I picked up a one-year-old Jeep Rubicon.

The OEM tires are about to wear out after only 19K miles and with gas getting so high priced, I was trying to figure out what the optimum size replacements would be.

BUT I must be doing the math wrong... can anyone help me with this? I would like to see 2000 rpm on the Tach, when I'm doing 65 MPH on the highway in top gear.

So here is the math:

65 mi. per Hour X 5280 Ft per mi = 343,200 ft per hour.
divide that by 60 min per hour = 5720 ft per min.

Now if the engine is turning 2000 rev's per min. and the trany is in high gear ( 1:1 ratio) then only the differential are giving me any reduction in the gearing to the wheels, right?

I have 4.11 diff.'s thus 4.11 rev's at the engine to one turn of the wheels, hence 2000 rpm = 486.618 turns of the wheel per min.
(2000 / 4.11 = 486.618)

Now for the geometery. If the wheel has a diameter of 33"...
then you multiply 33 by pie (3.14) to find the distance you'd travel for each turn of the wheel... but that is in inches, so divide the result by 12 to convert to feet.

Only I want to go the other way... I need a wheel that will move me 5720 feet after 486.618 turns.

5720 / 486.618 = 11.75 feet per turn.

11.75 x 12 = 141 inches per turn... which means

141 / 3.14 = 44.9 in diameter wheel.

If I've done that right... this means I'd have to put on wheels that measure almost 45 inches high to be able to tach 2000 rpm at 65 mph.

That can't be right.... Can it?

( it would give me some great ground clearence tho... Hmmm)

 

that dont sound right but i am not the best in math

if its right you will need a big lift kit lol

 

You got the math part down, just a couple things missed that I can spot. First, the high gear is not 1:1 for the manual or auto... I want to say its 1:.87 for the manual and 1:.64 for the auto but could be wrong. This would bring down the size to 39 inches for the manual which sounds about right, in theory. However, the engines "sweet spot" is at 65 is certainly not that low because of the Jeeps horrible drag and the power spin those huge tires.

Hope this helps a little!

P.S. I have a manual with 35's and 4.11's and am aroun 2300 at 65 i think

 

I didn't check your math, because that hurts my brain at this hour.

I did notice as I was glancing across your post that you were using a 1:1 ratio for top gear. I think 6th gear is like .84 and OD on the automatic is like .67 or something. Somebody will correct me if I'm off a little.


Punching the info in a gear ratio calculator told me this:
37's=2033 rpms@65
36's=2089
35's=2149
34's=2212
33's=2279


This info is for the 6 speed. RPM's may be a little lower for the auto.



BTW, getting taller tires won't significantly change how long they last. Even if they do last a little longer, that will be offset by the higher purchase price and maybe even more so by the worse fuel economy.


That said, get the biggest tires you need/want/can afford/can fit. Just don't justify it by saying they'll last longer, unless you're trying to convince your wife!!!



Edit:
Tyler posted above while I was checking the calculator, so some of my post may sound redundant.

 

I also do not feel like tackling the math, but I do have some things to consider.

A) Bigger tires = worse mileage in 99% of situations.
B) You will not necessarily get better mileage just because of an rpm drop when doing 65. It has more to do with wind resistance and the torque curve in relation to the fuel needed.
C) SLOW DOWN! The easier you are on the right pedal, the more gas you save. Cruising speed, accelerating, all of it.

 

This may sound like a silly question:

(But I did some web-research) and still not sure..

What exactly do we mean by 33's or 35's?

Is that a direct measure of the tires diameter?

(Already looked up the 235/65 R15 jazz & got that down, but no one bothered to explain the other form... <grin>

 

I have 245 75 16s on mine which are like 30 inches tall and mine tachs like 1700 at 65 MPH.

 

WOW this thread is WAY to smart for me GOOD LUCK guys........

 

I am getting 37s installed on my manual rubi as we speak so I guess I will be looking at 2033rpms @65 according to the post above. When I get it back and drive it a bit I'll let you know how the MPG are affected. Now it averages 16.7mpg. I am not getting new gears yet.

 

to get to the 33's you would divide the circumference of the tire by pi.

therefore the tire would be 103.62 inch if you rolled it across the ground and measured that distance. which divided by pi (3.14) would be 33" diameter.

Hope that helped.. otherwise the best tire size is 37" on a 2 dr and 40 on a 4dr... hey you only live once... maybe twice...lol