Sorry if there is already a thread on this, I tried searching and wasn't able to find anything.
My wife and I each have Wrangler unlimiteds. She has a 2014 Sahara with 30K miles and I have a 2016 Sport with 2,500 miles. Both have the Pentastar, both have 3.73 gear ratios, both have the same exact lift kit, both have the same tire/rim setup, both have been programmed with the SuperChips flashcal.
I live in AZ where the highway speed limits are 75mph. Her Jeep cruises at 75mph at 2K rpm while mine cruises at 2,400 rpm. I know that isn't a big deal, it does cause my Jeep to get about 2mpg less as an average (which I don't care about...let's face it, it's a Jeep...going to get bad mpg).
I was just wondering if anybody has an explanation as to why there is a discrepancy. Is my Jeep just not broken in...is there a break in period? Again, it's not a big deal, just curious.
That's the first thing I thought too so I checked both the window sticker and the factory build sheet for the VIN and they both said 3.73 ratio. I work with the previous owner and she said she never switched out the gears.
Gearing and rpm is not affected by lift. MPG can be, but rpms at a given speed is pretty much fixed. However, if you don't have the speedometer adjusted correctly for your tire size you may think your rpms at a given speed are different.
The other option is either the tire size or the gearing are different. I assume both Jeeps have the same transmission. That would mean either the tires are a different size, or more likely her Jeep has 3.21 as Jedi9 suggested.
You can actually measure the gear ratio by counting how many times the drive shaft rotates for one revolution of the tires.
Maybe she's treating hers to premium fuel? Just Kidding but have to eliminate all possibilites.. Are both of them running the same "tune" with the programmer?
What tire size is each jeep running? Also which brand and model of tire? Do you have a ton of heavy stuff in your jeep?
Tire size can be a cause, different gear ratios, or improperly entering the tire size will throw off the speedo. Was the tire size measured under the weight of the vehicle the height entered? If you just put in what the tire says it will be wrong.
Did you verify both jeeps speedometer calibration against a GPS speed app on your cell phone? Do you have 2 Flashcals? Aren't they good for only one jeep?
if you have stock 225/75-16 on you sport and she has 255/70-18 on her sahara that would make quite a difference and of course with the small tires you will turn higher rpms.
They both have BFG ko2 315/70/17 with same wheels and everything. They were both programmed with their own SuperChips flashcal and I put in the actual measurement of the tires under the weight of the Jeep.
If they are essentially the same Jeep and you're sure that the tires and gearing are the same on both, but are seeing a 400 rpm difference at the same speed, then it seems to me that it has to be either a resistance thing or a maybe a calculation thing.
For resistance, I guess I would wonder first if there's a weight difference between the two, air dam missing on one, tire pressure difference between the two, stuff like that.
For an indication problem, I wonder if the speedo is off on one of them? 2000 rpm for 75 mph seems low to me. I wonder if maybe when she's indicating 75 she's actually doing more like 70mph?
Everybody loves a good mystery, so please keep us posted.
Assuming all else is equal (e.g. both are in top gear with the torque converter locked, in addition to tires, wheels, etc), the rpm difference you are seeing matches the difference between 3.73 and 3.21 gears. I'd expect your crusing RPM to be closer to 2300 than 2400, but thats tough to gauge on an analog dial.
I do have more stuff in my Jeep usually...but usually just one strap, a couple of shackles, simple tools, stuff like that for the day to day because it is my daily driver. I do obviously carry more than that when I'm off road.
She does have an aftermarket front bumper while I have factory. Maybe that's it. It is a little more "aerodynamic", but I would have thought the extra weight of hers would compensate for the less wind resistance. I don't know.
A buddy here at work thinks it's probably just that my Jeep hasn't broken in yet. Mine only has 2,300 miles on it while hers is close to 30K.
Aerodynamics, weight and "break in" don't matter, unless yours is such a brick, so heavy, or so gutless that it won't hold 75mph in 5th gear, and downshifts to 4th. Or, I suppose, the torque converter could unlock and stay so, but you'd see that on the transmission temp gauge in a hurry.
The ONLY things that matter to this discussion are:
-gearing
-speed
-RPM
Mods, weight, aerodynamics--none of that matters. The amount of throttle required to get to a certain RPM is affected by those things, but RPM vs speed is unaffected by those things.
First I think you should verify your speed readings with as GPS (Waze app is easy). After that, if the speed readings are accurate, I would jack up the rear end and do the old "count tire revolutions and count drivershaft revolutions" drill. That will tell you the actual gear ratio installed.
My guess is that your wife's jeep mistakenly got 3.21s. But that's just a guess at this point.
/\ This. If you do the math backwards and solve for tire size with known speed (75mph), gears (3.73) and rpm (1900), she'd need to be running 40's. That tells us that one of those "known" values, is not, in fact, known.
As was said before;
If the tires are the same diameter, the gears are the same gears, and the rpms are the same, then the only other factor is the torque converter. If one of your jeeps has a slipping torque converter, then the rpms would be higher on that one at the same speed.
I agree that probably your gearing is different.
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