Would changinf from 235 Stock to 31 Stock effect Speedo/Odo reading? [Archive] - FSB Forums

: Would changinf from 235 Stock to 31 Stock effect Speedo/Odo reading?


JohnMcD348
11-28-2010, 05:45 PM
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

theramsey3
11-28-2010, 05:51 PM
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://fullsizebronco.com/forum/showthread.php?t=55005&highlight=psom for a how to it is super easy.

JohnMcD348
11-28-2010, 06:39 PM
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.

RebelRider2010
11-28-2010, 07:58 PM
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...

theramsey3
11-28-2010, 08:30 PM
31's will be fine don't worry about it. I wouldn't run anything bigger than 32's on those gears.

Dustball
11-28-2010, 08:49 PM
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.

offroadkarter
11-28-2010, 08:53 PM
its so easy to reprogram it, i would

RaZOrFLy
11-28-2010, 08:54 PM
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.

RaZOrFLy
11-28-2010, 09:10 PM
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.

Dustball
11-28-2010, 11:01 PM
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.

JohnMcD348
11-29-2010, 12:30 AM
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.

El Kabong
11-29-2010, 03:46 AM
Here's a good Gear Ratio Calculator (http://www.grimmjeeper.com/gears.html) 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.

RaZOrFLy
11-29-2010, 01:58 PM
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.

miesk5
11-29-2010, 03:52 PM
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.jsp?tireMake=BFGoodrich&tireModel=All-Terrain+T%2FA+KO&sidewall=Blackwall#
Click on Specs for Revs Per Mile, Overall Diameter etc.

and compare it to Ford's Conversion Constants for validation
http://www.supermotors.net/getfile/646264/thumbnail/conversionconstants.jpg (http://www.supermotors.net/registry/media/646264_1)

Calibration (http://www.supermotors.net/registry/media/76023)

justin
11-29-2010, 09:56 PM
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.

RebelRider2010
12-01-2010, 08:12 PM
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.