F-head street gearing

For the guys who drive their f-heads on the streets what gears do you run???????????????? right now in my 64 cj-5 it has 5.38s no overdrive and my grandpa claims he had it up to 60-80 mph when it was new and i just dont see this hapening unless it was downhill.

 

45 to 50 comfortably in most. 60 btdt but it's not gonna last long if done very often. But 80, probably if it was pushed out the back of an airplane but only once.

 

Mine will do 80 no problem!!! .... when its on the trailer riding behind my powerstroke diesel!

 

60, maybe. The problem is wind resistance and power:weight ratio. Most don't realize that wind resistance increases like the square of velocity, so 60mph has 4x the wind resistance of 30mph, and so forth.

If you look at the horsepower and torque curves for the F4 http://www.film.queensu.ca/CJ3B/Tech/EngineDyno.html you'll see that the toreque curve starts to drop off above 2200 RPM (HP ~ torque*RPM), so the HP flattens out. You need more HP to go faster, so you can see how these factors conspire to make the F4 poop out at much above 3000 RPM (about 60 mph). The HP is increasing sub-linearly, while the wind resistance is increasing quadratically. It's a substantial barrier.

80 mph is more than 5000 RPM - no way.

 

55 mph no problem. 4.27 gears with 31" mud terrain tires. It will go 60 but it likes 50 - 55 much better.

 

yeah we didnt really beleive it would do 80 especially with the 5.38s. but just wondering what you guys were using what you were getting with speed.

 

Mine with 4.27s would push 60 unhappily. With the OD I did get Lil' Blue up to 80 going downhill once.
Once was enough.

 

Sunday I was trolling along at 35 and thinking that at 50 it is a handful in mine since it wanders so much.

 

I run the 3B at 50 tops most of the time. 31's, 5.38's, no O/D

 

I have 4 point-something gears in mine, and 30" tires as I recall. 50-55 without the overdrive, max, and that's really pushing it. 60-65 with od, but only on level ground and not too much headwind. It would probably go faster with the od but I like to stay at or below 3000 rpm when on the freeway.

 

In 1976 I had my Jeep Chug clocked at 90 mph 134L. I know no one believes it but I did it for about a mile and a half it took about 3 miles to get up to the speed.

With no OD 55 mph is 3314 RPM w/30" tall tires 5.38

With No OD 65 mph is 3916 RPM w?30" tall tires 5.38 -

Max HP accours at 4800 rpm

With OD the RPM drops to 2485 and 2937

 

I run my about 35 on the back roads and Old School is very happy and thanks me by slowly drinking gas. When I drive to work I have to drive 301 for about 4 miles , I run 45 on the speedo but Im catching up with traffic some days so I guess Im running close to 55 with the 33s. Fast as I have gottin it was 55 on the speedo and that was down a long grade and she was howling.