how to correct speedo for taller tires?

[Jim Ross]

Just rec'd my new '01 and will be installing
235 85R 16's and I know from experience the speedometer will read slow with the taller tires, so what is the cure to correct the speedometer on these newer trucks?
Thanks, Guys!

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"J. R. "
Lifetime NRA Endowment
Lifetime AMS/OIL dealer
Purdue Univ. alumni
'98 SLT 4x4, V10, AT, dual exhaust, Smittybuilt nerf bars, custom wheels, tall tires, LSD, tow pkg, tow hooks;
'98 Honda Valkyrie Tourer, all black (fastest color and chrome, 4 degree trigger wheel, Rattlebars kickshift, Rattlebars QD bag drop kit, 4" exhaust tips, LaMonster risers,
Rivco spot lights, upgraded bulbs thru out, AMS/OIL thru out, Chrome, Chrome, Chrome (it truly is a disease!);
'65 Kaiser CJ-5, white, F-head, Warn OD, Smittybuilt tow bar, 54,000 orig. miles.

 

[JRG]

REPLACE THE SPEEDO GEAR IN THE transmission. YOU CAN GET DIFFRENT TOOTH GEARS FROM THE DEALER, THEY CAN TELL YOU YOUR ORIGINAL COUNT AND MAY EVEN MATCH YOUR NEW TIRE SIZE TO A NEW TOOTH COUNT. ONLY TAKES ABOUT 20 MIN. COST ABOUT $15, REAL EASY.

JRG

 

[work2muchplay2little]

Jim, check out this thread. https://www.turbodieselregister.com/ubb//Forum17/HTML/002260.html

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Kurt Voss
********* 1999 2500 24VCTD AUTO 4x4 QC LB Laramie SLT, Luverne stainless steel nerf bars, 3 piece bug deflector, mopar mud flaps, Line-X bedliner, Blue Chip Fuel Management System (Premium), Psychotty Air, DD3's,TTPM, DD 4" exhaust, HX40, DTT VB/TC(91%) , Mag-Hytec transmission pan... YEEEEEHAAAA!!

 

[Deezal Man]

JRG,

The new trucks don't have transmission/speedo gears. They use an electronic pulse position indicator in the rear Diff.

Only way around it is to buy a "black box" to adjust the number of pulses the computer sees per tire revolution.

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98. 5' Emerald Green 24v 2500 Auto/3. 54 4x4 SB QC 285. 75. 16's Bridgestone Dueler A/T's on 16" x 8" Alcoa's under Rancho 9000's. Everything but leather. PIAA 1200's, AMSOIL dual filter relocation system, Smittybuilt Stainless Steel Nerfs, Rhino Liner(Junk).

 

[joel]

Deezal -

Or if you have a good dealer, they'll hook up their computer scanner and enter your new tire size - well, actually, the new number of rotations per mile of the new tires - and you're good to go. Mine did it for me, but I think I gave them the wrong number... .

Oh, and the writer said they couldn't do it, it was the tech that offered to do it for me when I was in there one day and he saw my new tires. Since I think my writer is pretty good, too, I suspect she just didn't know the computer had that capability.

 

[Paul Herioux]

The pulses per mile can only be adjusted up that of the largest production tire. This will be better than what you have but still a little off.

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The smiling ex-dealer technician
89 w250, 47rh trans, cleaned up exhaust, Dynomax bullet muffer, 3" lift, unknown oversized injectors

 

[John]

Originally posted by Paul Herioux:
The pulses per mile can only be adjusted up [to] that of the largest production tire.

This is correct. That is why I added an Abbott E. R. A. (Electronic Ratio Adapter) after adding my M/T Alcoa/285 BFG combination.

This was originated by TDR member Jim Alexander in the Member2Member section of TDR Issue 21, pages 20 and 21. A very comprehensive installation article, pic and wiring diagram accompany the text.

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John Treibel
'98. 5 2500 QC 4x4 Sport, Glasstite Vision II, 285 BFG A/Ts on M/T Challengers, Warn 4X Boards, Mopar fender flares & tow hooks, NW Custom s/s-rubber mud flaps and accs. , Mag-Hytec diff. cover & DD trans. pan, Edelbrock IAS shocks, Reese Titan V hitch, SPA gauges (3), Dynamat, '00 Sport grille and headlights/PIAA Super White bulbs, PIAA Dual Sport 900 aux. lights, BD exh. brake, Amsoil Dual Remote Bypass w. trick billet block adapter and Aeroquip Teflon-s/s hoses, Optima Yellow Tops, DD3s and DD TTPM, Aeroquip AQP s/s fuel hoses, tricked out Cummins valve cover, tricked out Banks High-Ram intake manifold and 14 cm2 turbine housing w/Big Head wastegate actuator, ATS 3-piece exh. man. , Banks 4" dia. s/s exh. sys. , DTT 93% TC, DTT custom tuned VB, BD modified trans. front pump (enhanced by DTT), Air Bulldog induction hood (with NACA ducts), plus MANY other trick modifications

NRA Life (Patron) member

[This message has been edited by John (edited 05-11-2001). ]

 

[Jim Ross]

Thanks all of you guys for your replies. Now I have a clue how to handle the situation!
John,... . I may want to discuss some of the extra goodies you have after I accumulate some miles! I plan to try to get a local shop to do the TQ and transmission mods after a few miles, then I will likely go to larger injectors, turbo housing, etc. The first things going on are: Diablo puck and VDO gages.
Thanks, again everyone.

 

[OILBRNR]

The Abbott is no longer neccesary. As JoeL stated, the new software in the DRB at the dealer can correct for any size. You just need to provide the revs per mile of the tire. It used to be the case that the dealer could only adjust for factory tire sizes. I confirmed this yesterday at my dealership (Earnhardts) and will be taking my '99 in next week when the new *shoes* are installed.

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'99 2500 4x4 • SB • QC • 3. 54 • LSD • AT • Leather • Mag-Hytec • Westach Guages • Line-X • Luverne Nerfs • Tommy Gate

Towing:
'99 28' Airstream Excella (~7000# dry)
'69 24' IMP Inca Twin Screw (~6000# wet)

 

[Posm]

I also changed to 235-85/16E and my speedo is off 5. 3% against the mile markers on the interstate. I have been to my dealer twice to have it changed and have gotten nowhere. The have been very knowledgable in the past. The service writer even offered to pay for the recalabration if another dealer performed it. Help me train them on the change. What is a DRB ( the little hand held or the laptop) and what program or update do the need to have to install the new size. They told me, until recently, the just went in and pulled up the tire size to set the calabration.

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Y2K 2500, QC, 4X4, 3. 54LS, ST, 5 speed, SWB,Cummins E-brake,Westech Pyro and Boost Dual guage, 12 Disk JVC CD

 

[Deezal Man]

Yes, I would love to know as well.

Went to my dealer also, and they said you couldn't adjust the speedo.

Additionally, is this something that will only work on a certain year and above? Meaning my 98. 5 isn't eligable for the upgrade?

 

[Loren]

Something that hasn't been mentioned yet is the calibration for four wheel ABS. My understanding is that there are additional sensors that send signals to a computer for the ABS. If these sensors don't all agree within some tolerance, the ABS faults.

If the dealer can recalibrate the speedometer for different (non-stock) tire sizes, can the dealer also calibrate the ABS system?

I intend to install Rickson 19. 5" wheels this June and would like to know if there is a way to do the calibration with the factory computers that will not require installing additional pulse correcting boxes in the system.

Loren

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2000 Quad Cab, 4x4, short bed, six-speed: U. S. Gear exhaust brake, MileMarker Select Drive viscous coupling, Velvet-Ride spring shackles, Painless Wiring circuit box, Optima yellow-top batteries, Super-Glide 5th wheel hitch, Reese folding ball gooseneck hitch, Drawtite front hitch receiver, Tekonsha Sentinel brake controller, Westin nerf bars, Clarion sound system
1995 Standard Cab, 4x4, long bed, automatic: Firestone Ride-Rite air springs, torque converter lock-up switch, 4" cat-back exhaust, Reese 20K 5th wheel hitch, Reese gooseneck hitch, Hayes Micro Control trailer brake controller, 100 gallon bed-mounted fuel tank with 12 volt electric pump & fill nozzle, Swiss Cap fiberglass cap

 

[Mopar-muscle]

235/85/16 on a 2001 should not be a problem , that size was available on the 3500's ...

 

[Red360]

I went to the dealer yesterday and here is what I found out.
https://www.turbodieselregister.com/ubb//Forum4/HTML/005762.html

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2000 24V, 6-speed, Flame Red, Agate leather, 3500 QC, 4X4, SLT+, 4:10's, LSD, AMSOIL Air/Oil filters and all Fluids, 275 RV squirters, S. S. nerf bars, Line-x, Autometer Ultralites Pyro/Boost gauges, MOPAR tow hooks, Rancho 5000 steering Stabilizer, Rickson 19. 5's with Cooper C-140's, Sky-Jacker 2 1/2" lift in front, Upper & Lower control arms.

 

[machuri]

JIM,
I LOCATED A SPEEDOMETER CALIBRATION SHOP IN SAN DIEGO AND THEY INSTALLED A ELECTRONIC RATIO BOX. IT WAS THE FIRST 2001 THEY EVER DID. TOOK ABOUT 7 HOURS. COST WAS ABOUT $275. NEEDED TO DO THIS FOR MY BFG 315'S. GOOD LUCK.

 

[RADdodge]

Hey OILBRNR - So whats the story? Was Earnhardts able to change your revs per mile with their DBR? The suspense is killing me.

 

[K_Arts]

I just heard something funny that I thought you would get a kick out of.

I was at my dealer the other day getting my tires rotated, and there was a guy in there asking the techs if they would recalibrate his speedo to accomodate his 315's. The guy even went out of his way to tell the tech that the tires speedo was x% of... bla bla bla.

Well at the end of the converstation the tech said that they refuse to recalibrate the speedo because he can't get the speedo to read "Exact" like what DC does. LOL. . you mean DC did something right?? LMAO

To make a long story short... DC has directed their 5 cruddy star dealers not to recalibrate the speedo for any reason (according to this tech and the service manager)!! LOL

Kev

[This message has been edited by K_Arts (edited 05-18-2001). ]

 

[OILBRNR]

Bruce,

I haven't had the time to take it in since mounting the new tires yet.

I'm getting suspecious after seeing some of the comments above, but the guy was pretty insistant that it was possible with any tire size now.

It could be another way that DC is trying to discourage modifying their trucks out of original configurations. You know big tires distroy bearings, suspension parts, etc. #ad


I'll try and set an appointment for next week and let everyone know the outcome.

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'99 2500 4x4 • SB • QC • 3. 54 • LSD • AT • Leather • 16x8 M/T Alcoa Challengers • 285/75 BFG A/Ts • Mag-Hytec • Westach Guages • Line-X • Luverne Nerfs • Tommy Gate

Towing:
'99 28' Airstream Excella (~7000# dry)
'69 24' IMP Inca Twin Screw (~6000# wet)

 

[fortunate1]

When my stock 245 75R 15's wore out, I changed to 235 85R 15's for a taller tire and smaller footprint. I checked the speedo/odo accuracy on some long trips & found that my odometer measured 98. 3 miles after travelling past 100 miles of mile markers. This has checked out to be consistant time after time. By using my handheld GPS, I found that the highway speeds were pretty much dead-on to what the speedo displayed - never more than 0. 1 to 0. 2 mph off the GPS speed. Also, I found out that the DOT surveyers were pretty good at laying out the mile markers. 100 mile markers showed up as 100. 1 miles on the GPS. I haven't bothered to change any gearing; I just know the differences in my head.

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95 2500 SLT, clubcab, 2wd, longbed, a/t, 3. 54, 152K miles, "all the usual refinements"

 

[RADdodge]

Well I thought I had a brain storm but it turned out to be a brain cramp! I tried to have the dealer change the rear end ratio in the CAB with the DBR. It was a no go. Apparently the rear end ratio doesn't matter to the computer since the exciter ring tells the CAB the speed of the wheel in RPM and in turn is converted to MPH in the PCM. The DBR has the ability to change the type of rear end but ratio is not a programable parameter. Oh well. Anyway, Dan at Rickson Tire told me they are expecting a shipment of devices similar to the Abbot boxes, only these splice into the speedometer sense wire between the CAB and PCM. Thus it doesn't affect the ABS Controller. The price is a bit steep, but I'm willing to try it. I'll let ya know.



[This message has been edited by Bruce Radford (edited 05-29-2001). ]