Incorrect tire diameter makes a huge difference on Vdyno results, how to correct it.

[RAAMaudio]

Extreme example and a bit embarrassing, I forgot to change from 24.3 to 23" diameter when I put on my summer tires which are far stickier and lower gearing so the car is much quicker.....

24.3" showed 319 WHP and 297 ft lbs.
23" diameter shows 285 WHP and 266 ft lbs.

-34WHP and 31 ft lbs!

That is a huge difference to say the least but what about smaller errors?

23.2" shows 290WHP and 271ft lbs.

+5WHP and 5ft lbs added just from a .2" difference

----------------

Using a tire calculator is NOT the way to get accurate results for several reasons.

1) Manf specs are rounded up, down, etc
2) Tires vary greatly in tread width no matter the label numbers, sidewall height is a ratio that uses width.
3) Air pressure you are running can change the diameter significantly
4) Heat in the tires can make a considerable change
5) Tread depth, it can change the diameter quite a bit as tires wear down.

If you want to really get it right the best way is to park the car on level ground and measure from the road surface to the center of the dust cap and double it. You can do this for different air temps, tire temps, pressure, etc....if you want to get anal about it.

BUT, tires expand in diameter the faster you go....and they slip a bit as well so using RPM versus speed and gearing, tire diameter, etc....still not going to be precise. A 4th gear run instead of 3rd gear will likely show a bit more power due to the diameter change, load change of course from a taller gear will effect the real power as well of course.

All that said, Vdyno like any physical dyno is going to be a bit off are but great to compare tuning results which is why you want to run in the same conditions as much as possible.

 

[RAAMaudio]

Added a bit more data, info, etc....

 

[BoostBumps]

Yup...that's one sure way to get over-inflated (pun intended) numbers from V-dyno...Over estimating occupant and vehicle weight will do it as well...

Also for those with diameters / circumferences different than stock remember to change the "Tire Size / Tire Circumference (default)" value entered in your tuning program to maintain speedo calibration....

 

[JPGC]

Yep and so does the correction factor and vehicle weight

 

[RAAMaudio]

At least I put my car on scales and myself as well so got that pretty close.

My car has only been tuned on the 24.3" tires spec value but I should be able to run the summer only tires when the GTX is installed as well will be in AZ nice and warm but not hot

 

[Sourskittle]

I have a scale and run giant front tires. I've recommended a few people now NOT to run the 235 because its so tall and takes a lot of power from the gearing. If you don't have the power to make up for loss of gearing, def don't get a 235. That being said, back when I was on the stock size tires, I could demonstrate, with ease, the brake based E-diff trying to do its job in 3rd gear corning. It was funny and scary at the sametime.

Measuring the tire from center to thread is a good idea though Rick. I've always just entered the tire manf. claimed height. Which... Does change from the time a tire is new vs old.

 

[RAAMaudio]

I can see where a tire that tall would compound the issue of load on the rods when under boost at low RPM, in the top gears especially.

It most likely helps in first to launch better and grab in in second as less tendency to spin due to the change in gearing, more grip from wider and longer tread...

It would also seem likely the smaller diameter tires would have less tendency to stretch at higher speeds so the diameter would be less prone to increase resulting in more accurate power numbers so using a stickier lower diameter tire should be better overall for most users.

The 23" diameter, 7.8" tread width, 225/45/15 Rival S tires I made my last run with would light up easily in first so I had to take it easy to launch well but they did not even spin in second from banging the next gear or from a roll when the EFR turbo kicked in. The car just tore off like mad down the road with me hanging on and correcting torque steer which is annoying and fun at times as well.

They also only weigh 20lbs and were on 13.4lb 9" wide wheels and much lighter brakes. I highly recommend these tires for those that can fit the size they need, their are a few other tires that would fair well also.

The are available in two more good sizes for this car.
205/50/15 at 23.1" diameter and 6.9" treat width good for 7-8" wide wheels
215/45/16 at 23.6" diameter and 7.5" tread width that would be great on the 16x8 wheels PCA is working on which I hope and likely will fit over the WW BBK.

------------
Other good tires include:
Bridgestone RE-71R
205/50/15
205/45/16

Dunlap DZII
205/50/15
205/50/16 a bit tall but great tread width and at 21lbs a great weight
225/45/16 23.9", 23 lbs so a bit heavy but 8.5" tread width, works on an 8" wheel but 9" would be even better.
215/40/17

Hankook and Kumho have some good tires that will work well.
There are a few 17's but none in wider than a 205

----------

Got a bit off the main topic but tires and wheels, diameter, width, rim support, compound, weight, etc are so critical to overall performance in every aspect, which I have repeated many times, are the single most important part of any car setup to be it's best in whatever level you wish to use it.