2013 gt convertible with auto 3.15 -3.73 gear change

I recently purchased a 2013 GT convertible with 50K miles on it. I am planning to change out the rear gears this week and after researching it on this and other sights observed that there is a lot of opinions on this subject but not much data. I will attempt to apply some data to the change out.

goals from the changeout are
-drop a few tenths on the 1/8th mile
-more fun as a daily driver
-no major change in the MPG. a little would be fine.
-no major change in highway noise

Pre-purchase mod's
- axle back flowmaster american thunder exhaust
- 255/35/20 wheels and tires

Post purchase mods
- Dyno Tune by JBA speed shop
- no CAI as I can not find one that is California smog legal.


Pre-Change out data

best 1/8 time = 8.6 (using sport mode- some spinning but hooked up pretty well) crappy track at the stadium here in San Diego.
gas mileage = 17 - 18 MPG. mixture of highway and roads. it always seems to fall in this range no matter how I drive it.
gear/rpm/speed on highway - i did this by setting the cruise control at the speed and then manually selecting the gear
3rd/3750/60 3rd/4250/70 3rd/4750/80
6th/1750/60 6th/2000/70 6th/2250/80

I am getting the new 3.73 gears installed on wednesday and will post the after data in a few weeks after a break in period on the new gears.

The new gears are in and the break in period completed. The installer (JBA SPEED SHOP) recommended 10 heat cycles as a break in period. with a preferred heat cycle being 15 - 20 miles at various speeds. My drive to work is about 20 miles with some roads and some highway. I simply made sure to vary my speed on the highway.

The car now drives more like a sport car and less like a sport sedan. Everything just seems more severe. acceleration, shifting everything seems firmer and rougher. If you are looking for pure drive-ability I would say that Ford had it right with the original gears. If you want to drive it harder than this feels great. you definitely feel the gas pedal response more.

the gas mileage appears to have taken a slight drop. every tank of gas i ran on this prior to the change came out between 17 -18 MPG. The first tank on the new gears was 16.9. First tank ever below 17. I can certainly live with this slight change.

Noise was certainly increased slightly also due to higher revs at both city and highway driving. Since it is a convertible I likely notice this more than most would. If I was spending all day in the car or using it frequently for long trips this might be an issue. Since I use it as a daily driver to and from work and around time and would never take it for a road trip this is not an issue for me. I would consider it a minor change.

It is definitely more difficult to hook up and will likely be more of a challenge at the track. I do feel like it accelerates better so I think with a little practice my track numbers should be better. I did a test hit on a local country road this morning and it spun more than it did with the original gears but once hooked up was hot as hell. I was into the low 90's before backing off and still in 3rd gear so I do not expect to get an extra shift. I am planning to take it to the local track (stadium parking lot) this Friday so i can can some actual data in the 1/8th.

NEW DATA for GEAR / RPM / SPEED (this is me visually looking at speed and rpm so could be slightly off)
3rd / 4250 / 60
3rd / 5000 / 70
3rd / 5750 / 80
6th / 2000 / 60
6th / 2300 / 70
6th / 2600 / 80

80 MPH is a full 1000 RPM higher in 3rd gear, it will be interesting to see what effect this has at the track. My Dyno (pre gears) shows that my HP is best in the 5000 - 6500 RPM range so in theory this should give me more power on the big end. since I was completing the 1/8th in the low 80's I was not reaching this power band in 3rd gear.

I'll post again after the track this Friday.

any advice for launching with the new gears?

It was $1040 from JBA speed shop here in San Diego. I called around to 3 other places and they were all pretty close. I trust the guys at JBA speed shop so went with them. They do a lot of the high end racing mods here. This price covered everything including changing the settings to make the Odometer accurate.

I had previously had them do a Chassis Dyno Tune. I think the price on that was $700. It made a huge difference.

took it to the 1/8th track last night. Definite improvement. times at this track are considered to be slow since it is simply a stadium parking lot with a short concrete starting skid.

best time dropped from a 8.659 to 8.544
60' times were consistent at 2.1xx vice typically around 2.3xx
trap speed increased from 84 to 85 MPH
was crossing the line over 6000 RPM.
was consistently beating the rest of the 5.0's that i lined up against. I did about 20 runs and only lost three. Most of these shouldn't count as they were against ricers, but i lined up against a few Mustangs, and one Camaro. I lost 3 times. The GT500 put up a 7.3, a BMW SUV put up a 8.0 and was just nipped by a infinity IS F. His times were consistently 8.4 - 8.5.

I know there is plenty left as I did all my better runs with traction control in (Sport Mode). I did a few runs with traction control off and was able to get my times down to 8.6xx but when i lined up next to a GT 500 I just let it rip and the car was all over the place. came in at 8.9. So I know there is plenty of power being reduced by the traction control and I need to find a way to get it to the pavement.

I hope this applies some actual data for anyone considering this change. I highly recommend it but it does make it less of a smooth daily driver.