Dirt Hammer

In this video I will show you how my 2015 Jeep Wrangler JKU 3.6L (non Rubicon) manual trans with 4.56 gears and 35x12.50 tires performs at 70mph. I can’t express how important it is to re-gear your drive line after going to oversized tires. You will regain lost performance and reduce stress on your drive train components. My Jeep is riding on 35x12.50 BFG MT and we all know tires don’t measure out to their advertised size. My speedo has been recalibrated based off the mounted/measured tire size (just shy of 34in) and confirmed with a gps. I selected my Yukon Gear Set based off the data found in these charts https://www.extremeterrain.com/regea...explained.html To my surprise my gas mileage went up 2.0 mpg and increased my Jeeps drivability and off-road fun.

resharp001

May I ask, had you calibrated your tire size prior to regear, or just at the time that you regeared and programmed the new gear ratio? I only ask cuz if you hadn't, then you're fuel economy was understated previously, which would account for a bit of your increased economy post-regear. (if you answered this already in the video I apologize as I have not watched it. I can't watch youtube here at work).

Dirt Hammer

I reprogrammed the tire size and new ratio like two days after the regear using the OBD JScan App and Bluetooth adapter. The regear was done is March of this year, 5 months later still getting 2mpg average over my old set up.

resharp001

What I'm saying is that the mileage on your old set up (larger than stock tires but not having programmed the computer for them) was understated, and therefore your fuel economy was understated. You traveled further on a tank of gas than the computer thought you did. Saying that a regear gained you 2mpg better fuel economy is not that cut and dry.

Jay2013jk

Very interesting and informative.. I do have a question or 2... what was the original gearing in the JKU and what MPG ? Did I miss this in the video? My reason, is 2013 6spd JKU with 3.73's on 33's now; and will be going up to 35's (Falken or Toyo) in a couple of months. Looking at a Gear ratio chart currently I pull 2120 rpm (tach is close) and will drop to 1998 rpm at 70mph .. In SE Pa. We are more around town and hwy 18.6 - 18.9 mpg) than Off roading; we feel we can live with this performance change. Thanks for the link to the gearing charts, not planning to re-gear at this time will give it a while operating

Dirt Hammer

I understand the question now and you are correct but fuel mileage is calculated by engine rpm and throttle position and mapping, so shouldn’t the old fuel mileage before the regear be correct?, I think but could be wrong. But yet I guess miles traveled is used as well, which would be off due to larger tires.

Dirt Hammer

stock gearing was 3.21 and reported 14.8/15 miles per gallon but speedo wasn’t set for proper size tires. Today’s mpg with new gears, calibrated speedo etc is max 17.2 my Jeep is heavy and getting heavier as I add parts and gear, we mostly ride hilly pavement backroads when not trail riding.

resharp001

I would bet some of the old timers could tell us exactly down to the detail how the actual mileage is calculated (rotation of hub via speed sensor or whatnot), but the bottom line to me is that the jeep is factoring in tire size to both calculate speed and mileage driven. With a larger tire than programmed in the computer, the jeep is traveling further than it thinks it has, and in that scenario the fuel economy is understated. It's really not a big deal either way. I'd really like is to see someone scientifically lay out the details how lower gearing and a higher-revving engine equates to better fuel efficiency, all other things being equal. It's not a challenge, I've just seen a few people claim that. I can't wrap my head around it, but admittedly, I'm not a very scientific person and it gives me tired-head to think about. I'm sure there is some break-even point where that is true to an extent, but I have a feeling that is more an anomaly in some small power band range. In the end, all that really matters is the smile on your face when you drive.

Jay2013jk

Thanks .. as resharp001 said .. It is about the Smile ! BTW.. I check out your videos too.

Sunnysideup

No science behind it.... but It's about keeping the engine and tranny from lugging, and keeping shift points closer to stock. I would venture to say those who have improved mileage only have an improvement over running large tires with the wrong gears... not improved over stock tires and gears? My Jeep had 3:73 gearing, going to 35's reduced gas mileage by 5 mpg. Re-gearing to 5:13 brought it back up three mpg... still lower than stock but still better than running large tires with the wrong gearset. I started at 18... dropped to 13...went back up to 16. In town at lower RPM I gained more as well. The right gears and - and on an automatic tranny...the right shift point- brought the fuel milage closer to stock. At least that was my experience... then I went to 37's... and dropped 1 more mpg... LOL!