Originally Posted by kona stang10

Naturally aspirated -yes to 3:55's , Power added ie: sc or turbo-no

Originally Posted by kn7671

What.... That response does not address the OP's question regarding a taller rear tire and its affect on gearing.

OP - making some assumptions since you didn't post, if you had the factory all-seasons, 245/45/19, which are 27.7" tall, and you have an Automatic with 3.15 gears, the change in gearing is pretty small. With the taller tire, you will only have a 3.4 mph increase around 120 mph.

@6800 rpm with 3.15's, 27.7" vs 28.5"
1st = 43.68 vs 43.91 mph
2nd = 76.05 vs 78.25 mph
3rd = 117.08 vs 120.46 mph
4th = 156.11 vs 160.62 mph
5th = 204.56 vs 210.47 mph
6th @ 60mph = 1600 vs 1550 rpm
6th @ 80mph = 2125 vs 2050 rpm

In other words, no change in gearing required.

Originally Posted by blairnr67

Ah ok cool, thanks. where did you find that info?

Originally Posted by blairnr67

I am upgrading from the stock pirellis to 305/35r20 drag radials. The wheels are 12.5 inches wide and 28.5 tall. Being so much bigger, will I need to switch gears from my current 3.15s?

Originally Posted by Five Oh Brian

Your factory tires should be 27.25" tall. If your drag radials are 28.5" tall, that's a 4.6% increase in tire height, so you'll want to go to 3.27 or 3.31 gears (Ford Racing offers both) to make up for the extra tire diameter. The 20's are heavier, too, so going up one more ratio (to 3.55's) makes up for not just the tire height, but also the extra rotating mass.

I went with 255/50/19's for my daily tires (29" tall) and I run a 28" tall M&H drag radial on race days, so I opted to go a bit overkill for gearing with 4.56's (up from the factory 3.73's I had). Car launchs awesome now at the track and is so much funner to drive on the street.