Speedometer fix for bigger tires?

Sthornton1986

New Member
Hey just got my TB lifted and got the new tire and wheel setup put on. My speedometer is now roughly 6-7% off at highway speeds. Anyone know the fix for this? Is it a computer reprogram or is it a simple speedometer gear swap?

Any help is always appreciated and yes I did search before posting 😉89216
 

TequilaWarrior

Well-Known Member
There are many users on here that are more knowledgeable on this subject than I am, so I would expect them to correct me if I'm wrong, BUT:
I know of no way to dial in the speedo.
There is no speedo gear to swap, it has an inductive pickup or a tone ring.
The tire size changes that are possible in the PCM are limited to factory tire size options - i.e. if you throw 33's on you're SOL.
 

Mooseman

Moderator
AFAIK, nope. Depending on your gearing, you might be able to get it close by reprogramming your PCM to different VINs that had different gearing and there were only 3 choices, 3.42, 3.73 or 4.10. You would need someone with a Tech 2 and Tis2000 to do this. @limequat might be able to confirm this.
 

Redbeard

Well-Known Member
Well for me I use my Garmin GPS unit for my speedo. What I like the best about it is that I don't have to take my eyes off the road to glance down for my speed. I've rebuilt the gauge with new stepper motors, but they like to fail way to early so I have given up on them and the speedo is one that flaked out first.
just my 2¢
 

Shdwdrgn

Well-Known Member
Since someone else mentioned the 2004, I'll assume that's what year you have? The ECM only has programming options up to a 30.5" tire, so you can get a little closer if you have a Tech-II or a dealership willing to make the change.

Changing the gear ratio to be different from what you actually have can cause major issues with ABS warnings. A lot of people have been able to change the gears by one step (E.G. going from 3.42 to 3.73) without any issue, but when I swapped my 3.42's for 4.10's I couldn't drive more than 5 minutes without the system completely freaking out. So if you want to try changing the gear ratio to compensate for the tire size, I wouldn't recommend going more than a single step. Note that gear ratio can be changed with HPTuners by simply grabbing a tune file from someone with the gears you are looking for, however this file will also overwrite any tire size adjustments made with the Tech-II. I do have a stock HPT file available for 4.10 gears and 30.5" tires.
 

Sparky

Moderator
This is going to sound a bit CRAZY... But there MIGHT be a much simpler "ANALOG" solution for this problem... as long as you intend upon remaining with using those Larger Tires... and Change things BACK to Nominal if you intend upon Selling the Vehicle. Here goes nothing:

(1) Take a Test Drive on a Level Road with a Passenger using a Reliable GPS Unit to read your Actual Speed closest proximity to some Known, Posted Speed Limits ... like at at 30-45-65 MPH... and have him be ready to write down what you are about to say for each accurate GPS MPH reading. Perform this test at LEAST 3-5 Times so you can record an Average of these Speedo- vs. GPS variances

(3) The Driver should call out the Speed of the Truck -SUV showing on the Speedometer just as soon as the Passenger with the GPS Unit advises that the Vehicle is moving at a Steady Posted Speed Limit.

(4) Take the Truck/SUV Home and after taking a picture of the Dash Panel Speedometer Dial... Get a Cup of Coffee and compare the notes the Passenger was taking of the Speedometer-vs.-Actual GPS Speeds.

(6) Then... Pull your Instrument Cluster Panel out of the Truck/SUV and bring it inside where you can disassemble it and work on it where you can concentrate.

(5) Using those Recorded MPH Differences... Count Off How many Line Segments are present on the Speedometer Dial in between those Two Speeds.

(7) CAREFULLY... Measure the Rotational Distance using the same Line Segment Counts in between those Two Speeds and lightly place a piece of Tape in line with the last Speedometer Gauge Dial Position at REST. Then Place a Second Piece of Tape in line with the EXACT what the Desired Speedo Dial Position will be in AFTER this adjustment in the Analog Speedometer Dial is achieved.

(7) Then Physically and CAREFULLY ...Slowly Rotate the Red-Orange Dial Armature either Clockwise to DECREASE the MPH or Counter- Clockwise to INCREASE the MPH reading using EXACTLY the Same Number of Line Segments distant in between the Driver recorded Speedometer Readings versus the GPS MPH recorded by the Passenger.

(8) This might be Very Tedious to do... but from that Adjustment Point on... your Speedometer WILL be an Accurate Analog to the actual Posted MPH speeds in line with the Larger Tire Outside Diameter variance. If this Idea Works... No More GUESSING at how fast you are traveling and having to Look Down at the Speedometer and Calculate with MPH Math, taking your eyes and your attention off the road any more than the Average Driver.

Hope this Works!
This isn't going to do it as the speed off is a percentage not a hard number. The faster you go the more off the speed. Sure you can get it maybe less off, but it is still off.

On my 02 I had almost 10% taller tire, so my speed was about 10% more than the gauge, so easy to quick math it. 55 if speedo reads 50, 77 if speedo reads 70, etc. Basically did not worry about compensating for the discrepancy until around 50 or so I'd begine to adjust mentally.
 

MRRSM

Lifetime VIP Supporter
If you want to play around with "Tire Size Marking Calculations" down to the finest grain details...You can Download the Cross-Platform Programming, Scientific and Engineering Application called "SCILAB" and after installing it... Just Copy-Paste in this Environment Script (...after making some Basic Changes to the Inputted Tire markings) and then Run the Script to see what happens:

Here is where to Download "SCILAB" for FREE:


Then read the information from this Tutorial Link to become familiar with how "SCILAB" works on this Automotive Engineering Problem:


Then... Run whichever version of "SCILAB" you just installed (Windows, Oracle, MAC, Linux) after you Copy-Paste in this Environment Script onto the "SCILAB" desktop and after making your Tire Dimension Edits to suit your particular vehicle "sizeMarking" Parameters ... Run the Script:

clc
sizeMarking = "225/55R17";
W = eval(part(sizeMarking,1:3));
AR = eval(part(sizeMarking,5:6));
D = eval(part(sizeMarking,8:9));

H = W * AR / 100;
wd = D*25.4 + 2*H;
wr = wd/2;
C = %pi*wd;

mprintf("\nwd = %.2f mm\nwr = %.2f mm\nC = %.2f mm\n",wd,wr,C)
 

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