235/75R15 tire is 28.9inches

new tire / old tire * gear ratio = new gear ratio
(34.8(typ 35) / 28.9) * 3.07 = 3.7

for reference, same calc with 32.8 inch new tire(typ dia of 33 tires) comes out to a new ratio of 3.48, so 3.73 would be over gearing a little for 33s

I'm assuming 27" stock tire diameter. I'm not using any formula, I am just using ratio's.

27/3.07 = 33/X solve it and you get 3.752222......

If you assume 28's

28/3.07 = 33/x you get about 3.62

This also works for tire size and gear ratio.

28/3.07 = X/3.73 x = 34.01ö 27Æs stock would be 32.8ö

I'm sure that's what rock jock is doing.

Sorry BJ, I just read your post to fast... and wrong. I though you meant bigger tires are harder to turn from size and weight difference not load and RPM differences....

The whole post is regarding the latter.

Carburetors loved going lean during the initial transition to the power system, then richened out once they were fully on it (hence the worse gas mileage). EFI is a lot more linear (not relying on a mechanical system for AFR) so that the transition stays fairly close to center adding a bit of gas mileage.

Ok Sunder you say: Carburetors loved going lean during the initial transition to the power system, then richened out once they were fully on it (hence the worse gas mileage). EFI is a lot more linear (not relying on a mechanical system for AFR) so that the transition stays fairly close to center adding a bit of gas mileage.

I have no quarrel with the fact that modern fuelies are much more sophisticated in what they do. HOWEVER the fact remains that adding a load, such as an incline or larger tires, will decrease the timing and increase the fuel to air ratio. Or am I incorrect? I cannot see how even a fuelie can maintain timing and mixture when adding a load.. For sure it would clatter and the EAR would retard the timing at that point.. does it not richen the mixture too?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Ps.. Did you mean ADDING a bit of gas milage?

Sunder "I'm not using any formula, I am just using ratio's."

The formula he is using is the same you use to solve for x=gear ratio in your ratios. <img src="/forums/images/graemlins/lol.gif" alt="" />

We are using the same formula. If you plug in your numbers

New Tire / old Tire * gear ratio = new gear
33 / 27 * 3.07 = 3.752

The difference is I am saying the stock tire P235/75R15 has a diameter of 28.9 inches, and the typical diameter of a 33 tire is more like 32.8 inches, so:
32.8 / 28.9 * 3.07 = 3.48

We are using the exact same math, just starting with different numbers.
So for 35s:
34.8 / 28.9 * 3.07 = 3.7

This means the 3.73:1 gear ratio would get me back to stock with 35 inch tires. I have 33 inch tires so I am over gearing a little.

I know that at 69MPH (using GPS) I am currently at 2200 RPM,using similar math, with 4.11 I will be just over 2900 RPM and with 3.73 gears I wil be around 2600 RPM at 69MPH.

I usually figure from 27" as a stock tire. I think mine came with 205/75-15 or 27.1" I would have thought your 2001 came with either 215/75-15 (27.7") or 225/75-15 (28.3). I would still go with the 4.10 <img src="/forums/images/graemlins/notooth.gif" alt="" />

Well, if your tire numbers are right, go with 3.73's. My experiance with 31's and 3.73's would tell me to go 4.10's and 33's.....

A friend is running 34" Super Swampers with 3.55 and his stock speedo is right on. That means his RPM's are running like stock also. Right? (his stock gears were 2.73's). My CJ with 3.31's and 32's (31.8" nominal dia. new) turns 2200 @ 60 (GPS) in 4th and I like it. At 60 my speedo reads 5mph slow. I get good mileage in 5th @1850 cruising about 65. I got no problem downshifting when I need more power. I always figured if I went to 33's i'd like 4.1's but only with overdrive. <img src="/forums/images/graemlins/cheers.gif" alt="" />

Couple gear ratio charts and formulas and stuff:

http://www.4lo.com/calc/geartable.htm

http://www.raingler.com/jeepReference/speedoChart.html

http://www.4lo.com/calc/gearratio.htm

http://www.us43.com/tipspage.htm

http://www.pirate4x4.com/tech/bible/index.html

http://www.bgsoflex.com/auto.html

Well I decided on 4.10s. I just hope the D35 will give me a few years of service before I blow it up. I will have to start saving for a Ford8.8 now