Help me understand gearing and re-gearing

finchmeister08 said:
this is what i don't get. in my mind, higher RPMs means more fuel is being burned at a faster rate (with lower gears). which would make your mileage drop. my gas light just came on after a full tank and i've only gone 164 miles on it with it's current set up. a majority of it is around town mileage.
Click to expand...
Have you corrected the speedometer for the 33" tires? If not, you're going farther than the odometer says.
 
Just listen to the advice being given about lower gearing due to the .69 overdrive on the 42RLE. Do not fall into the 30" tire/3.73 gearing = 33" tire/4.11 gearing because of the math fallacy. It does not work that way. Even 4.56 is too tall.
 
finchmeister08 said:
this is what i don't get. in my mind, higher RPMs means more fuel is being burned at a faster rate (with lower gears). which would make your mileage drop. my gas light just came on after a full tank and i've only gone 164 miles on it with it's current set up. a majority of it is around town mileage.
Click to expand...
It is very common for the fuel gauge and fuel light to be inaccurate on our TJs. My fuel light typically comes on with 6 or 7 gallons left.
 
  • Like
Reactions: Barefoot Productions
glwood said:
Just listen to the advice being given about lower gearing due to the .69 overdrive on the 42RLE. Do not fall into the 30" tire/3.73 gearing = 33" tire/4.11 gearing because of the math fallacy. It does not work that way. Even 4.56 is too tall.
Click to expand...

You don't really need to regear at 30" do you? I'm planning to run 30x9.5.x15 eventually and I thought I remembered reading you could go to even 31x10.5x15 without regearing. I also have the 42RLE with 3.73 gears.
 
Not_Invented_Here_ said:
You don't really need to regear at 30" do you? I'm planning to run 30x9.5.x15 eventually and I thought I remembered reading you could go to even 31x10.5x15 without regearing. I also have the 42RLE with 3.73 gears.
Click to expand...
3.73 with 30" tires would be fine with a manual transmission. The problem with the 42RLE auto is the steep 0.69 overdrive. Even with 30" tires, you'd probably be best to have 4.56 if you want to utilize overdrive.

From grimmjeeper with 42RLE/29.5"/70mph:
3.73: 2052 rpm
4.10: 2256 rpm
4.56: 2509 rpm

The other option is to turn O/D off, which would give you 2974 rpm in 3rd gear at 70 mph.
 
Not_Invented_Here_ said:
You don't really need to regear at 30" do you? I'm planning to run 30x9.5.x15 eventually and I thought I remembered reading you could go to even 31x10.5x15 without regearing. I also have the 42RLE with 3.73 gears.
Click to expand...
I have 32" tires on my Rubi with the 42RLE with the stock 4.10 gearing, and it's adequate. Not great, just adequate.
 
finchmeister08 said:
filled up at 170 miles just to see how much i put in. 16.8 gallons/19 gallon tank. that's roughly 10mpg. OUCH!!!
Click to expand...
That is a bit low. Part of it is definitely due to the speedometer not being accurate. Even just 5 mph can make a significant difference in the odometer reading over the course of a tank. The other part may be due to factors like gearing and past maintenance.

Your SE 4.0L probably has 3.07 gears, which is definitely an issue trying to turn 33" tires.

First thing to do would be to give your wear parts a review. How many miles does it have? Perhaps the O2 sensors need to be replaced. Maybe the cats are clogged. Spark plugs could use a refresh. You get the idea.

Next is to decide if a regear is in your plan. If so, it is important to decide what tire size you ultimately want and stick to it. Then, once you've made a decision either way, replace the speedometer gear to the appropriate one (which is dependent on gears and tire size) to get your speedometer and odometer back on track.

TJs aren't known for their spectacular gas mileage, but you should be able to squeeze a few more than you're currently getting.
 
nk24 said:
3.73 with 30" tires would be fine with a manual transmission. The problem with the 42RLE auto is the steep 0.69 overdrive. Even with 30" tires, you'd probably be best to have 4.56 if you want to utilize overdrive.

From grimmjeeper with 42RLE/29.5"/70mph:
3.73: 2052 rpm
4.10: 2256 rpm
4.56: 2509 rpm

The other option is to turn O/D off, which would give you 2974 rpm in 3rd gear at 70 mph.
Click to expand...

Interesting thanks for the info. Yeah I have no plans to regear nor do I have the budget for it. Perhaps I'd be fine just rocking the 235X75rX15's and switching to A/T's. I can't imagine 30x9.5x15 would cause that much trouble with the way I drive (mostly city, light highway, camping and light trails is worst it'll see) I have some decent Michelin all seasons that came new with the Jeep and I'm just running those until they reach end of life.
 
Top Bottom
Back