I appreciate the offer to calculate my rpm, I actually found a good resource for that. Do u know what a good rpm for highway running would be? Or what it would b stock for like 55 mph. None of my wranglers had stock tires so I have no point of refrence.
Check this website out. Put your info in and it will calculate you MPH and RPM for gears and you can compare different sets of gears. That way you can decide your highway speed and make sure that you are in the 4.0's power range.
I can't tell you what stock RPMs are, but for me personally anything just above 2000 RPM to 3000 RPM I would consider acceptable. I prefer somewhere between 2500 and 3000 though. At 75-80 I'm right around 3k RPM. 65 is usually usually the fastest I go though and sits right at about 2800 RPM.
CU Boulder Aerospace Engineering
Basing on 30" tires with my stock 3.55 gears, at 65 I would be at 2584 in fourth and 2042 in fifth. With 35's and 5.13 gears, 65 would be 3201 in fourth and 2529 in fifth. It wld be like losing overdrive. I guess the gears are a tad to low. I then ran it with 3.73 gears as that's the more common ratio, and that's 2715 and 2145 which is about 300 to 400 difference across the range. What do you guys think?
You don't really want to base things on stock. Stock isn't exactly the greatest gearing, also increase in tire size is not linear in terms of momentum and un sprung weight. You need the higher RPMs and 2500 would be perfect cruising RPM IMO.
Contrary to popular belief these engines are not afraid of RPM, 3k RPM is nothing. Factory gears high for efficiency. You putting big tires on, throw efficiency out the window. The difference in MPG between 2500 RPM and 2000 RPM is virtually zero. In fact it could even be worse if your lugging the engine.
CU Boulder Aerospace Engineering
I just wonder still if 2500+ rpm is to high in fifth on the highway for the engine.
That is not even close to being too high, I consider those RPMs just right for tires that big. You're not driving a car with tiny tires where the engine doesn't have to work much. My previous TJ with 35" tires and 4.88 gearing/5-speed transmission was pretty close to 2700 at 70 mph and it couldn't have been better. Don't worry about the rpms, they won't be the same as a car & the engine is loafing at that kind of an RPM. Heck after I converted that TJ from its OE 5-speed to a 3-speed automatic, it was right at 3000 rpms at 70 mph. I didn't like the 3000 rpms so I slowed down a little for most road trips but many times I would stay at those rpms if I was a little behind schedule.
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Thanks guys the reason I was asking was I am looking at a set of axles, tj rubicon 44 with cromoly shafts and some other stuff and a tj 44 with an arb locker and someother goodies both with 5.13 gears for 3500 plus a bit for shipping. Kinda outta my budget, but.if.I just got a. 8.8 from ecgs had my 30 geared and both with lockrites is be pretty close to that anyway. Seemed like a good deal but didn't want to mess up w to low of gears. And I know.it will be kickass on the rocks, I just can't wait to get back out on the trails again.