3.73 gears with a F134

I just bought another 1966 CJ-6 with a V-6 in it ,and it looks like it has the 3.73 gears. My question is, can I run this gear ratio in my 1965 CJ-5 with the F134 motor that had the 5.38 gears? It is being used as a daily driver so would this help with the problem of only going 45 max speed . More on the CJ-6 later after I get pics to post because WE all like pics

 

The F-134 came from the factory with 3.73 if you wanted it. Or, at least the parts manual called out the 3.73 gearset for use with the f-134. Most all DJ jeeps had the 3.73 ratio in them.

I don't think it would be a problem. In fact, if you could get lucky and put a t-98 in it, your first gear ratio would be lower than a t-90 with 5.38 gears.

It would about like using a 5.38 geared jeep with an overdrive.

 

AFAIK the F134 CJs only came with the 4.27 and 5.38 gears. I expect that you won't gain a lot of top speed with the 3.73s, because you will run out of power. What kind of top speed do you need? An overdrive will make an effective 4.04 final ratio - you're going to have to hit the gearing sweet spot to get that added 10-15 mph ...

Two factors needed to go faster - revs and power. Let's consider 3 cases, where you increase your top speed to 1) 55, 2) 60 and 3) 65.

Here's what gearing you need to go each speed at the same engine speed that you have at 45 mph now:

1) 5.38 * 45/55 = 4.40 final gear ratio
2) 5.38 * 45/60 = 4.04 final gear ratio (same as Warn overdrive)
3) 5.38 * 45/65 = 3.72 final gear ratio (axle swap)

So, if your top speed is 45 now (rev limited) and you want your new top speed to be 65 (again rev limited), you need a 3.72 final ratio.

You'll also need more power. Since wind resistance goes like the square of velocity,

1) ((55/45)^2 - 1) * 100% = 49% increase in power
2) ((60/45)^2 - 1) * 100% = 77% increase in power
3) ((65/45)^2 - 1) * 100% = 109% increase in power

Wind resistance is a considerable barrier - you can see why there's so much mpg change between 55 and 65 mph.

Are you driving at less than half-throttle at 45? If so, you may have enough power to go 65 at the same engine speed. You will need twice as much throttle, and you'll only be going 44% faster.

I think you'll be dissappointed by 3.73 gears with an F134.

Hope this is clear (and helpful) ...

 

Even on a good day my :hurrican: doesnt like rolling the 33x13.5's with the 5:38's. I would be worried about going under the 4.27 set.

 

Let me disagree....maybe. I drive my Willys truck regularly with the O/D in it. It has 4.88s in it and with the O/D, it is roughly 3.75s. It has the 6 Cyl, in it that is supposedly rated at 105 hp. it does fine. Not very peppy, but it does alright. Tires are 29" tall. the 4 cyl has less power, but the jeep weighs 15 to 1800 lbs less than my 1 ton truck. my question is this; Does your jeep max out at 45 because of lack of power, or do the revs bother you? lack of power = don't even think about gears. Fear of revs = maybe think about gearing, or an O/D.

On the other hand, if you already have the other rear end, swap out the 5.38 one in your DD with the 3.73 one and have at it. what have you got to lose besides time, and all of our speculation put together won't be worth what you will realize when you try it out for the first time yourself. Just don't try to put it in 4WD and try that out too. Won't work with different ratios.

 

105 hp seems low for the Super Hurricane. I thought the F134 put out that much.

 

I stand corrected. Again.

 

I have a f134 with 4.27 and t90. Idle gives around 15 mph or so.
I believe the 4.27 f134 combo used the T90-C. The c denoted a lower first gear ratio. Giving a reasonable first gear a bit of a step to second a somthing aproaching blazing speed (remember designed in the 30's and built in the 40's thru the 70's) As "Cutie Pie" gets sorted out I am gradually increasing speed. The 4 speed t98 would be the best accomodation that and reducing the Delta R (wind resistance) by driving with the windshield tied down to the hood.

 

You thinking the T-98 would split the ratios better?

 

Doesn't a T98 have roughly the same spread in 2nd through 4th gears as a T90 does in 1st through 3rd? I can't see using the granny gear in a T98 much on the street.

 

I am running about half throttle so the revs are keeping me from going faster .I am turning about 3500 rpm;s and showing about 47 mph. I also have LT 235/75/15 tires on it so I thought maybe going to a 31 " tire would maybe help but didnt want to put them on and try the 3.73 gears until I got some feed back . My consences would be to find a set of 4.27 gears to gain my added speed to about 60 . The only reason I was going to do this is that I have a complete jeep with donor rear ends.

 

Ya know Jason if you were to put OIL in the tranny it'd run faster. R)

 

Mine had the F head and the 4.27s witha T90 a.

 

I have personally hit 110 kph with the f-head, t-90, and 5.38 gears -
running 36 x 12.50 r15 tsl's aired up to 40 psi.............
this was paced by my brothers chevy...... and my foot on the rad cap!!!

 

how fast is that in english?

 

repost

 

110 klicks is ca 66 mph. 3314 PRM per my calcs.

Downhill? Drafting?