Gearing/wheel/speedo questions

Does anyone know what the stock tire size on 1975 CJ-5s were?

Does anyone know the stock gear ratio?

Does anyone have a source for speedometer gears for these and, even better, a reference chart (likethe ones for YJs and TJs) that shows what gear you need for a given gear/wheel size combination?

I'll have my CJ on the road soon, I hope, maybe next week, and I need to fuigure out what to do as far as fixing the speedometer for the 35s, and then, soon, to decide what gear set to choose and the correct speedo gear for that setup.

 

My 74 brochure list F78-15 but my76 brochure said H78-15. I can remember seeing G78-15 on some. So thats 26"-28" all Goodyear Polyglas Surburbanite XG's. hth

 

What I remember is H78-15 was the optional size. There were plain street tires standard that may have been the F78-15s. Almost everyone got the optional tires - considered good tires in their time.

Final drive ratios were 3.73:1 standard, 4.27:1 optional.

Maybe the factory parts manual...

 

Thanks, fellas!

 

quadratec has a speedo chart, but its listed as for 87's and up. Don't know if this would track for yours...
35" tires and 4.56/35 teeth; 4.10/33 teeth; 3.73/31 teeth; 3.55/30 teeth; 3.07/26 teeth.

 

Is the F78-15 a 205/75/15 or am I thinking of something else?

 

Fyi

 

What do the rest of you do to correct your speedos?

 

There's a Buick site that gives the F,G,H,L78 equivalents, but I can't find it now. I posted it before; can't find that either! IIRC H78-15 was P235/70R15, so 205 or 215 is close for F78-15.

 

Look-up table is the easiest.

 

You, sir, are a genius. I don't know why I didn't just look it up. When you said there was a ste, I just dropped H78-15 in Yahoo's search blank and found a site in the list. Here you go:

http://www.geocities.com/MotorCity/2398/tires/tirediam.htm

 

but f the gears for the Dana 20 are made out of unobtainium, how do you correct it, regardless of what the table says?

 

a.

 

Well, all you need is one point. If you have a GPS, or a measured mile and a stopwatch, you can find your actual speed at one reading of your speedomenter, say 30mph. Say it's really 36.5 mph, and you're actually going faster than the speedometer says - that's the usual case with tall tires. Then, your conversion constant is 36.5/30 = 1.217. Make a table of values in 5 mph steps (a look-up table), multiplying each value by the conversion factor. Thus, say you read 45 mph on the speedometer, then look at your table and that's 54.75 mph. This isn't as goofy as it sounds - after a while, you'll remember what actual speeds the indicated speeds correspond to, and you won't need the table.

 

Will the speedo gear from a Bronco Dana 20 work? I know there are differences int he linkages, but, other than that, a Dana 20 is a Dana 20, right?

And, if so, I have options for 16, 17, 18, 19, 20, and 21 teeth. Is there a way for me to calculate what gear I need or should I just buy one of each, test them all out with my GPS and pick the one that is closest?

 

Second vote for the GPS method. I make a table of actual to indicated speeds and keep it handy. It doesn't take long to remember the table, and if it's handy, you can refer to it quickly if you haven't driven the vehicle in a while. I've got three different vehicles I do this with.

 

My speedo cable is a part-time thing, and most of the time, it does not work

I made a chart in Microsoft Excel with all the gearing of my tranny, t-case and axles. Then I enter the tire size and the rpm of the engine.

For my drivetrain (stock 1977), I know that when I go 2500 rpm in third high, it is about 60 mph.

Do you want the chart ??? Send me a PM, and I'll send it to you. I have an aftermarket tachymeter to give me the RPM


Bob in Sherbrooke

 

Actually, you have emailed it to me and I have it on this computer... i'm just trying to get the shiny new instrument cluster to be worth something since i paid 118 for it. Plus, I wouldn't mind glancing at the gauge and being close