Lame question, First Time Jeeper: bigger tires, no 8th gear on highway. What to change?

[sandor.g]

Hi,

So I have a JLU Sport S, I have lifted it with the MOPAR kit and put 285/70/R17 BFG A/Ts on. Changed the tire size to 33" as I read in other threads. The speedometer is a little off, but what bothers me more is that on the highway I barely get into the 8th gear. Riding at solid 70-75 mph on a flat highway road, still in 7th at like 2200rpm. If I shift it manually to 8th, it goes back to 1900 rpm, my mileage immediately improves accordingly.
If I wan't to correct the speed I know I have to adjust the tire size to the actual 32,7", but what else do I need to change?

thanks!

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[LUV2JEEP]

Hi,

So I have a JLU Sport S, I have lifted it with the MOPAR kit and put 285/70/R17 BFG A/Ts on. Changed the tire size to 33" as I read in other threads. The speedometer is a little off, but what bothers me more is that on the highway I barely get into the 8th gear. Riding at solid 70-75 mph on a flat highway road, still in 7th at like 2200rpm. If I shift it manually to 8th, it goes back to 1900 rpm, my mileage immediately improves accordingly.
If I wan't to correct the speed I know I have to adjust the tire size to the actual 32,7", but what else do I need to change?

thanks!

Either live without 8th gear or regear both axles. I regeared from 4.10 to 5.13 and got 8th gear back and gas mileage went up.

 

[sandor.g]

Either live without 8th gear or regear both axles. I regeared from 4.10 to 5.13 and got 8th gear back and gas mileage went up.

re-gear for 1 inch difference in tire size? interesting

 

[RockyMtnHigh]

re-gear for 1 inch difference in tire size? interesting

Your tires are nothing but the last gear in the chain to the road. Bigger tires require more torque to keep them spinning. 8th gear provides the least amount of torque out of the transmission, and it was only designed to provide enough torque to keep you moving at 75mph with the factory tires. To offset this, you need to add more torque to the system, either with an engine swap, or through the magic of gear reduction.

In your case, you'll want to re-gear to 3.73, front and rear. This will actually be a bit deeper than the stock ratio, and you should notice a bit more pep than stock. See here: https://tiresize.com/gear-ratio-calculator/

Also yes, you need to do a flash to correct the speedometer.

 

[sandygar4]

Probably a really lame question, but how do you tell what gear you are in when in drive?

 

[RockyMtnHigh]

Probably a really lame question, but how do you tell what gear you are in when in drive?

Drive a manual

Else, check here: https://www.jlwranglerforums.com/forum/threads/gear-indicator-for-automatic.6127/

 

[plex]

If you constantly drive with overdrive (force 8th) but less power, you will hurt your transmission. So either regear or live with it. Or at least correct your speedometer, that will get to the right shifting point. Right now your speed and shifting points are all incorrect.

 

[ChattVol]

Your tires are nothing but the last gear in the chain to the road. Bigger tires require more torque to keep them spinning. 8th gear provides the least amount of torque out of the transmission, and it was only designed to provide enough torque to keep you moving at 75mph with the factory tires. To offset this, you need to add more torque to the system, either with an engine swap, or through the magic of gear reduction.

In your case, you'll want to re-gear to 3.73, front and rear. This will actually be a bit deeper than the stock ratio, and you should notice a bit more pep than stock. See here: https://tiresize.com/gear-ratio-calculator/

Also yes, you need to do a flash to correct the speedometer.

It seems not worth the $ to regear to only go from 3.45 to 3.73. I'd go to 4.10 running 33s to have ratios the same as a rubicon.

 

[DrJoeSchmoe]

It seems not worth the $ to regear to only go from 3.45 to 3.73. I'd go to 4.10 running 33s to have ratios the same as a rubicon.

I can't imagine spending money to go to 3.73, but I guess if you're really not interested ever running bigger tires, then it's ok. The JL has made re-gearing much more critical with the 8-speed trans. On the JK 5-speed, you could get away with not having the correct gearing. I'm still running 4:56 with 37-inch tires on my JK, and It doesn't bother me. My JL needs 5:13s to get into 8th easily and it would drive me crazy if I couldn't get 8th.

 

[ZombieMojitoJL]

Either live without 8th gear or regear both axles. I regeared from 4.10 to 5.13 and got 8th gear back and gas mileage went up.

I am in the process now of regearing I have an 18 Sahara with 37s going 5.13 I am getting 16/18 mpg how much will mpgs be affected approximately? Thanks in advance!

 

[Uhdinator]

I used a GPS phone app with the phone not plugged into the jeep to compare speed against the speedo. Used a flash cal like this that plugs into a terminal behind bottom of the glove box. https://www.quadratec.com/p/rough-c...PE7cDFDqKkqBqpgJXT9g5AeBRmHXxB3BoCiEQQAvD_BwE

I actually did it a couple times and ended up setting it at 31.75 for a 32" measured height to get it at 1 mph within what the speedo says. It can read codes too as it has to clear them before setting the tire size.

Keep in mind also that the Trans adapts to your driving. It you are mashing the skinny pedal a lot the shift points gradually will change to higher RPM shift points to match your driving habits.


Pull the Trans Contr Module fuse for a few minutes to reset it. Re cal the speedo, Drive without aggressive excelleration to get where you can test it at 70 mph. Try it with cruise control also. See if it makes a difference.

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