Differential Gear Swap

[JStinson]

I'm thinking about swapping the diff gears in my M37 to the higher 4.88. I figure this will up the road speed a little bit and make everything run at a lower rpms, while still keeping the gear ratio low enough for good offroad performance.

What are your experiences with this? Where can the components be purchased from? Is it a good idea?

Thoughts?

[Charles Talbert]

The gear change is one of the better upgrades that are possible, IF your engine is in good enough condition to support the extra load that will be applied as a result. I would highly recommend the change, however some that drive over hilly terrain are less than happy with a few extra down shifts as a result of higher ratio and the load you may be hauling. Most are really pleased, but with the few it is usually a combination of an engine suffering from low compression as a result of excessive wear, plus the other factors I mentioned that brings on dissatisfaction.

[JStinson]

The gear change is one of the better upgrades that are possible, IF your engine is in good enough condition to support the extra load that will be applied as a result. I would highly recommend the change, however some that drive over hilly terrain are less than happy with a few extra down shifts as a result of higher ratio and the load you may be hauling. Most are really pleased, but with the few it is usually a combination of an engine suffering from low compression as a result of excessive wear, plus the other factors I mentioned that brings on dissatisfaction.

No trouble on the engine. It's going to get a rebuild soon. From somewhere.

The concern I have is the off road performance. How much will it be affected?

Also, what is the cost associated with doing such a swap?

[Charles Talbert]

Off road performance in the lower gears will not be noticably affected at all.

The best way to do the swap is in conjunction with a complete differential overhaul. It is the highest cost to rebuild of any gear box on the truck, the gears are custom made and the wholesale bearing cost for 1 unit is now over $600. Total parts cost for 1 unit to do it the best it can be will tear up $1,500, with labor at around $650, if your cores are good rebuildable units. Total cost for both front and rear diff rebuilds with 4.89 gearing will be in the $4,000 - $4,300 range. You can do it for a little less if you cut corners, but with the cost of custom gearing, I wouldn't at all recommend taking that risk. You could easily wind up losing a $450 set of gears if something went wrong as a result.