use the RPM calculator to see how tire size affects your rpm at a given speed
Calculators

you want to select a gear ratio that will put your final drive ratio close to what the factory setup was. that will give you close to the same MPG as the factory tire size and gear ratio.

by going to a larger tire you are going to lose some MPG from the change in wind resistance, and larger contact area of the larger tire as well as the additional weight.

your MPG while towing will always be lower simple because you are moving more weight and depending on the trailer, wind resistance.

 

4.30's and 37's seem to be the best match up for the 7.3. If it's already taking a lot of throttle to get moving, might I suggest delving into that first? At least ensure everything is in top shape before throwing giant tires under there.

Personally I'm on V's with a 2" drop kit and mod-B's. 35's are a good, somewhat snug fit.

 

If you're rocking 35s, then 4.10 gears are what you should have if you want an effective ratio of 3.73.

I've found those online calculators to be wrong more than they are right. If you need to find out what gears your need for the tires you're running, the formula is very simple:

New tire size/old tire size x Desired effective ratio= correct gear ratio

In my case with Rudolph:

34.6/31.6 x 3.73 = 4.08 or 4.10 gears.

With 4.10 gears and 35s Rudolph gets good fuel economy but is also able to tow the heaviest of loads and not break a sweat or anything else. Running a 6.0 trans cooler, the temps rarely go above 150 even with it being 400 degrees outside.

37/31.6 x 3.73 = 4.36 or 4.30 gears

If you're a v10 guy who tows a lot:

37/31.6 x 4.30 = 5.03 or 5.13 gears

35/31.6 x 4.30 = 4.76 or 4.88 gears

 

^^^ Truth! ^^^
I'm a V-10 guy that pulls 11,000lbs about 70% of my EX's miles, the combination of my 35" tires and 4.88 gears is great, much better performance AND (towing) mileage!

 

[QUOTE=Toreador_Diesel;15604922]

I've found those online calculators to be wrong more than they are right. If you need to find out what gears your need for the tires you're running, the formula is very simple:

/QUOTE]

The formula for rpm is also simple math but you need to know the gear ratios in your transmission to calculate it. But to demonstrate how a diff gear change affects your engine rpm the one I linked to is close enough for government work

 

I've done a custom 4" lift with the larger block from an F-350 in the rear and I run 295 70 18s and the only adjustment I had to do was create a bracket to suck the front inner fenders back a bit to stop the rub. I have the same offset the truck came with so I'm tucked nicely and I clear all around. With 4", 37s would be a real pain for me.

I pull heavy so I had 4.30s installed giving me an effective 4.0 and I'm in the fat of the power. I do lose out on top end speed so I don't push the Ex beyond 70mph but where I live, 60mph is the common. I'm happy with the choice to re-gear to cope with the larger tires.

 

I have X codes up front and modded C's in the rear. I could prob fit 305s without any mods but I currently have 285's.

 

I think you could no problem, but remember the op is using 18" wheels,
2" bigger diameter.

Good looking EX!

 

MrB, where have you been. I have been looking for this set up since iv been on here and have yet to see your EX. Im doing close to the same as this set up just with 17's.

I have 285.70.17s if that helps the OP. I have no mods yet but will be doing modded x/b codes. Supposed to be around 4inch but we will see. iv got the leafs, just gotta get springs and other parts before i do the mod.

 

Just for kicks. Same truck, 4" lift running 18s.