Quick question about gearing.

[babysilk]

Since I did my body lift Ive wanted to run bigger tires. I talked to mudryder and he said he ran 38.5's for 2 years but then blew out his rear end. What do you guys think about running 37's on my stock h9 3.55 lsd axle?

 

[panda24619]

nuts pretty much does, but has 3.73s. personally i like the 35s. they look better than the 37s. and when you flex out youll have more room before tire rub.

but with 35s i would also regear. 4.10-4.88 i would go with. with 37s i would say you have to regear. or just be like nuts and not care. haha

 

[walnuts4x4]

I ran stock 3.55 with 38x15.5 swampers on my f150 with the little 8.8 rear end for almost 2 years without problems..until i could get 4.88s.... and now I have 37s with stock 3.73 gears... and still waiting for cash to get 4.88s.. but as long as you don't beat the hell out of your expy you will be fine....

if you stick with 35s I would go with 4.56 but if you wanna go bigger I would look at 4.88s.... I want them but cant do the install myself so gotta save the cash for someone to install them

 

[babysilk]

I think that Ill stop at 37s or 38.5s but if i could run 37's for a couple years without destroying anything that sure would be nice!

 

[SkyJumper]

I got to beg everyone differ. When running a tall tire like that, without at least a reprogram or gears, your trans will take a beating constantly shifting out of necessity. Running a 37 or hell a 33" tire, the shift pattern are disrupted and the trans will shift up and down constantly. If you use the Expy on the road alot, you will see gas mileage drop drastically, and the possibility of a trans wearing out prematurely is high on the chart.

Now I run the 305/70/16 size and my shift pattern was all whack and I too run 3.55's. But a 3.55 is a good gear to run if on the road alot as long as you reprogram the trans to match the tire size upgrade. That is why I bought the Edge Programmer. It alowed me to update tire size and now the trans shift as stock, the RPMS are spot on and the speedo is correct. Only reason people regear in the first place is to compensate for the tire diameter. but if you reprogram the ECU, then a 3.55 is a perfect gear to run 33's thru 37's.

Now just because someone else did this and experienced no breakage does not mean it is true for all to do. The manner of how your truck has been maintained and how it acts now mechanically will tell alot. But if you have a slight issue with the driveline, then upsizing this will just intensify this unknown or known problem.You can take 5 of the same year and model Expy's and do the upgrades as tires and rims, and I will tell you now, none will act or drive the same.

Just be warned that this could cause issues down the road, a block down the road or 2 yrs down the road, either way it is not healthy for the transmission.

 

[Thermo]

from my experience (running 35's with 3.73 gearing), while it works, it isn't necessarily the best on the tranny (the weak link in all of this). It requires a lot more shifting from the tranny due to the low RPMs that it is going to keep the motor at. Also, you will notice that unless you are up at 70+ MPH, your mileage is going to suck (ie, need to get the RPMs up over 1800 RPM). With the big tires and the stock gearing, it is hard to do.

Also, do not plan on towing anything or putting heavy loads in the truck. If you look closely, the towing rating is directly tied to the gearing in the axles. Trying to run 37's with say 3.55 gears, that is pretty much going ot limit you to around a 3,000 pound trailer as your max. For some this is a big deal, for others it is not. But, the same can be said about how much you can put inside of the truck.

If you are looking at tires over 35's, regearing when you get the bigger tires is going to be vital as I see it. Will the truck take it, sure. But, like Panda and Walnuts say, it can work, but be gentle.

With 37's, if you chose 4.56 gearing, it will be effectively like having the truck on stock tires and 3.73 gears in the truck. If you run 38's with 4.56 gears, it will be like having your truck completely stock with the 3.55 gears. That will give you something to base what gearing to go with.

 

[toms89]

Here is a helpfull chart to determine gear ratio.

http://auto.howstuffworks.com/auto-...wing/maneuvers/gear-ratio-tire-size-chart.htm

Keep in mind the stock tire size on an expy is near 30".

Also that this the rpms in 3rd gear (1 to 1 ratio). Multiply by .71 to find rpms in overdrive.

I have been through many ratios with my mustang through various stages of its build. I can tell you it is a great tool / modification to get the most out of your combination.

I would actually prefer a lower gear ratio for my expy right now as it would put less stress on the tranny and improve acceleration.

I would consider a change in tune necessary when changing tire size without changing to the appropriate gear ratio because of what SkyJumper stated. I also agree with thermo 100% that taller tires with stock ratio will put more stress on the tranny. It has the same effect as installing higher gear ratio. Just for an analogy....Think of how hard it is to pedal a ten speed starting in 10th gear... Much harder on your legs, pedals, chain..etc..

Can you imagine trying to tow starting in 2nd gear...this is why it is important to match gear ratio with tire size. The lower the gear ratio (higher numerically) the easier it is on the drivetrain, other than your axles. This is of course at the expense of top speed and fuel efficiency (once your out of the sweet spot).

 

[edinphx]

Skyjumper,
I really appreciate the advice you give on the forums. with that
said.. I have the exact same vehicle you do with the exception of no CAI or Exhaust or chip. I just put 315 75 16 on my expy. My next move was gears
4.56 to be exact is this what you would do? Also would you do the Edge Tuner also?
thanks
ED

 

[SkyJumper]

Skyjumper,
I really appreciate the advice you give on the forums. with that
said.. I have the exact same vehicle you do with the exception of no CAI or Exhaust or chip. I just put 315 75 16 on my expy. My next move was gears
4.56 to be exact is this what you would do? Also would you do the Edge Tuner also?
thanks
ED

Thanks for the comment. To answer your question, the gear seems to be a good choice if you go that route. And if you want to do the tune only with the programmer from Edge than that is good also or both at the same time. If you go the gear route only, then your RPM and speedo will be off slightly and the tranny is still the weak link in all this. If you do the tune with the programmer, then the assurance of RPM's and speedo will be within 1% of stock reading. If you are running a 3.55 gear as I do, I see no reason to spend all the money to regear and labor for someone else to do if you can not yourself do the install. When you can buy the programmer and and within 5 minutes, be back to stock configuration, and the trans will be just fine. 315/75 is pretty much a 33" tire and will do just fine with the 3.55 gearing. Especially on the highway, great gear. My 305/70's are just alittle taller than a 32" tire. After I tuned, and drove on 2 tank full of fuel, I increased my MPG rating from 11 to 16.5 MPG, that is a hell of an increase in fuel economy. And that is with the 3.55 stock gear, and my transmission shifts perfect, especially in OD, and shifts firmer with the reprogram.

Either way you go is fine, but, you still will be off slightly with new gears alone without a tune to reprogram the tire size, and it is in my eyes more economical to due the tune, run the 3.55's and be done with it. Better fuel mileage, and better shifts with the trans, and not to mention, more horsepower.

Glad I could be of assistance.

 

[edinphx]

Thanks Sky!

just oredered the Edge, should be here early next week.

Ed