I have compiled a spreadsheet that details RPMS in each gear at a given speed for 3.31, 3.55, and 3.73 rear gears. There have been many posts recently asking which gears should I choose. This spreadsheet answers all of those questions.

 

Your calculations appear to have some errors. For instance you list 7000 rpm in 4th as 115, 121 and 130 depending on gears.

3.73 - 111.5 mph (18" All-Seasons 765 rpm) - 111.1 mph (19" All-Seasons 768 rpm) - 111.8 mph (19" Summer Rubber 763 rpm)

3.55 - 117.2 mph (18" All-Seasons 765 rpm) - 116.7 mph (19" All-Seasons 768 rpm) - 117.5 mph (19" Summer Rubber 763 rpm)

3.31 - 125.7 mph (18" All-Seasons 765 rpm) - 125.2 mph (19" All-Seasons 768 rpm) - 126.0 mph (19" Summer Rubber 763 rpm)

As you can see you appear to be off by 4 mph on average.

 

It all depends upon what you believe is the correct tire diameter / revolutions per mile. I calculated everything from the stock 18" tire size, which is 235/50R18 (27.25196 inch diameter). We all know tire sizes can vary, but I didn't have any solid info on the actual diameter so I went by the size.

765 rev per mile yields a diameter of 26.36355. That's nearly an inch difference, and the reason for the differences in our calculated speeds. Where did you get your information?

 

I don't, and am sure others don't, have the software to open the spreadsheet on my computer.

I just use this calculator: http://www.f-body.org/gears/

Easy to use and you can make all the changes you like to it.

 

It's called OpenOffice. And, the 21st century. It's not OP's fault that people can't figure out how to use a computer for basic tasks.

 

Well mr. smartguy, just hoped to offer the less computer proficient among us an alternative.

 

I'm sorry, but this is like *****ing at a guy who brings you a gallon of milk because you don't have a fridge in your house. It's 2010, you should have some method of viewing spreadsheets on your computer, especially if you're a car guy (because you might need to, oh, look at gear/speed charts).

 

Don't need to look at gear/speed spread sheets, I just plug the numbers into the calculator I posted the link to and it gives the info I need.

 

Tirerack lists the 235/50/18 at 765 rev/mile. Click on "Specs"

Pirelli's site has 761.9 revs/mile.

I've been through this discussion with Gene....believe he's got it down pretty good!

 

So you used unloaded diameter as opposed to loaded radius or revolutions per mile? That likely would explain the difference.

The listed rpm varies for several tires. I believe the OEM Ford 235/50R18 is 765 rpm (I believe the generic is 762 rpm). However the 763 rpm Summer Tire is for the 255/40R19 XL OEM MB Tire. Ive seen the Ford OEM listed as being 763 rpm / 769 rpm / 769.5 rpm in various places. Im not sure which is acccurate. I simply used the one that gave the closest to your numbers.

 

I hear others saying that 1st gear is much less usable with 3.73 but this confirms that it shouldn't be an issue. I ordered it for but-o-meter in 2nd, 3rd, and 4th.

 

I've made a few different Lotus 123 then MS Excel spreadsheets to calculate engine RPM's at various speeds in all gears too and wondered why the RPM's were slightly higher in the "real" world. The spread sheet does almost ALL the calculations, all you need to insert is the gear ratios of the trans, axle, wheel size, and section height.

My '11 theoretically should be turning 1823 RPM @ 75 MPH, but in reality it is closer to 2k.

Doug

 

Amazing that folks offer some good info, then get attacked for doing so. To coffeejolts, thanks for posting the info.

 

Is it possible to use 5th gear on these and gain any time, or is it a loss between shifts? Just curious. I'm thinking 4.11 and using 5th.

 

That will completely depend on where you're running out of gear. For instance, if you can up your rev limit to 7400 on the new 5.0L and stay in 4th across the traps, that's what I'd do for the best times. If upping your rev limit won't quite get you there, then you may want to consider going with a shorter tire.

What size tires are you currently running?

 

Good stuff for the purpose intended, and dead on accurate for my 3.73s with 19 inch Brembo wheels...

Ignore the whiners who want to measure with a micrometer when the part is going to be cut with a chainsaw...

 

RPM 1st Gear 2nd Gear 3rd Gear 4th Gear 5th Gear 6th Gear
500 3 4 6 8 11 17

im lost in the first line, what does 3,4,6 etc mean? goes to 90 i have no idea what it is

 

That is the mph in that gear at that rpm with that ratio.

 

Do these numbers work for a 2013 GT with the 6 speed manual?

Joe C.

 

Appreciate the info. The spreadsheet does provide an answer to my inquiring mind. Namely, what the hell good is the 1st gear in my transmission with 3:73 axle gears? Answer, I might decide to install a trailer hitch someday so I can tow heavy stuff! Any good tow truck with a MT needs a granny gear. I have less than 300 miles on my Boss and I am already sick of constant shifting in/around town. I'm already considering a set of 3.31:1 axle gears. Then maybe the trans ratios would make sense. The Tremec in my Shelby is much more user friendly and is supposedly stronger as well. Anybody here have the inside track on Ford's thinking/purpose for the granny gear? I guess I'm wondering if there is anywhere on a road course where such a steep first gear is actually an advantage. There, I feel better now.

Bill