gator's 99TA

iTrader: (2)

is this true? right now my car is programed to shift nicely on a stock 245/50/16 tire but i will run my 28 x 9 ET Drags at the track. will i be shifting lower than i had it running on street tires? the factory tach is a POS so i dont go by that. also the roads are too salty and sandy right now so i am just wondering. i cant just put them on and hammer it. thanks!

2001 Pewter WS6

iTrader: (43)

A 28" tire is going to change your effective gearing. It will still shift at a certain RPM or MPH which will probably happen later in a measured out distance. A smaller diameter tire will increase your gearing and will shift in a shorter distance at the same RPM.

gator's 99TA

iTrader: (2)

ok, so, um..... can someone explain this to a dumb redneck like myself? <img border="0" title="" alt="[Embarrassed]" src="gr_emb.gif" />

quickWS6

It'll be the same as going to a higher gear. You will be shifting too early if it is set to shift at a certain MPH.

sited12

Bigger tires chop down the gear so it would be like going to a smaller gear. So your car will shift later.

Gary Z

I just want to mention that some of the terminology we use is confusing. “Lower gear” usually means higher overall ratio. “Smaller gear” is used inconsistently and is often ambiguous. This is the first time I’ve seen “chop down the gear”. The way I remind myself that a larger tire is equivalent to a higher gear (lower ratio) is by seeing that a car will travel farther for each revolution of a larger tire. At any given engine rpm the car speed will be higher with the larger tire.

<small>[ March 11, 2003, 11:31 PM: Message edited by: Gary Z ]</small>

gator's 99TA

iTrader: (2)

so with my rev limitor right now being taged in 3rd but running nice and smooth through all the shifts in 4th (or overdrive) should i back off the shift points with the bigger tire?

jfm

iTrader: (4)

I have a set of MT ET Drags 26x9x15 with my 3:42s,my shift points were perfect.I tried my buddy Petes,26x10x15 ET Drags,it hit the rev limit on the 1-2 shift and the 2-3 shift[barely].I lowered my shift points 1 mph on the 1-2 and 2-3,worked OK then.

I bought a set of Hoosier Drags 26x10x15 and with the same shift points set for the 26x10x15 ET Drags,hit the 1-2 and 2-3 rev limit again.I had to reset my shift points again.The Hoosiers are a larger diameter tire[26.4 vs 26 ET Drag].

My 1999 A4 shifts by MPH not RPM,so when the shift points are perfect for a 26" diameter tire[let's say it shifts 1-2 at 6500 RPM/33MPH/6600 RPM rev limit] and I put a 26.4 diameter tire on it will try to shift at 33 MPH,it will need more RPM to get there,lets say 6680 RPM.Bang it now hits the 6600 RPM rev limit.

Hope this helps.

Top Fuel

iTrader: (8)

JFM,

I'm switching from 3.73's to 4.30's using the same size tire. How many mph should I raise each shift point to keep the same shift rpm? Is there any type of science to this? or is it just trial and error setting up shift points?

thanks,
Steve

<small>[ March 12, 2003, 08:03 AM: Message edited by: Top Fuel ]</small>

jimmyblue

iTrader: (11)

The way I understand it, you will upshift at
MPH or RPM, whichever happens -first-. The
PCM has scaling built in, you can change
this with Predator, LS1Edit, maybe HPP3?
But if you leave the ratio / tire size
alone then you will see a lower indicated
speed, and a later MPH upshift (good) and
the same RPM upshift threshold. You can
change the MPH and the RPM thresholds, by gear,
if you want & have the tool.

jfm

iTrader: (4)

I use my HPP III to raise and lower my shift points.I forgot to mention that.

Top Fuel,you will need to get your speedo recalibrated for the 4:30 gears.That should keep your shift points as they were with the 3:73s.Did you get it reset for your 3:73s ?

Automatics with changed gear ratio not only have the speedo off but it also skews the shift points.

Top Fuel

iTrader: (8)

yeah I'm going to adjust the computer for the 4.30's. I didn't know that doing that alone would keep the shift points the same.

thanks,
Steve