Any idea why my car is slower on 28s than 26s

[Sweet6]

I ran the 275/60 BFGs then went to a 26X10 MT ET Drag and picked up 2 MPH with a similar 60' time. This was on the same day and I did not change anything on the car between runs. I want to go out and test again, but I know the car had a good tune on it. So I am not sure anything will change. Just seeing if anyone else has any thoughts on this.

 

[Sweet6]

I posted this in the lounge by accident. Can the moderators please move this to general tech?

TIA

 

[BLACK6PACK]

One reason is the you have more rotating mass further out from the center of the rotation. Put a 15 pound weight on your elbo and stick your arm strait out, now put that 15 lb weight in your hand and try holding your arm strait out. It takes more power to do the latter. Another reason is you've changed your gearing by going to the taller tire, and your car may have liked the shorter gearing. With your setup though, you should be able to turn the bigger tires better. What kind of tune were you running and what mph did you run?

 

[Welby]

Going to a smaller dia. tire steepens your gear ratio slightly...

This is informative

http://www.chevyhiperformance.com/techarticles/148_0208_gear/

 

[Sweet6]

I know it changes the gearing to a taller gear. I thought that helped our low reving motors? I ran 114 MPH on the 28s then 116 on the 26s. 24lbs of boost EGTS around 1700 with 116.

 

[Unsafe6]

I had similar results when I switched from my 26" QTP's to 28", The 26's were worn and the car was wanting to go into OD before the end of the 1/4, so I jumped up to 28's and lost a few mph. I tried my friends 27" QTP's on similar wheels without tubes and I ran my best ET and MPH with that setup.

 

[strikeeagle]

I don't think there's anything magic about it. Based on your MPH, your car runs at perhaps the high end of what's generally accepted to be the break point between 26" and 28" tires. Which is to say, things could go either way.

You've simply proven the validity of "common knowledge". And as your car gets faster it will likely do better on 28s. Likely being the operative word. Generalities are just that. No two cars are the same.

Hell, somebody on this board was running 5.13s or thereabouts for the 1/4, shifting into OD for sure, and he was REAL fast...