ubengineer
New member
I keep hearing that if I increase my tires to 35's I'm going to need to re-gear. I currently have stock 3:07. Can someone please explain to me why I would need to regear?
Gears vs tires
- Thread starter ubengineer
- Start date
ubengineer
New member
Can someone please let me know your setup regarding tire size vs gears and what rpms are you getting on the highway.
Bounty__Hunter
Super Moderator
Larger tires will greatly drop the engine RPM at a given speed, causing the engine to struggle. Power and mileage will decrease. If you want an idea of how bad it could be, try shifting from 3rd to 5th when accelerating. The drop in RPM and engine struggling would be similar to shifting to 4th with oversized tires.
Check out this chart to help determine what axle ratio you should have. I recommend finding the RPM for 3.07 gears and 27" tires, then find a similar RPM for 35" tires. That's your new recommended ratio. I always go a little lower (higher numberically) with gears than recommended.
ubengineer
New member
That's chart is for manual I have an automatic
What year Wrangler do you have? I just ask because my 2005 came stock with 3.73 gears.
That chart is good for an automatic with this caveat:
from here:
Jeep Parts, Jeep Accessories & Jeep Soft Tops From The Jeep Parts Experts - Quadratec
ubengineer
New member
I have a 97 auto straight six with 3:07.
ubengineer
New member
Can u give me an example of converting this to an auto
Bounty__Hunter
Super Moderator
Doesn't matter. Find the RPM for your stock tire size and gear ratio. Then work your way across the 35" row until you find a similar RPM, that's the proper gear ratio for that tire size. Regardless of transmission type.
I recommend going one ratio lower (numerically higher) than recommended. This will greatly improve performance on road and off. Regearing only compensates for the added tire diameter of larger tires, but does nothing to compensate for the added weight and rotational mass. For this reason it's best to go a bit lower.
ubengineer
New member
I have no idea what stock tire rpm would be I currently have 31 tires
Bounty__Hunter
Super Moderator
Stock tire height is 27" or 28", simply follow that over to 3.07 ratio.
69jeepcj
New member
im with jester32, 4.56 to 4.88 would be good. I am running 4.88 with a v8 and 4spd trans and my rpm on the highway are right around 3000 rpms, I dont have a speedo so Im not sure how fast I am going but I would say around 55-60 tops. I had a overdrive in the jeep and with it on the highway the rpms were around 21-2300 if i remember correctly running with traffic. Your auto will not require as low (numaricly higher) of a gear as the converter will compensate for a slightly higher gear, where as a clutch would slip alittle, but I would still go with at least 4.56. probably a poor explanation for a gear change but heres one way to look at it, If you have a 30" and a 35" tire and put them side by side and roll them one revolution you will see that the 35" tire traveled a further distance than the 30" due to its larger diameter. This takes the engine out of its normal operating rpm band, (lowers it) so a gear change will put the engine back in its normal power band. Its sorta like driving a stick shift car in one gear too high at all times vs driving it in one gear lower all the time, it raised the powerband, (rpms)
carnuck
New member
If the power is good with that setup, then going to a 35" tire and not losing any power 3.73 should be the minimum change, but I'd go 4.10 (I have 33" tires with 3.73 and the power is just the same as if I had stock tires and 3.07 gears) 6 cyls are bottom end torque unless they've been modded or re-cammed.
The reason you need to re-gear is because it changes the leverage. Picture a seesaw. Small tires equal a short plank on the other side of the fulcrum. Big tires equal a longer plank on the other side and a bigger guy. When the other side gets too heavy/long then you can;t push on your end anymore and the same goes for your Jeep.
The reason you need to re-gear is because it changes the leverage. Picture a seesaw. Small tires equal a short plank on the other side of the fulcrum. Big tires equal a longer plank on the other side and a bigger guy. When the other side gets too heavy/long then you can;t push on your end anymore and the same goes for your Jeep.
