- Location
- NJ, U.S.A.
Ok, I know this has been asked before, and I have read several recent threads on the matter as well as CRASH's missive in the stickies collection above. That being said:
I have been running 30x9.50x15s for a couple of years now on my '92, using the stock axle gearing (3.55). It's not been all that bad, but at times it has felt like the engine was laboring a bit on hills (I have on occasion run around town in "3" instead of "D" to keep the rpms up, but stick with "D" on the highways). The tach reads about 2K rpms at 60mph. Never really thought much about regearing (expensive).
Now that I'm starting the process of going over the vehicle to prep it for the run to Moab in October, I've been once again thinking about the gears. While I don't expect any issues out in the plains, it's the mountain climbs nearer the west end of the trip that concern me - whether I am going to run into trouble being slightly undergeared relative to stock. As I am already at 30s, I figured 4.10s would be the most logical choice, since the notion of going up an inch in suspension and maybe going to 31s when the 30s wear out has been on my mind since NACFest.
Obviously, I have done a little research on having my existing gears replaced (instant sticker shock, just add water). The guy I spoke to about it (local mechanic/speed shop) suggested I call a junkyard that specializes in Jeeps. I have done this, and they quoted my $250 for a complete D30 (HP, non-disco, non ABS) and $350 for a C8.25, both with 4:10 gears. When I mentioned to him why I was contemplating this, he did indicate that I could see a drop in MPG from the increased RPMs.
Certainly, even when you factor in installation, this is far cheaper than installing new gears in the existing housings (pity this is the case), but then I got to looking at GoJeep's site, which has an interesting RPM chart at the bottom of his speedo drive gear change. This chart indicates that there's only a change of about 100RPM between 28-inch tires (P225/75/R15) to 30-inch tires. I am aware of the implications of increased tire width and mass, but am uncertain as to how much weight they carry in the equation, if you'll excuse the pun.
The last time I took the Jeep on the daily commute for a day or two, I checked the mileage and was getting around 19-20 (pretty good, IMHO, considering the oversized tires, skid plates, and rock rails), which is consistent with what I was getting back when it was stock and serving as my DD. Of course, that was before I got the A/C fixed, so I do need to try that again once I get the holes in the driver's side floor addressed.
Another observation I have recently made is that since I had the radiator replaced (7-8 years old, fins rotting away from age and salt), the truck feels a little peppier - maybe better flow through the cooling system has restored some accumulated parasitic loss?
As a result, I'm now of two minds - swap in 4.10 axles to get the hill climbing power back at the risk of annoyingly high highway RPMS and possibly decreased mileage, or stand pat on the stock 3.55s for now?
Thanks in advance for thoughts.
Rob
I have been running 30x9.50x15s for a couple of years now on my '92, using the stock axle gearing (3.55). It's not been all that bad, but at times it has felt like the engine was laboring a bit on hills (I have on occasion run around town in "3" instead of "D" to keep the rpms up, but stick with "D" on the highways). The tach reads about 2K rpms at 60mph. Never really thought much about regearing (expensive).
Now that I'm starting the process of going over the vehicle to prep it for the run to Moab in October, I've been once again thinking about the gears. While I don't expect any issues out in the plains, it's the mountain climbs nearer the west end of the trip that concern me - whether I am going to run into trouble being slightly undergeared relative to stock. As I am already at 30s, I figured 4.10s would be the most logical choice, since the notion of going up an inch in suspension and maybe going to 31s when the 30s wear out has been on my mind since NACFest.
Obviously, I have done a little research on having my existing gears replaced (instant sticker shock, just add water). The guy I spoke to about it (local mechanic/speed shop) suggested I call a junkyard that specializes in Jeeps. I have done this, and they quoted my $250 for a complete D30 (HP, non-disco, non ABS) and $350 for a C8.25, both with 4:10 gears. When I mentioned to him why I was contemplating this, he did indicate that I could see a drop in MPG from the increased RPMs.
Certainly, even when you factor in installation, this is far cheaper than installing new gears in the existing housings (pity this is the case), but then I got to looking at GoJeep's site, which has an interesting RPM chart at the bottom of his speedo drive gear change. This chart indicates that there's only a change of about 100RPM between 28-inch tires (P225/75/R15) to 30-inch tires. I am aware of the implications of increased tire width and mass, but am uncertain as to how much weight they carry in the equation, if you'll excuse the pun.
The last time I took the Jeep on the daily commute for a day or two, I checked the mileage and was getting around 19-20 (pretty good, IMHO, considering the oversized tires, skid plates, and rock rails), which is consistent with what I was getting back when it was stock and serving as my DD. Of course, that was before I got the A/C fixed, so I do need to try that again once I get the holes in the driver's side floor addressed.
Another observation I have recently made is that since I had the radiator replaced (7-8 years old, fins rotting away from age and salt), the truck feels a little peppier - maybe better flow through the cooling system has restored some accumulated parasitic loss?
As a result, I'm now of two minds - swap in 4.10 axles to get the hill climbing power back at the risk of annoyingly high highway RPMS and possibly decreased mileage, or stand pat on the stock 3.55s for now?
Thanks in advance for thoughts.
Rob