Help after changing tire size

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05-14-2016
TWS

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Help after changing tire size

Another newbie question about modifying tune for taller tires.

My 1998 Z28 was tuned with 26" tall drag radials last time I had it professionally tuned. I recently switched to 28" tall slicks and took the car to the drag strip last night, thinking I was going to lower my 60' and improve my ET. Well, that wasn't the case. Besides the warmer and more humid air than last time that slowed me down, my car was short shifting a little, especially noticeable on the 2 -> 3 upshift. It would actually bog down a bit after the shift as the RPM dropped too low and got me out of the power band for a second causing it to not pull as hard as it usually does down the track. I have looked at a few other threads of guys posting for advice and couldn't quite figure out what I need to do. Could be the head cold I'm fighting or a lack of intelligence. Anyway, it's a '98 Camaro with an RPM 4L65 trans and 3.73 gears in the Moser 9 inch.

I pullled up the table that I believe needs to be modified (screen shot) but not sure what new value I need to enter.
Attachment 58941

Any assistance would be greatly appreciated.
Thanks!
05-15-2016
MarkTucker

1 Attachment(s)

That is the table you are looking for.

You say you have 28" tires and a 3.73 gear. So (28 / 3.73)=7.5067024129

If the tire is in Metric, you can use the Unit Conversion table in the editor or scanner of HPT to convert metric to find actual tire height.
Attachment 58955
05-16-2016
TWS

Thanks! Good to see it is that simple. My tires are Hoosier slicks (not P metric) so no need to use the unit conversion table.

The threads that I found made it more complicated than that. I'll measure mine with a tape measure to verify exact size, but it's an advertised 28 x 10.5 slick. Understand that doesn't account for "squish" but should be close enough.
05-16-2016
MarkTucker

Quote:

Originally Posted by TWS

Thanks! Good to see it is that simple. My tires are Hoosier slicks (not P metric) so no need to use the unit conversion table.

The threads that I found made it more complicated than that. I'll measure mine with a tape measure to verify exact size, but it's an advertised 28 x 10.5 slick. Understand that doesn't account for "squish" but should be close enough.

Yeah, close enough. Don't think it is actually need to bust out the tape measure.