Stock Tire Size 73 Cj5

Hi, I have a 1973 jeep cj5 with 31in tires. I am having problems with my speedometer and am looking at getting it repaired at united speedometers. I'm wondering if I need to purchase a ratio adapter from them because of my larger tires. Most of these jeep's I've seen don't have the larger tires not the goodyear suburbanites. Any ideas if the 31's came on the jeep from the factory?

 

Nope, no 31's in 1973 CJs. Largest factory tire was a H78-15 Goodyear Suburbanite.

 

Thanks, so it sounds like I'd need to get a ratio adapter to make my speedometer accurate. Any ideas about how much the 31's would be throwing it off?

 

Whatever the ratio of the diameters is. Taller means that you are going faster and further than indicated.

 

The original suburbanites were about 27.5" in true diameter. You'll need to search (possibly the mfgr's website, or an online tire outlet) to find the true diameter of your present tires. Then you can enter the data into an online calculator to determine speedometer error.

Most supposed 31 inch tires are closer to 30", especially if they are radials. When I replaced the originals on my jeep with 15-7.00 bias tires, the true diameter was 30.1" for the new tires. The change made my speedo read about 5-6 mph slow at 60. In other words, about 10%.

 

You don't need a special calculator. If the current tire is 31" diam and the Suburbanite is 27.5", 31/27.5 = 1.127 ... Then if you have an indicated 55 mph, 55 * 1.127 = 62 mph.

I've bookmarked this page http://www.geocities.com/MotorCity/2398/tires/tirediam.htm which says that a H78-15 is 28.34" in diam. I expect Dave knows better, or has actually measured one of these tires, but that's what the page says.

I'd suggest you measure the circumference of one of your tires with a tape and divide by 3.14 to get an accurate measure of the height.

 

Yep Tim, I did actually measure the size of the old tire with some country boy calipers I made from wood. Obviously, the tire was really old... I guess the point I wanted to make here was that the same "size" tire can actually be quite different in true measurement. Depends on tire type (AT, MT, etc) inflation pressure, and mfgr, to name a few variables. Just one more example of how much variation can be found in a "standard".

As usual for me, I probably went overboard due to my Germanic genes. In the end, a ballpark correction should be close enough. The reality is that a 3 mph difference in speedo reading is almost impossible to detect with the bouncing jeep, having to look away from the road down to the dash to see the speedo, and the necessity to do it quickly in order to keep from driving off the road. R)

I remember when I was an apprentice carpenter years ago, and the crusty old journeyman who was teaching me framing used to yell at me "you're not building a piano, here!". I've had to apply that advice many times over the years...

 

I had United Speedometer make me up a ratio adapter. They had me drive 10 miles worth of mileage markers and had me send them the odometer reading. They made me an adpater based on that info and now my speedometer reads right on.

http://www.speedometershop.com/

 

Just curious, how much did that ratio adapter run you?

 

Yeah, I'd be curious to know too.

 

Yeah - me three!

 

Just as a further point, I know guys with very agressive rigs that have 36" tires that are the same size as other guys with 35" tires. Can't remember the manufacturers, but the 36 from one was in fact a 35" tire.

So check `em yourself, best way to know for sure.

 

Get on the freeway and drive next to a friend who has stock tires. Ask (yell) how fast he's going, and compare your speedo...R)

.......:?what??? worked for me......