dont know where to post this. hopefully a bunch of people who use VD will get this tip. I dont know if you guys actually measure your tire height or if you are using tire manufacture listed heights such as can be found on tirerack.

my tip is you really should measure actual rolling distance. measure on ground how far tire travels in one complete turn, then divide that by 3.1416. failing that you should use the revs per mile of tire and calculate diameter from that. at least that number will be correct for new tire.

tire heights listed on tirerack are WAY OFF. so far off i dont even know why they have those specs listed. my last tire purchased was listed at 23.6. actual current measure with 5k on them is 22.55. when I found that out and started measuring all tires I had and compared them to listed tire heights and most were at least .5 inch shorter.

the revs per mile figure is spot on. but only when tire is new. treadwear quickly reduces that figure.

so imagine how far off calculations would be if I had just used the listed 23.6 height.

 

Very happy to hear you care enough to be as accurate as possible. This really will make a difference. Keep up the good work on promoting good use. Some people will post any number they get without analyzing what variables they used or if they are even correct.

Good Job!!!!!

 

yes did some calcs on 235/45/17 tires . they are actually 24.5-24.6 when new. everyone seems to use 25.33 no matter what.

would like to see what it does to the hp curve with correct numbers.

 

well if you are measuring them you are doing it wrong. dont know how you calculated 25.2-25.5?

they are actually 24.5-24.6 when new. everyone seems to use 25.33 no matter what.


the 25.33 is indeed calculated from 235/45/17 ..BUT ITS WRONG

true tire size comes from rev per mile spec. thats where I got the 24.5-24.6 when new specs from.
so even when tire is new you are already .7 inches off. 10k miles and you are likely 1 inch exaggerated.

SEEMS PRETTY stupid considering VDR derives a HP number solely based on math.

5280ft / 819-823 rpm = how many feet tire rolls per one rev. = 6.455-6.416 ft
then just multiply by 12(inch) and divide by 3.1416 (pie)
=24.5-24.6= true tire height for 235/45/17 tires being sold in USA

then consider tire tread is 11/32 when new , used at 7/32 is already 1/4 inch shorter than when new

 

Or you could not use VD and use a real dynometer instead?

 

 

Some of us don't have AWD dyno's within 2-3 hours, seems a bit silly to drive almost 3 hours each way when I can get a very close approximation of power with VD if I make a change to my tune and just want to verify its effectiveness. I do have miles of open farm roads just by my house that I can do tuning runs on without bothering a soul.

So, put another way, you could understand not everyone has an AWD dyno down the road.

 

a real dynojet isnt any more accurate than VD. they are both just doing math and spitting out numbers. Dynojet is just more consistent due to not being able to change parameters.

I personally would choose VD any day of the week as its actually taking data from real world conditions. something a dynojet simple cant reproduce.

cant tell you how many times I have heard someones boost needed to be adjusted as boost on dyno didnt match boost on the street. HELLO maybe dynojet is not simulating load very well? didng dindg ding thanks for playing

 

That's a hell of a good point... hmm

 

Correct - however, Mustang dynos DO measure load.

But the funny thing is, almost all VD settings are matched to DynoJet settings which read higher than a mustang dyno, which measures load... why is that?

 

Vd also does mustang dyno #s just in case you didn't know.

 

Totally agree what you said here.

 

Dyno's as we all know are simply tuning tools. It is too bad that we all try to measure our e-peens with them. Improper settings can make it look bigger, which some need.

Let the track do the talking. Making power is only one part of the equation.

Good info Jerry, thanks for posting it up.

 

Just to be clear here, you are saying that the tirerack measurement (probably given by the manufacturer) is different than what measurement?
A) none mounted tire diameter
b) mounted tire with X psi of pressure on Y amount of vehicle weight?

There are many tire diameter measurements which will change, for instance a SLR, static load radius and DLR, dynamic load radius. Depending on how much your vehicle weighs, how much tire pressure you are using and what speed you are driving your tire diameter will change. For instance, tire diameter at 30mph is certainly different than 75mph.... how accurate are you trying to figure?

 

Very good post. I'm going to measure mine tonight.