Re-gear advice for newly purchased 2008 JK

[Genesiss]

Hello, I’m new to owning a Jeep and the one I purchased seems to be very incomplete. I need advice on what most recommend for gear ratio for Ironman 37’s on a 3.8l with Dana 30 and 44 and what I assume is 3.73 gear ratio. I need mpg over power but dependable is priority. It’s the wife’s daily so it will not see much off road action. I need to know first what gear ratio is needed? 4.88? 5.13? do I have to go 5.38 and switch out the front Dana 30 to 44? I need reinforcement for the axels? An upgraded steering pump? Any advice us greatly appreciated.

 

[Vman]

Hello, I’m new to owning a Jeep and the one I purchased seems to be very incomplete. I need advice on what most recommend for gear ratio for Ironman 37’s on a 3.8l with Dana 30 and 44 and what I assume is 3.73 gear ratio. I need mpg over power but dependable is priority. It’s the wife’s daily so it will not see much off road action. I need to know first what gear ratio is needed? 4.88? 5.13? do I have to go 5.38 and switch out the front Dana 30 to 44? I need reinforcement for the axels? An upgraded steering pump? Any advice us greatly appreciated.

Answered some of this one the other thread but wanted to respond to the steering question.

The PSC kit is amazing. I put in the big bore full kit w/o hydraulic assist and I can turn my 35's with one finger easy. The steering is perfect on the road.

 

[MacGuyver]

37s on a dana 30 is.. possibly fine if no off-roading is going to happen. I would verify that it actually is 3.73 ratio that is on there. Those years of jeeps often came with 3.21.

You didn't mention if it is a manual or auto. 37s are pretty big for a stock 3.8 auto, and it would feel pretty powerless. Here is a link to a post I made the other day addressing something similar. Your first step is establishing what the ratio is now.

loosing power going up hill

Hey Guys, i have a 2009 JK 3.8 manual trans with 35'' tires, it looses power driving up hill. even downshifting fells like it takes a while to catchup. any advise? Thanks

wranglerjkforum.net

 

[Anybodyhome]

Enter your VIN on this website and get a build sheet for your JK:

https://www.nathanson.org/davesays/2019/buildsheet-for-jeeps-vin/

Please do this rather than take someone's word. The 2008 Sahara came with 2 transmission options: 3.21 and 3.73.

And here's the gear chart I feel like I'm posting about every other day, but use it, please:

I think the other issue that needs to be addressed is using 37s on a daily driver that will see little off-road use. The optimum gear, based on the chart above with a 3.8 auto trans starts at a 5.38. Next up is a 5.86.

 

[Genesiss]

37s on a dana 30 is.. possibly fine if no off-roading is going to happen. I would verify that it actually is 3.73 ratio that is on there. Those years of jeeps often came with 3.21.

You didn't mention if it is a manual or auto. 37s are pretty big for a stock 3.8 auto, and it would feel pretty powerless. Here is a link to a post I made the other day addressing something similar. Your first step is establishing what the ratio is now.

https://wranglerjkforum.net/threads/loosing-power-going-up-hill.29543/post-443695[/URL

At the moment it’s in the shop but I was able to locate the original factory sticker online which leads me to believe it’s got the 3.73 ratio with the towing package.[/url]

 

[Vman]

Enter your VIN on this website and get a build sheet for your JK:

https://www.nathanson.org/davesays/2019/buildsheet-for-jeeps-vin/

Please do this rather than take someone's word. The 2008 Sahara came with 2 transmission options: 3.21 and 3.73.

And here's the gear chart I feel like I'm posting about every other day, but use it, please:

View attachment 120310

I think the other issue that needs to be addressed is using 37s on a daily driver that will see little off-road use. The optimum gear, based on the chart above with a 3.8 auto trans starts at a 5.38. Next up is a 5.86.

yep.

And with a D30, it starts AND ends at 5.38.

That's why I went 35's. 37's are a lot of weight to stick on that axle with that transmission, not to mention the gearing issues.

 

[Vman]

At the moment it’s in the shop but I was able to locate the original factory sticker online which leads me to believe it’s got the 3.73 ratio with the towing package.[/url]

View attachment 120313

Wait...that says it's a manual transmission..

If that's true, you can go 4.88 no problem.

 

[Anybodyhome]

Wait...that says it's a manual transmission..

If that's true, you can go 4.88 no problem.

The sticker lists the base, but look at the right column with all the option upgrades: he's got a 3.73 w/automatic trans among other installed options.

I think OP needs to gear up to a 4.88- a good, comfortable place to be if you want to do at least some mild off-roading and still have daily driver quality.

 

[Vman]

The sticker lists the base, but look at the right column with all the option upgrades: he's got a 3.73 w/automatic trans among other installed options.

I think OP needs to gear up to a 4.88- a good, comfortable place to be if you want to do at least some mild off-roading and still have daily driver quality.

Ah I see now.

If he's got an automatic 3.8 he'd need 5.38's for 37's though. 4.88 in a manual would be ok, but 4.88 in an automatic would be awful.

I'm running 5.13's for 35's in my '07 and stayed away from 37's due to the gearing required in a 3.8 auto for that size tire.

 

[Anybodyhome]

Ah I see now.

If he's got an automatic 3.8 he'd need 5.38's for 37's though. 4.88 in a manual would be ok, but 4.88 in an automatic would be awful.

I'm running 5.13's for 35's in my '07 and stayed away from 37's due to the gearing required in a 3.8 auto for that size tire.

You're right, I was looking at the manual trans.
I have no experience with the 3.8, but looking at this chart, there's no way I'd put 37s on this vehicle unless I was using it off-road more than a DD. Even then, I'd be looking at a Dana 44 swap.

 

[MacGuyver]

You're right, I was looking at the manual trans.
I have no experience with the 3.8, but looking at this chart, there's no way I'd put 37s on this vehicle unless I was using it off-road more than a DD. Even then, I'd be looking at a Dana 44 swap.

The link I posted was encouraging them to figure out the gearing themselves. Because they BOUGHT it this way. With an auto trans and with 37s and original gearing, it would probably not drive well. And we all know what strange little things get changed by previous owners you discover buying a used jeep.

So I'm guessing that it already has been re-geared. and before OP does anything, they need manually figure out what is in there. Hopefully, by looking at the rear pumpkin, but they should also verify by doing the wheel and axle spin.

 

[Anybodyhome]

The link I posted was encouraging them to figure out the gearing themselves. Because they BOUGHT it this way. With an auto trans and with 37s and original gearing, it would probably not drive well. And we all know what strange little things get changed by previous owners you discover buying a used jeep.

So I'm guessing that it already has been re-geared. and before OP does anything, they need manually figure out what is in there. Hopefully, by looking at the rear pumpkin, but they should also verify by doing the wheel and axle spin.

True that.
I agree they should verify the gearing for reelz with a hands-on check.

 

[Genesiss]

You're right, I was looking at the manual trans.
I have no experience with the 3.8, but looking at this chart, there's no way I'd put 37s on this vehicle unless I was using it off-road more than a DD. Even then, I'd be looking at a Dana 44 swap.

You're right, I was looking at the manual trans.
I have no experience with the 3.8, but looking at this chart, there's no way I'd put 37s on this vehicle unless I was using it off-road more than a DD. Even then, I'd be looking at a Dana 44 swap.

Thanks everyone for the assist and input. So I take it that the best overall choice is the 5.38 on the automatic transmission. Is it worth it to upgrade a Dana 30 to 5.38 or swap out the front end Dana 30 for a Dana 44 then upgrade to 5.38 gearing?

 

[Anybodyhome]

Thanks everyone for the assist and input. So I take it that the best overall choice is the 5.38 on the automatic transmission. Is it worth it to upgrade a Dana 30 to 5.38 or swap out the front end Dana 30 for a Dana 44 then upgrade to 5.38 gearing?

Do you know what the actual gearing is now? Not what the build sheet says or the tag on the differential, but have you verified physically, for yourself, what the gear ratio is? That is step 1 and here's a couple ways to accomplish that:

 

[Genesiss]

Do you know what the actual gearing is now? Not what the build sheet says or the tag on the differential, but have you verified physically, for yourself, what the gear ratio is? That is step 1 and here's a couple ways to accomplish that:

Thank you for the video! I got it back last night and will check today.

 

[Genesiss]

I just checked turning the rear wheel and counting revolutions and it’s a 3.73 gear ratio.

 

[Vman]

Thanks everyone for the assist and input. So I take it that the best overall choice is the 5.38 on the automatic transmission. Is it worth it to upgrade a Dana 30 to 5.38 or swap out the front end Dana 30 for a Dana 44 then upgrade to 5.38 gearing?

If swapping out the D30 is an option for you I'd take it.

Not only do you have the gearing issues with a D30 and 37s, but that's a lot of weight and stress to expect of it.