way to the distance per revolution (circumference).

[jackedupcanyon]

Right. Measuring tire circumference should be the most accurate if you are careful but takes a little more time and more maneuvering to do. Measuring actual tire height is fast and usually will get you closer than nominal or even manufacturer specs (if not using the same width as the manufacturer's measured rim the specs were established around). What ever method you use, you should verify it with a GPS unit.

[tording99z28]

I will measure the circumference tomorrow with a soft measuring tape and try that and then verify it with my gps. Hopefully this will work. Thanks guys.

[miker2]

i used a gps to get actual speed--calculated 5 error--and used that percentage to correct the tire size.results were perfect.

[fishlips]

Place a chalk mark on the bottom of the tire and on the ground. Pull the car forward until the chalk mark is on the bottom again and make another mark on the ground. Measure the distance between the two marks on the ground. This is the roll out of the tire. Just measuring the circumference of the tire does not always give you the real roll out. Then you can do your math.
Plus it is much easier on the tape measure!