Would changinf from 235 Stock to 31 Stock effect Speedo/Odo reading? - Ford Bronco Forum
 
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post #1 of 16 (permalink) Old 11-28-2010, 04:45 PM Thread Starter
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Would changinf from 235 Stock to 31 Stock effect Speedo/Odo reading?

OK, so I'm thinking way ahead about switching over to the 31x10.5x15 tire when my factory size 235's wear out. If I calculated right, my 235's are about 29-30" tires. If I switch to stock size 31" tires would I need to adjust the PSOM or would the 1-2 inch difference not be enough to throw the speedometer and odometer off?

I looked around online for a calculator for this and all I could really find were tire size calculators.

Thanks

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post #2 of 16 (permalink) Old 11-28-2010, 04:51 PM
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Yes you will need to program your psom. It is a relatively simple process and should only take a few minutes. It will probably take longer to do the math than to make the change. Look herehttp://www.fullsizebronco.com/forum/show...highlight=psom for a how to it is super easy.


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post #3 of 16 (permalink) Old 11-28-2010, 05:39 PM Thread Starter
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OK, thanks. I think about going up to the other stock size but have concerns about it since I've got 3.08 gears. I can't justify spending $4-500 on new rubber and then having to spend more to get slightly lower gears (3.55-3.73) to go with it. I wonder what the average weight difference is between the two sizes.

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post #4 of 16 (permalink) Old 11-28-2010, 06:58 PM
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I've wondered about this as well, My truck has 31's on it now, pretty sure they weren't stock?

Looking through that link, it looks sort of confusing, I'm not a math person so all those equations throw me off...

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post #5 of 16 (permalink) Old 11-28-2010, 07:30 PM
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31's will be fine don't worry about it. I wouldn't run anything bigger than 32's on those gears.


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I was always taught u got to lube it up before you stick it in. My friends said the teacher yesterday said that too.
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post #6 of 16 (permalink) Old 11-28-2010, 07:49 PM
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30"/31" = 0.97 so your speedo and odo will read approx 3% low. At 70 mph indicated on the speedo, you're really going 68 mph.
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post #7 of 16 (permalink) Old 11-28-2010, 07:53 PM
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its so easy to reprogram it, i would

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post #8 of 16 (permalink) Old 11-28-2010, 07:54 PM
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Quote:
Originally Posted by Dustball View Post
30"/31" = 0.97 so your speedo and odo will read approx 3% low. At 70 mph indicated on the speedo, you're really going 68 mph.
You are correct that the speedo/odo will read a little low, but the last bit is actually the other way round. A taller tire has a slower rotation, so the truck will actually be going a little faster than what is indicated on the speedo.
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post #9 of 16 (permalink) Old 11-28-2010, 08:10 PM
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Originally Posted by JohnMcD348 View Post
OK, thanks. I think about going up to the other stock size but have concerns about it since I've got 3.08 gears. I can't justify spending $4-500 on new rubber and then having to spend more to get slightly lower gears (3.55-3.73) to go with it. I wonder what the average weight difference is between the two sizes.
Are you running 15 X 6" or 15 X 7.5" wheels?

If you've got 7.5" wheels, go with the 31/10.50 (or 265/75/15--it's the same size, but you get a UTQG rating, lighter weight and additional load rating, for whatever reason), but don't try to shoehorn a 31/10.50 onto a 6" wheel.

As per your question about weight difference (good question, BTW), assuming we are comparing all terrain tires and not mud terrain tires, you can expect the following:
235/75/15 Versus 31.10.50/15:

36LBS Versus 42LBS from BFG
38LBS Versus 46LBS from Goodyear
32LBS Versus 43LBS from Michelin.
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post #10 of 16 (permalink) Old 11-28-2010, 10:01 PM
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Quote:
Originally Posted by RaZOrFLy View Post
You are correct that the speedo/odo will read a little low, but the last bit is actually the other way round. A taller tire has a slower rotation, so the truck will actually be going a little faster than what is indicated on the speedo.
Ah, yeah- you're right. Brain fart on my part.
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post #11 of 16 (permalink) Old 11-28-2010, 11:30 PM Thread Starter
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Originally Posted by RaZOrFLy View Post
Are you running 15 X 6" or 15 X 7.5" wheels?

If you've got 7.5" wheels, go with the 31/10.50 (or 265/75/15--it's the same size, but you get a UTQG rating, lighter weight and additional load rating, for whatever reason), but don't try to shoehorn a 31/10.50 onto a 6" wheel.

As per your question about weight difference (good question, BTW), assuming we are comparing all terrain tires and not mud terrain tires, you can expect the following:
235/75/15 Versus 31.10.50/15:

36LBS Versus 42LBS from BFG
38LBS Versus 46LBS from Goodyear
32LBS Versus 43LBS from Michelin.
I think they are 7.5" wheels. They are the stock factory Aluminum Alcoa wheels. I'm planning on going with the BGF All Terrains when I make the switch. THey came stock on my Ram and I've been pretty impressed with them so far. Granted the ones on the Ram are E rated at 265/70/17. I don't get it much since I can't seem to get an equivalent rating on basically the same tire. If I go larger they are rated at D. I think the 31"(265) are only a C rated tire. I'll be getting new rubber on the Ram before I get it on the Bronco so I've been doing alot of looking around.

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post #12 of 16 (permalink) Old 11-29-2010, 02:46 AM
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Here's a good Gear Ratio Calculator where you enter your info & it'll tell you the RPMs at various speeds, along with a ton of other info. It has 2 calculators side by side for easy comparison. Be sure to hit "Calculate" for each side after any changes. Plugging in an M5OD, 1356, 3.08 gears, & 29s it shows 1832 rpm at 65 mph. With an M5OD, 1356, 3.08 gears, & 31s it shows 1846 rpm at 70 mph.

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post #13 of 16 (permalink) Old 11-29-2010, 12:58 PM
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Quote:
Originally Posted by JohnMcD348 View Post
I think they are 7.5" wheels. They are the stock factory Aluminum Alcoa wheels. I'm planning on going with the BGF All Terrains when I make the switch. THey came stock on my Ram and I've been pretty impressed with them so far. Granted the ones on the Ram are E rated at 265/70/17. I don't get it much since I can't seem to get an equivalent rating on basically the same tire. If I go larger they are rated at D. I think the 31"(265) are only a C rated tire. I'll be getting new rubber on the Ram before I get it on the Bronco so I've been doing alot of looking around.
It's a safe bet that they are 15 X 7.5", then. Pull off one to be sure, and look at the JJ stamp in the aluminum. It will confirm it or prove otherwise, but if memory serves, all 15 X 6" wheels for Bronco were steel.

Look at the Goodyear Wrangler Silent Armor "Pro Grade" line. They come in the same sizes as the regular "c" range SA tires, but have load range capacities between 116-123.
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post #14 of 16 (permalink) Old 11-29-2010, 02:52 PM
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yo John
Tire Rack has the info you'll need when you calibrate your PSOm when you go to large diameter tires
This is for da BF Goodrich All-Terrain T/A
http://www.tirerack.com/tires/tires....all=Blackwall#
Click on Specs for Revs Per Mile, Overall Diameter etc.

and compare it to Ford's Conversion Constants for validation


Calibration

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post #15 of 16 (permalink) Old 11-29-2010, 08:56 PM
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Originally Posted by RebelRider2010 View Post
I've wondered about this as well, My truck has 31's on it now, pretty sure they weren't stock?

Looking through that link, it looks sort of confusing, I'm not a math person so all those equations throw me off...
265/75/R15 was a stock option, check your doorjamb sticker.

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post #16 of 16 (permalink) Old 12-01-2010, 07:12 PM
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Originally Posted by Dustball View Post
30"/31" = 0.97 so your speedo and odo will read approx 3% low.
Ok, I'll save the reprogram for when I go to 33's.

Thanks.

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