Craig King

iTrader: (11)

Yes, but 99% of VD users use the dynojet settings for the bigger horsepower numbers.

CurseDSM

iTrader: (3)

You would if you want closer to real dyno #s. But I agree

94AWDcoupe

iTrader: (125)

what I am saying is the tire size printed on tire doesnt tell you what the diameter of tire is. it tells you what it 'should' be. tire manufactures make more money selling you a tire that is smaller in diameter than it should be, less rubber = more money.

the revs per mile (rpm)data will give you true diameter. but you will have to calculate yourself. or you could measure yourself how far tire rolls (40psi no flex) in one rotation then divide by 3.1416. this is best because its also calculating for tire wear.

currently tires are when new about 3/4 inch shorter than the tire size says they should be.

wizzo 8

iTrader: (35)

Ill have to measure mine. I changed my tire diameter by .5 just for curiosity and it changes my hp by about 30hp.

GTijoejoe

iTrader: (25)

..... ok 0.75" smaller than posted, that's close enough for me to use

94AWDcoupe

iTrader: (125)

thats a big WOW for me.

thats serious HP change for such small diameter change.

mrm95

iTrader: (1)

When we did the VD for my car the best run was 421/370 on my Evo IX on E85 with basic bolt ons. This was using the stock 25.33 tire diameter. I realized after that my tires are 245/40/17 which makes the diameter 24.71 and the tires only have about 1,300 miles on them. How much do you think this would affect my numbers? just curious

Fireescape

When I did my VD logs I always marked the tire where it touched the ground and the ground at that point then pushed the car foward one full wheel rotation and mesured the distance along the ground to work out the tire diameter setting. My question is, at 7500rpm in 3rd (or at that speed) what's the circumference of your tyre, taking into account centrifugal force. As stated, the smallest difference in diameter makes a big difference in power output.

mrm95

iTrader: (1)

How much though in my case?

twabtoxer

iTrader: (39)

Haven't measured mine since i was a young lad...

Fireescape

OK I adjusted one of my runs by dropping the weight of my car until I got to 421hp with a 25.33" tire. I then adjusted the tire size to 24.71", this gave me exactly 400hp. Tire size makes a big difference in VD. You would, in my mind, get at least 10hp back due to centrifugal tire expansion at speed (assuming your tires will expand back out to 25" diameter). So I'd say your putting out about 410hp.

mrm95

iTrader: (1)

Yeah I had several runs all ranging from 404whp-411whp so I think its safe to say im around the 400 mark.

94AWDcoupe

iTrader: (125)

street tires do not balloon at all. if they did they would wear out in the center. roll per mile data is accurate for all speeds.

Fireescape

Not sure if your logic holds true. The whole tyre is spinning so the whole tread is still contacting the ground just the static sidewall sag is removed. If you look at your tyre when your car is parked their is a certain degree of crush at the base. My theory, and it's only that, is that an amount of this crush is pulled out of the tyre when it spins and even if this is only a small amount it would only have to be to make a difference to VD's accuracy.

I guess one way to see if my theory holds true is to watch a car carefully on an actually dyno to see if there is any movement in the sidewall at wot. If not then I'm happy to have my theory proven wrong.

mrm95

iTrader: (1)

Ive heard that if your tire diameter is slightly smaller (.5 or less) then what is used on the VD it wont make but a 5 to max. 10 hp change if that.