Jason Moore

I purchased the 5.13 gears. I have not installed them yet. I ran a couple long highway drives with OD off as jpop told me that would simulate 5.38 gears. My mpg actually got better with OD off, even running the higher rpms. I was typically between 2500 and 2700 driving 68 mph.

Just so you know
2010 jku auto
2. 5 inch lift
Superchips Vivid linq
Actually measured tires 34.7 inch

Wgrin

1st gear 3K rpm at 9mph
2nd gear 2800 at 18 mph
up to 6th gear 70 mph right around 3k rpm
I personally would have been happier with 70mph and around 27-2800

nthinuf

3k @ 70mph? From referencing the charts, that would put your 35's down around 32" actual height? Sounds really short for those tires, so have you 'verified' your speedo calibration?

JPop

Yep, that sounds like your tires are running at about 33" tall in actual rolling diameter.

My 35s measure out at 34.25" and I'm running about 200rpms less with 4.88s. I'd like to be spinning a little less at cruise speeds, but I'm closer to what I would consider to be ideal with the 4.88s than I would be with 4.56s. I'm fine with it, and not much additional torque to be had as you approach 3k. An inch more tire height and I would be nearly perfect for my on road driving.

RhuntMT

Auto or Man trans? And how is off road performance?

DadsCJ

Here is a bad picture of my dash at 75ish.



By the time I took it I was headed up a slight incline and had a headwind.

Normally the ECO light is "ON" at this speed and the RPM is 2600 at 75 solid.

GunDude

So let me throw this out for thought. Do you think the mfg. of tires make the measurements when the tire is not static, say at 55 mph. So, that a 35" measures 34.5" sitting still but rolling down the road, the centrifical force expands the tire to 35". So say thin walled 35" light BFG KM2, that measure 33.5 increase to 35" when rolling down the road.

So, do you set your Procal, etc. for the stop dia. or rolling dia. if this does happen? Also, might say the same for which gears you choose.

JPop

It appears that the manufacturers often come up with their specifications in an alternative universe. They are all over the board, although a lot of it has to do with tire construction. As mentioned here and elsewhere, BFG is one of the worst offenders for running short.

There isn't a measurable difference for centrifugal force applied to radial tires at normal cruise speeds. Even with bias ply tires you need to be running at drag strip or speedway velocity for the expansion to be noteworthy enough to add into calculations.

Sahara Lee

That's why I do NOT use tire diameter when calculating for speedo and regear. Tire revolutions per mile (trpm) as listed by the tire manufacturer are the ultimate in accuracy. As stated on the TireRack website:

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"Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire’s published overall diameter (which would only reflect the tire’s unloaded radius), calculating the tire’s absolute rolling circumference isn’t possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate."
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So, as you can see, revolutions per mile (trpm) on a given vehicle at highway speed is directly proportional to engine Revolutions Per Minute (ERPM). That's why my engine RPM calculations are so simple but extremely accurate:

(new trpm / old trpm) x (new gear ratio / old gear ratio) x old ERPM = new ERPM for the same speed

This equation works great when you already know your ERPM at a given speed.

Cheers!

JPop

Tire Revolution per mile isn't accurate either. It can be measured or calculated by the manufacturers, then of course you don't have all the various wheel widths, inflation rates or vehicle weights.

Total crapshoot, and it's best to seek out tire measurements from someone that's running the same tire on similarly configured vehicle.