jimmyg

My car weighs 4050 . I trapped 128 bouncing off the limiter in 3rd (6750) tires were 25.6 tall . What will I trap if I go with 28 in tall tires?

a05c

What transmission and rear gear is in it?

Doug G

Depending on when you hit the limiter ....maybe not at all ?

25.5" Vs. 28" is about a 200-300rpm +/- difference....but the rolling resistance and inertia to get rolling.....could be equal mph, but better ET since you're not on the limiter.

Can you borrow a pair for a test ?

01ssreda4

iTrader: (96)

In a properly setup car, geared right with the right converter I bet it doesnt cost you anything.

jimmyg

4l85e and 3.73

jimmyg

I'm hoping to trap higher and I bet you I will. It's going to change my gear ratio enough to go thru traps in 3rd. Last night went 11.0@130 going thru in 4th gear . They weren't too happy about it lol . ( no cage)

speedtigger

iTrader: (16)

MPH x Rear Gear x Trans Gear x 336 ÷ Tire Diameter = RPM

Example:
148 x 3.73 x 1 x 336 ÷ 28 = 6624.48 RPM

If it is an unlocked automatic, then multiply your final answer by estimated torque converter slip and add that to the RPM
Example: 6624.48 x .06(6%)= 397.4688.
6624.48+397.4688=7021.9488 or 7022 RPM.

jimmyg

140x3.73=522.2x1x336= 175,459.2 divided by 28 =6266.4
6266.4x.07= 438.6
6266.4+438=. 6704 rpm

So I should trap 140 @ 6700 rpm in 3rd with 28 in tires . (Hopefully)

GARY2004Z06

No. Unfortunately, you will not automatically pick up 10 mph. You need a great deal more power and/or weight reduction to do that. Depending on the atmospheric conditions, your 130 mph trap speed in fourth will most likely be indicative to what you will trap with the 28s.

SoFla01SSLookinstok

I like this calc. http://www.pszweb.com/car/gears.htm

jimmyg

I fig . But it's got to trap higher. If I had raised limiter to say 7500 instead of 6750, I probably wouldve gained another 3-5 mph. It was pulling like a freight train.Instead , I started bouncing off limiter and had to shift. I'll put the 28s on and give them a shot .its just going to be hard to get a good run in without getting kicked out.

Tuskyz28

Tire calculaters are great but they not exactly accurate. Reason I say that cause some tires GROW going down the track which throws caculations off some. Something to think about....

jimmyg

Well I put 28s on. Used to rev 3200@80mph now it's down to 2700. The nittos looked better, but the m&h racemasters hook way better and I should be able to go thru traps in 3rd @peak torque
Look weird?

SoFla01SSLookinstok

Looks good!

jimmyg

Thanks ! I was worried they looked too big

Doug G

Looks fine to me....but, yes, nittos won't hook as good.

ElkySS

iTrader: (6)

Just food for thought: I was running 26 inch radials and trapping 122. I switched to 28 in bias tires (among several other small changes) and can now only trap 117. the general consensus is that my loss in speed is from the tire change. I wasn't hitting the limiter before but I was right at it. When I get my speed back, I'll let you know what it took.

jimmyg

Buddy of mine went from 26 to 28s and gained 4mph.( Tbss w/ whipple) .I've got an ly6/whipple 2.9 so I hope I have similar results. I guess it all depends if you have enough power to pull it off. That's why all these turbo guys that run tall tire and 3.08s are trapping so high. Same with that demon trapping 140, 28 in tire and 3.08s

T6Rocket

Trap speed indicates HP. All things being equal, to increase trap speed, you have to increase HP.

If someone sees a significant gain in trap speed from just a tire diameter change, then the prior gearing/tire combo was inadequate.

Al

jimmyg

Exactly.this is what's up with my combo. It hits redline in 3rd about 3/4 track.