ok i have 3.08 now and stock tire size was 235/75/15 i put 31/10.5 on it, so my question is if i do change to 3.55 will that improve its performance on trailering and driving. i have a 300 six and have to drive town alot and i have seen a big loss of milage after the tire change..will i get this back with gear chnage? it is also a manual.

 

Remember that your speedo will now be off and that will affect your mileage calculation. If you do change gears you will notice improvement in performance of the truck. However you did not go up in size that much only about 1.5" taller than stock. If you go to a 3:55 it may be to much. I am sure somebody has this setup and will chime in for a definate answer.

 

ya i know by speedo is off...but its not costing me 3 mpg only that way...my guess is its around 6% off and i just add 1 mpg to my figure for milage. i am going to adjust my speedo but only after i decide weither or not i change gears so i dont have to do it twice
thanks for the reply

 

Ford should be slapped for putting those gears in a truck, what were they thinking. How big a load do you haul? If it's considerable I would suggest you go to 3.73 or even 4.10 gears.

 

I would run 3.55 or 3.73 in your truck with a 300 and 31s. I would not run 4.10 gears in a 300 unless you had 33 or 35 inch tires. It would be a different story if you had a 302 or 351. The 4.9 is a low reving motor. That said. The 3.08 is a lousy gear for a truck with an overdrive transmission. So, to answer your question. Yes, the 3.55 is better than the 3.08 gears.

 

31"s OMG Those are such huge tires man. What ever made you decide on 31's Those even make my 33's look small and those gears. There perfect for your app.

But in all seriousness you shoulda gone 33's man. 31's are lil kids tires blike what youd find on a big wheel. Go 33's - 35's there perfect for you truck dude Then get a 4:56 gear ratio, Rear locker. then ya got a good running rig.

 

OMG 3.08's that must suck, I'm with Conanski Ford should be slapped for putting those gears in your truck. With your combo having an overdrive tranny, I bet you almost never use 5th gear, I would install 3.73's and enjoy all the getup and go you've been missing. It will make a world of differance towing, hauling and off road, also your clutch will last twice as long, don't hesitate it will be the best mod you ever made to your truck.

 

Your tire change took the gearing up about 6-7%, the change from 3.08 to 3.55 will take it down about 14-15% from stock. That would leave you a little lower than stock gearing, Maybe 6-7 %. I'm running the 300 with 3.55 , 31/10.50 , and a e4od. Works ok but gas mileage ain't great.
FRank

 

im running 3.08s withe 31s and the grannylow 4 speed no over drive
running 70 by my speedo [even if its wrong] i am turning 3grand its alow lower thanmy 87 tht has 3.73s im going to 33s this summer would i swap to 3.55s?
my truck is geared extremly low so much as i thot mayb sum1 swaped my 3.08s for something else but the p/o said its all stock

 

Dude please type better, this is vary hard to read.

Is there a question in there I saw a question mark?

 

should i swap to 3.55s when going to the 33s?
my truck is geared low now with the 3.08s and 31s.
how much is my speedo off?

 

I think you got the term "low" gears reversed, when someone says they have "low" gears think "slow" gears, low refers to a numerically high ratio, 4.10 is a low gear. 3.08 is a tall, fast, or high gear but is numerically low.

OK so your talking about the 89 in your sig? This truck has a 300 and a t-18 so you have alot of low end torque and a granny low when you need it, but no overdrive. This means you can get away with a taller (numerically lower) gear and still use your granny when you need the torque. So your biggest concern is high gear final ratio. This comes down to what RPM you are spinning at highway cruise, and if you dare top speed.

With the 3.08 gears and 31" tires my numbers say your turning 2177 RPMs at 65mph. And if you top it out at 4500 RPM your going 134mph.

If you keep the 3.08 gears and install the 33" tires you get 2045 RPMs at 65mph. And if you are crazy enough to spin 4500 RPMs your going 143MPH!!!

With 3.55 gears and 33" tires my numbers say 2343 RPMs at 65mph. And at 4500 RPM your going 125mph.

The question becomes is that good for you, how you drive and where your motor is happy? Do you want the engine turning slower at highway cruise? Do you cruise along faster then 65?

Personally I think if you do alot of highway driving, keep the 3.08s, I say this to you and not the OP because you do not have an overdrive gear but do have the granny. Having a low enough gear is never a problem for you but having a tall enough gear could be.

As to how far off your speedo is I need to know your factory tire size. And for all these numbers to be real acurate you really need a number published by the tire company called rotations per mile. I can say that with the example above, that your speedo will read 7.6% faster then it does now.

 

Sence I'm crunching numbers for fordguy300 I'll do it for the OP.

Your primary concern seams to be getting your power and mileage back after the tire change, lets see what the numbers say.

I'm fiquring this with the M5R2 5 speed tranny, it has a 3.75 1st gear and a .75 overdrive. I'm not sure if this is the correct tranny please correct me if I'm wrong.

I think a good way to measure take off power when looking at gear ratio is to look at rpms at a low speed in 1st gear, the higher the RPM at these low speeds the more take off power you have. So lets look at rpm at 10 mph.

In 1st gear I get a final drive ratio of 11.59, this means the engine is turning 11.59 times for every axle rotation, and with the old tires I got 1348rpm at 10mph, not great.

With the new 31" tires it drops to 1256rpm, even worse and a drop of 7.3% that's 7.3% less usable power to get you going. And your turning 1633rpm at 65mph in 5th, not enough to get that motor in an efficient range.

If you change to 3.55 gears you get a final gear ratio of 13.28 turning 1439rpm at 10mph, better and an increase of 12.7 % of usable lauch torque. And you'd still only be turning 1870rpm at 65mph in 5th gear.

But with 3.73 gears you get a final of 13.99, and are turning 1516rpm at 10mph an increase of 17.1% of usable power over the 3.08s. And are still only turning 1971rpm at 65 mph. This sounds pretty good, that is unless you think you want a little more and would consider 4.10s

With 4.10s it goes to 1666rpm at 10mph an increase of 24.6% of usable torque at the wheels, better still but the highway number gets up there to 2166rpm at 65mph, not to bad but up to you depending on how you drive.

Bottom line I think 3.73s sound pretty good unless you think the 4.10 might be a better choice for you and your driving habits. But I don't think 3.55s are a big enough leap especialy because it will cost the same and take the same work as going to 3.73s. 4.10s would be a little more work and expense though, the 44 up front in your truck uses a differant carrier for all ratios above 3.92, so changing to 4.10 in the front would require a differant carrier. The 8.8 in back though doesn't matter.

Hope this helps.

 

Opossum
Great job crunching numbers.
You need to consider the 300 is a low rpm motor. I don't know what the numbers are, but I know mine makes most of it's power between 1500 and 2000 rpm. Actually it is making plenty of power for city driving between 1000 and 1500. I have one long hill on the way to work. Love to go up this in the summer with the windows down. I'm running just under 40 in overdrive and turning about 1200 rpms. The sound bounces off a concrete wall and sounds really nice. Nothing sounds like the big 6 under load.
Frank

 

Thanks,

I kept all the rpm numbers under 2200 except the CRAZY toping out speeds for fordguy300. I haven't driven a 300 in a while so I'm not sure how much they like to lug, I'm just throwing the #'s out.

Are you saying you can highway cruise under 1500 rpm?

On the numbers I put together for Fox_Racing I assumed the M5R2 tranny with a 3.75 1st, but I just came across some info that it may be the M5R2-C which would give it a 3.42 1st making it even worse at take off. Any info on that?

Edit,
Just looked at your sig and realized you have a vary simalar setup, but with an automatic. Auto's will let you lug the motor alot more and will tolerate higher ratios because of it. I guess if my numbers are right you should also be turning about 1900rpm at 65mph, is that your experiance and is it to high or low?