Hey everyone, I’m in need of other people’s perspective prior to my gear swap next week. I have an 05 Ex with a 6.0 and last July when returning from Oregon (on my stock tire setup) I was constantly below the fat of my boost when doing 60 miles \ hr. I find with my truck, if I’m around 2000rpm while in over drive, I’m in the fat of the power and the truck doesn’t have to work too hard to accelerate while at that rpm. My truck tows a 10,500lbs trailer and when on the I5 at 60miles\hr, if there is a slight grade change, the truck will have to kick out of over drive to go up the grade with power. So with a stock setup and 3.73 gear, my rpm is too low for my liking (below 2000rpm)

Once summer ended I upgrade to 295 70 R18 wheels and tires and obviously my effective gear ratio has dropped putting my rpm even lower than where I liked it to be. On December 22 my truck will go in and either get 4.3 or 4.56 gears put in and I’m just trying to make sure I’m making the right choice because if I over gear the truck I could be running well over 2000 rpm at freeway speeds. The truck 70% is a run around town day care truck for my wife and the other 30% is driving my truck and trailer throughout the states.

Current RPM Numbers with stock gears:
60 miles \hr – 1650rpm
70 miles\hyr – 1890rpm

When I have my trailer hooked up I keep the speed civil at 60 miles\hr and don’t usually push to 70 so my goal is to have 2000 rpm at 60.

My numbers I’m working with for the example to follow:
1. Stock tire – 265/75R16 = 31.7”
2. New tire – 295-70R18 = 34.3”
3. Current Ratio – 3.73
4. Current Effective Ratio – 3.45 (4Lo.com :: Tire Size Change, New Gear Ratio Calculator)
5. Gear Choice #1 4.3 which has an effective of 3.97
6. Gear Choice #2 4.56 which has an effective of 4.21

The company that is doing the gear swap has given me two different simple math equations for determining rpm.

1. Take current RPM (for 60miles\hr) divide by current “effective” gear ratio and multiply by new effective gear ratio to get final rpm. So based on that, here are my choices
Gear choice #1: (1650/3.45) * 3.97 = 1899 rpm
Gear choice #2 (1650/3.45) * 4.21 = 2013 rpm
2. Second example provided by the gear company would be take the effective gear ratio, add a 0 and divide by 2.
a. My current setup is 3450 / 2 = 1725. Based on this math, the calculated rpm is 1.05% higher than what I’m actually doing. My actual is 1650 / 1725 = 5% difference. Do I use this 5% to reduce the found rpm for the next two answers?
b. Gear choice #1 3970 / 2 = 1985. Is it safe to say, multiple by 95%? 1985 * 0.95 = 1886rpm
c. Gear choice #2 4210 / 2 = 2105. Times this by 95% as well. 2105 * 0.95 = 2000rpm

The two different math equations do come up with similar answers; however my biggest concern is what will my rpm be at 70 miles\hr for when I’m not towing the trailer? At 60 I’m reving 1650 and at 70 I’m reving 1890 (current setup with 3.73s). That is an increase of 1.145%. Would it be a safe assumption to take my answers for the two different gears at 60 miles \hr and multiply be the % listed above to get possible rpm at 70 miles\hr?
Gear choice #1 1886rpm * 1.145% = 2159rpm @ 70 miles \hr
Gear choice #2 2000rpm * 1.145% = 2290rpm @ 70 miles\hr

I’m fearful for picking the wrong gear and spending the money to find out I under geared the truck for when it’s under load or I over geared the truck for when it isn’t under load. Does anyone out there have a 6.0 running 34 or 35s and done the gear swap? I have read many threads on people with the V10s and putting in 4.3 with bigger tires but haven’t tripped across a thread where they have the diesel, bigger tires and a gear swap.

I have 7 days to make up my mind and I’m currently leaning towards 4.3 as it is a safer choice at the moment.

Thanks for reading and any input you might have.
Jim

 

I think you're making it WAY too complicated.

You already know your actual rpm's with the tires you have on now; so you can forget about the tire heights and all that "effective ratio" nonsense.

(Edit: assuming you either corrected your speedo for the new tires or used GPS when you determined the rpm's with the new tires?)

You want 2000 rpm at 60 mph so:

2000 rpm / 1650 rpm = 1.212 change in gearing needed. 1.212 x 3.73 = 4.52 which a damn close to the 4.56 gears they offered you.

Now take your current 70 mph rpm of 1890 x 4.56 / 3.73 = 2310 rpm at 70 mph.

 

You guys are getting too deep into it.

 

When looking to spend around $2000, I want to be as sure as possible I'm making the right choice. Like I thought, 4.3 is looking like the right gear. Reving the truck at 2300 rpm just to do 70 miles/hr seems like a High fuel consumption if I was to use 4.56 gears.

 

More rpms = less MPGS, just keep that in mind.

 

Waste of money for 200 rpm, just take it out of overdrive when towing at 60 mph.

 

I don't see it as a waste, more of a long term investment. The rear diff is sweating and needs to be checked and the front diff has an inner seal leaking, so the time to fix that is nearly the time to replace the gears in the front.

As well, not looking for thoughts on how to spend money but more of thoughts on gear choices.

 

That`s not how it works!

 

I agree with Brent and learned an expensive lesson many MANY years ago.

There is a ridiculously easy formula you should use:

(New tire Diameter/Old tire Diameter) x Current Gear Ratio = New gear ratio

In your case:

(34.6/31.7) x 3.73 = 4.10 gears

You want to end up with an effective gear ratio of 3.73. If you go too numerically high with 4.30 or 4.56 gears, your RPMs will rev too high and the truck will run out of steam at the top.

Given that you're ending up with a 35 inch tire when everything is said and done, 4.10 gears should be your gear of choice, especially if you tow.

This all comes from first hand experience and making an expensive mistake.

It starts at post #54: https://www.ford-trucks.com/forums/6...y-caved-4.html

On page 5 at post #77, I had it right and should've trusted myself and my gut.

Post #102 is where the hard lesson was learned.

 

Thanks for the comments and the post! This for sure rules out the 4.56 gears! What I do know is that with stock tires and stock gears, I was missing the power band when pulling the trailer and based on your equation and what I have read, installing 4.10s will just get me back to the gear ratio I had with a stock setup.

On average the truck will see 60 miles\hr with odd stints hitting 70 so I'm hoping I'm not tacking out with 4.3s.

Do you pull a trailer with your 6.0L, 4.10 gears and 35" tires?

 

Hey there Toreador_Diesel

I just re-read your thread and most likely will read it a few more times. It appears before your gear swap you were running higher rpms than me with 35" tires.
At 60 I'm reving 1650rpm and at 70 I'm reving 1890rpm. Where you mentioned in your thread at 65 mph you are reving 1900 rpm. It seems you were reving higher than me prior to doing your gear swap?

My rpm was determined by OBDII reader and my speed was being confirmed by GPS.

 

Based on your current setup and what you are looking to achieve, I would go with 4.30's.

 

I have 35's, if I were to tow I'd consider 4.10's on the 6.0, I think your revs will be too high with 4.30's. Just my .02

 

By doing the 4.3 gears, I would only have an effective 3.94 ratio. So I would be going slightly higher in gears over the stock 3.73 however less than a 4.10 ratio if everything was left stock.

Final ratio with 4.30 gears


By doing the 4.10 gears, I would gain minimal rpms when I was already to low on rpms based on my stock setup.

 

Yes I have but with my 7.3 Excursion and it pulled night and day better with 4.10 gears. The speedo was also more accurate. Instead of it being 7-10 mph off, it was only a mere 3-5 mph off. All that was fixed once I bought the AE though.

You're reading too much into this and worrying over nothing. I worried, stressed, and ended up making an expensive mistake.

The RPMs were exactly where they were from the factory as opposed to numerically lower due to the larger tires. I.e. It was as if I had stock tires and gears again after the gear swap to 4.10.

4.10 gears and 35s go together giving you an effective ratio of 3.73
4.30 gears and 37s go together giving you an effective ratio of 3.73
and so on....

If you go numerically higher than you should thinking you're doing yourself a favor or compensating for something that really isn't a problem: You.Are.Wrong. You will also pay twice as much as I did since my 7.3 Excursion is a 2wd truck and your 6.0 Excursion is a 4wd truck.

Some of those calculators and charts are skewed and are geared towards gas racing applications than our trucks.

This formula is simple math and does not lie:

(New tire diameter / Old tire diameter) x current gear ratio = new gear ratio to achieve an effective gear ratio to what you had before.

You should also consider having a detroid tru trac installed. Other than having to pay for a gear swap twice, it was the only other regret I had. Do yourself a favor and get yourself a tru trac while you're having all this work done.

Absolute bottom line: Do whatever you want, I've learned from experience and am passing on some advice from a hard and expensive lesson learned.