Recalibrating speedo help
Thread Starter
|
Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
Likes: 2
From: Vancouver BC Canada
Recalibrating speedo help
I've read the posts regarding recalibrating the speedometer when tire sizes have been changed and I know how physically to change it. However I'm a little lost as to what to change it to. 31x10.5x15 tires (manufacturer lists o/d at 30.4") and 3.55 gears. Any help is appreciated.
Lead Driver
Joined: Aug 2006
Posts: 5,670
Likes: 13
From: Sand Lake, MI
Use, 30.4x3.14=95.45/12=7.95 5280/7.95=664x.0135= cal896 (x.0135 for 15" rim)
Be sure and double check my math, you can only change it 6 times (That is if no one has done it before you).
I'd check the 30.4 number with a tape measure too. Measure the tire with no weight on it, or just "front to back" to make sure of the tires actual diameter. All figures after that depend on that number being right.
Gearing changes do not effect speedo reading.
Be sure and double check my math, you can only change it 6 times (That is if no one has done it before you).
I'd check the 30.4 number with a tape measure too. Measure the tire with no weight on it, or just "front to back" to make sure of the tires actual diameter. All figures after that depend on that number being right.
Gearing changes do not effect speedo reading.
Thread Starter
|
Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
Likes: 2
From: Vancouver BC Canada
Hmm, thank you both for responding. Your numbers are different from each other though. I have observed you both to be very knowledgeable so that leaves me a little bit stuck.
Dan, I doublechecked your math and it is correct, I don't understand it though.
Bill, is that an official chart from a ford manual?
FWIW, the tires are Dunlop Radial Rover RVXT 31x10.5x15 and the 30.4" diameter figure is from their website. I guess I'd have to email customer service for a Revs-per-mile figure.
Dan, I doublechecked your math and it is correct, I don't understand it though.
Bill, is that an official chart from a ford manual?
FWIW, the tires are Dunlop Radial Rover RVXT 31x10.5x15 and the 30.4" diameter figure is from their website. I guess I'd have to email customer service for a Revs-per-mile figure.
Lead Driver
Joined: Aug 2006
Posts: 5,670
Likes: 13
From: Sand Lake, MI
<i>Ok, just found the revs/mile number on the Tire Rack site: 719
Would that make my conversion constant 9.71 (9.7065 rounded up)?</i>
Yes, using that figure provided "revs/mile 719"= Cal971
I would measure the tire with a tape just to be sure it works out the same, before locking in the setting change.
Would that make my conversion constant 9.71 (9.7065 rounded up)?</i>
Yes, using that figure provided "revs/mile 719"= Cal971
I would measure the tire with a tape just to be sure it works out the same, before locking in the setting change.
Trending Topics
Thread Starter
|
Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
Likes: 2
From: Vancouver BC Canada
So I locked in the 971 conversion constant for the new tires. After driving for a while I have the feeling that my speedo is still off. No proof, just a sense. I did some more poking around and something doesn't add up.
Tire rack lists the diameter of my 31x10.5x15 tires as 30.4" and the revs per mile as 719. The constant I ended up with was 971. The truck was already programmed for 972 so the change was minimal. I looked at some other 31's on tire rack for comparison and the numbers are really different.
Tire / Diameter / Revs per Mile
Dunlop RVXT 30.4 719
Goodyear SA/AT 30.5 681
BFG AT 30.7 677
Dueler Revo AT 30.5 683
Firestone AT 30.5 683
Michelin LTX AT2 30.5 682
Even the Michelin LTX AT2 215/85/16 which has the exact same diameter as the 31x10.5x15 RVXT at 30.4 lists the revs per mile as 684.
Something must be screwy here, don't you think?
Tire rack lists the diameter of my 31x10.5x15 tires as 30.4" and the revs per mile as 719. The constant I ended up with was 971. The truck was already programmed for 972 so the change was minimal. I looked at some other 31's on tire rack for comparison and the numbers are really different.
Tire / Diameter / Revs per Mile
Dunlop RVXT 30.4 719
Goodyear SA/AT 30.5 681
BFG AT 30.7 677
Dueler Revo AT 30.5 683
Firestone AT 30.5 683
Michelin LTX AT2 30.5 682
Even the Michelin LTX AT2 215/85/16 which has the exact same diameter as the 31x10.5x15 RVXT at 30.4 lists the revs per mile as 684.
Something must be screwy here, don't you think?
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Thread Starter
|
Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
Likes: 2
From: Vancouver BC Canada
Update: Ok I've solved the mystery. After contacting Tire Rack and not getting very far, I called Goodyear customer service and they told me that the proper spec for that tire and size is 681 revs per mile, not 719 as Tire Rack has listed. I called Tire Rack back and told them that their website is inaccurate. Hopefully they correct the mistake so that no one else burns through one of their allowed changes on their speedo.
Anyway, that makes the new conversion constant 919. Hopefully that clears everything up.
Anyway, that makes the new conversion constant 919. Hopefully that clears everything up.
Posting Guru
Joined: Oct 2005
Posts: 1,017
Likes: 3
From: Longwood, Fl
tire diameter
I have measured both 31X10:50X15 and 32X11:50X15's. neither tire actually rolled out to be the 31or 32 inch size (31/32 X 3.1416). Each one was ~1" short on height. I also did the conversion on my 93 bronco PSOM.
danr1 did some calculations (appear to be correct). Do your own measuring and math. Remember, the tire is a circle (forget any "rolling radius" arguments!) and the calculations for all the figures are "cut in stone". I checked my 32's at all pressures from the max rating of 50 psi down to 10 psi and the measured roll only varied less than 1/4 inch. Put a mark on the sidewall where it contacts the ground and roll it 1 revolution. Measure that distance and calculate your constant from that. You will have to calculate, in reverse, the height of the tire by dividing the actual roll by 3.1416. Usuing the actual roll you can then calculate the revolutions per mile. I have found that generally speaking, the mfg figures are not correct (wishful thinking?)
I have measured various car tires (michelin, bridgestone) and found that none of them actually measure what the sizing says. My 32's are bridgestone apt's and my speedometer is correct. I dont recall what my constant was. LOL
danr1 did some calculations (appear to be correct). Do your own measuring and math. Remember, the tire is a circle (forget any "rolling radius" arguments!) and the calculations for all the figures are "cut in stone". I checked my 32's at all pressures from the max rating of 50 psi down to 10 psi and the measured roll only varied less than 1/4 inch. Put a mark on the sidewall where it contacts the ground and roll it 1 revolution. Measure that distance and calculate your constant from that. You will have to calculate, in reverse, the height of the tire by dividing the actual roll by 3.1416. Usuing the actual roll you can then calculate the revolutions per mile. I have found that generally speaking, the mfg figures are not correct (wishful thinking?)
I have measured various car tires (michelin, bridgestone) and found that none of them actually measure what the sizing says. My 32's are bridgestone apt's and my speedometer is correct. I dont recall what my constant was. LOL
Senior User
Joined: Sep 2007
Posts: 336
Likes: 0
From: Iowa
P-metric sizing was supposed to "industy standard' the sizing (ie diameter) but that, in itself, varies by enough to vary the speedo depending on calibration specified. Mine, I tried to out think the specs on Tire Rack and ended up showing 3 MPH fast but at least I know what it is and adjust.
Thread
Thread Starter
Forum
Replies
Last Post
Ddaybc
1987 - 1996 F150 & Larger F-Series Trucks
10
Jun 1, 2013 10:02 PM
mueckster
1999 - 2003 7.3L Power Stroke Diesel
5
Jul 6, 2010 10:28 AM
