I am running 4.30's with 245/75/17 tires and it will get up and go. With the 37's you will need to go online and find the chart that will give you the effective ratio based on the gears / tires. IMHO it all depends on what your goals and uses for the truck are.

 

For 37s normally 4.88s are your corresponding ring and pinion set. You should even out as a daily driver with 37s, and regain some of your fuel mileage with 4.88s. If your going down the highway alot and want a little more top end go with 4.56. I think the 4.30 would be a little under powered but I could be wrong, these are diesels after all. I normally base my gear ratio on using 350s or an inline 6.

Gear Ratio and Tire Size Chart - HowStuffWorks

 

I'm running 4.10 with 38s..

 

Last time I looked, my rpms with 3.55 gears and 285/75/16 tires should be about the same with 4.10 gears and 37x12.5x16.5 tires.

 

I don't do a lot of highway driving.and when i do i pull a 26' rv, so 4.30's might actually work pretty well. I still need to see about the availability of lockers for those axles. other than the physical bolt up, does anyone know what else is involved in putting the newer axles under my obs?

 

There's plenty of threads about this (I'd be doing a disservice to the people that wrote them to give more details than this) but it is doable, depending on your skill set.

But, it you pull a lot you should also do research about lockers. The short answer is that only the electrically/pneumatically %100 locked/unlocked lockers are the ideal for towing.

 

373 gears on 37s, loved them with 35s. I may go lower to get back a little more bottom end but highway cruising rpms are fine.

 

430s and a true 40" tire turns almost the same rpm as a dually with 410s and stock tires.i was running 456 with 38s and 40s.now I am running 410s with 36s until I order new rims and put 37s back on.

I would not go lower than 456.if 430s are already in the axle it ain't worth the labor and cost to change I believe.

 

...it ain't worth the labor and cost to change I believe.

Some simple math, assuming $3.00 per gallon, and a MPG improvement from 10mpg to 15mpg, and $500 for a ratio change means it takes 50,000 miles to break even.

 

I have the stock 4.10s in my truck with 37" tires and it is perfect. I still start in 2nd gear (trans is a ZF5) 99% of the time and tow 6.5k without any issues. Also I still get 17.5 mpg average as well.

 

I have a 7.3 with 38"s. Been running 4.56 for nearly 20 years and by all means it runs great, but when the speedometer reads 60mph I'm actually doing 70mph via GPS. Been like that since the day I switched gears. As of now at 70mph actual, I'm at 2100rpm and at 60mph actual I'm at 1800 rpm with the 4.56.

 

You can reprogram your speedometer for the tires that you are running. It could care less about the gears.

Instructions are in this PDF about 3/4's of the way through it.

 

You can reprogram your speedometer for the tires that you are running. It could care less about the gears.

Instructions are in this PDF about 3/4's of the way through it.

Thank you for the pdf. I appreciate it.

 

Like I said it could care less about what gears you are running. The speed sensor gets its reading off of the ring gear in the rear end way before any gearing.

I just noticed that you have a 2000 but you are posting in the 94-97 forum. But still you need to reprogram the speedometer and not worry about the gears.

You need to go into your account and put a signature in that will show up at the bottom of your post. That way we will know what vehicle you have. The 7.3 engine came in trucks from the early 90's until 2003.