OE Stock Tire Size for '82 J10 -or- Accurate Speedos

[The PIG Smith]

I've searched the forum and there seems to be a lot of discussion about larger tires and what size tire will fit on a stock wheel,
but I did not come up with what the size tire that Jeep actually installed on an 1982 J10.
I do not have my J10 here to see what the door sticker reads.

I did go out and look at my 1986 Grand Wagoneer and the sticker reads 7.00R15.
Because my J10 and Waggie both 15" wheels and have 3:31 gears, I will assume that they both have the same tire size.
Okay, that sounds lame, but its the best thing I could come with.

My goal is the find the closest correct tire size for my J10 so the stock speedometer will produce the most accurate reading.
I know, I know...stock Jeep speedos are not accurate to begin with.
Without a detailed discussion, I have a real world need to have the most accurate stock speedometer possible.
I cannot afford to be caught speeding again.
Aftermarket speedometers are not in the budget and I would still have the issue of calibration.
I will need tires soon, so this where I am concentrating for now.

Questions:

1. What is the OE Stock Tire Size that Jeep installed on a 1982 J10?
This J10 has an 8' bed is powered by a 360 V8, 727TF, NP219 and rolls on a Dana44 front axle and AMC23 rear axle, both with 3:31 gears.

2. Is 7.00R15 a correct size for an '82 J10?

3. Is there a link to a web site or download to a Tire Calculator that will calculate old school tire sizes to modern Metric tire sizes?
Yes, I have bookedmarked many tire calcs, like: http://www.miata.net/garage/tirecalc.html
But none of these style calcs work with old school sizes.

4. What is the modern Metric size for a 7.00R15 tire.

5. What modern 16" tire would good size that would produce the most accurate speedometer reading?

[AlsChopShop]

why not just change the speedo drive gear of the transmission? they are cheap and very easy to replace.

Al

[The PIG Smith]

why not just change the speedo drive gear of the transmission? they are cheap and very easy to replace.

Al

Excellent point
I agree that swapping out the speedo gear on the end of the speedo cable is easy.

But, because the J10 is not currently at my house, I dunno what size tire it's rolling on now.
I know they are a metric size, 225? 235? I do not remember.
I do know, those tires needs to be replaced soon.
So, instead of guessing what speedo gear will work with what tire size, I am focusing on buying new tires of the correct size that will produce the results I need, that is the most accurate speedo reading possible from an OE gauge.

[Kali]

Stock tire size in 82 was an H78-15. Ive seen a number of conversion charts into today's tire size and its usually either 225/75/R15 or 235/75/R15. Im not sure which one is the exact match, but either way they are relatively close in size anyway.

Now if you had a trim package you could have had 10-15 Tracker ATs which I've read are essentially 31x10.50x15s.

AFAIK gearing did not have an impact on what tires you got. You either had stock gearing of 2.73 or the optional towing gearing of 3.31s.

As far as the speedo being accurate when your Jeep left the assembly line is anyone's guess. I wonder if AMC was making sure they had the proper gear in every t-case for each gear and tire combo leaving the plant.

[The PIG Smith]

Stock tire size in 82 was an H78-15. Ive seen a number of conversion charts into today's tire size and its usually either 225/75/R15 or 235/75/R15. Im not sure which one is the exact match, but either way they are relatively close in size anyway.

Now if you had a trim package you could have had 10-15 Tracker ATs which I've read are essentially 31x10.50x15s.

AFAIK gearing did not have an impact on what tires you got. You either had stock gearing of 2.73 or the optional towing gearing of 3.31s.

As far as the speedo being accurate when your Jeep left the assembly line is anyone's guess. I wonder if AMC was making sure they had the proper gear in every t-case for each gear and tire combo leaving the plant.

Thanx!
I did a Google search of "H78-15" and "Conversion" and found this trailer tire conversion chart:

http://www.greenball.com/tires/biastrailer.htm

According to this chart, the H78-15 is the same as a 225/75/15, just like you said.
Once again, I would have no reason to believe that auto/trucks tires sizes are any different than trailer tire sizes.
I do know trailer tires have different tread pattens and are not recommended for auto/truck use.

J10 Trim Packages...Hmmmmm...that I do not know about.
My J10 seemed to me to be loaded with lots of options.
It had bucket seats, A/C, Cruise, Tilt Wheel, Sport Steering Wheel, brushguard, towing package, to name a few.
It does not have the Laredo badge on the bed like I have seen on others.
But it does not have any badging in place, as the PO was attempting to do amateur body work, removed them and sprayed primer on the body.
So, I dunno if I have a package or not that would allow me to run with 10-15 Tracker ATs (31x10.50x15)
I do not have the Chevy Rally style or Steel Chrome Wagon wheels.
I have 15" Waggie style Turbine wheels, which I know are not stock.

When I am able, I will try to get some info of the door sticker to comfirm if I have H78-15 or 10-15

Now, I can take this size, 225/75/15 and run over to http://www.miata.net/garage/tirecalc.html
I now know that I could use 225/70/16 size tire with .04% difference in the size of the 15" tire.

[Don S]

..
The PIG Smith;

Here is a nice tire size link... I use it myself...
http://www.discounttire.com/dtcs/infoTireMath.dos tire CALCULATOR

http://www.4lo.com/calc/gearmetric.htmtire CALCULATOR #2



Have a good one while you can still laugh about it.. Don S..

[tgreese]

You could do this in reverse ... will a GPS tell you how fast you are moving? If not, find a measured mile or use the markers on the highway to travel some distance and record the time it takes at a fixed speed on your speedometer. Then, your actual speed in mph is (distance in miles) * 3600 / (time in seconds).

The speed you measure is linearly related to the circumference of your tires. Your speedometer will report too low a speed if the tire is bigger around (and taller) than the original - assuming the speedometer was accurate to begin with. The circumference of a tire is proportional to its height. So, if your speedometer says 45 mph, and you measure that you are actually going 50 mph, you need to reduce your tire diameter by (1 - 45/50) * 100% = 10% to have an accurate speedometer. Let's say you have 31" tires now, then your ideal height tire is 31 * .90 = 28" diameter.

[The PIG Smith]

..
The PIG Smith;

Here is a nice tire size link... I use it myself...
http://www.discounttire.com/dtcs/infoTireMath.dos tire CALCULATOR

http://www.4lo.com/calc/gearmetric.htmtire CALCULATOR #2



Have a good one while you can still laugh about it.. Don S..

Don,

Those are some nice calcs that I will add to my Tire Bookmark folder.
I have a bunch!

Sadly, the calc in the links you shared are for metric sized tires
Taking a metric sized tire data and getting real world size info.
There are many, many of those type of calcs.

I was looking for a calc that used old school tire sizes to see what the metric equivalent.

[The PIG Smith]

You could do this in reverse ... will a GPS tell you how fast you are moving? If not, find a measured mile or use the markers on the highway to travel some distance and record the time it takes at a fixed speed on your speedometer. Then, your actual speed in mph is (distance in miles) * 3600 / (time in seconds).

The speed you measure is linearly related to the circumference of your tires. Your speedometer will report too low a speed if the tire is bigger around (and taller) than the original - assuming the speedometer was accurate to begin with. The circumference of a tire is proportional to its height. So, if your speedometer says 45 mph, and you measure that you are actually going 50 mph, you need to reduce your tire diameter by (1 - 45/50) * 100% = 10% to have an accurate speedometer. Let's say you have 31" tires now, then your ideal height tire is 31 * .90 = 28" diameter.

OH YESS!
I have a GPS receiver for my laptop and I use M$ Streets and Trips.
I could calibrate using that.

[Lindel]

If the speedo moves, then it's "somewhat" accurate. A J-10 should have 31's or the equivalent for stock.

Your GPS is probably far more accurate than the speedo would ever be.

[Dirtball]

I was going to suggest using a GPS too, but you all beat me to it. My GPS goes with me everytime I take the Chero out. Its speedo reads slow, so the GPS let's me know just how slow. My 91 Honda reads faster than I'm going. Now I know, other than just people driving too fast, why people were passing me on the interstate so much.

[Woodweavil]

Mine is running 235s and pretty close to accurate

[WHSII]

1980 TSM tire sizes, yes I know you will not look at this because there was a brow...




1984 to 87. They appear to be similar.

The LT 10-15 is very similar to the 31 by 10.5 , height is within 1/8" or so depending on manufacturer. It seems this was the largest tire that AMC put on our rigs in this time frame.

Hope this helps!