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2002 gmc envoy_slt
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Discussion Starter · #1 ·
I have a 2002 Envoy with the 4.2 engine. I put 18" Boyd Coddington billet rims with some 285/60 tires which are larger diameter than the stock wheels and tires. Now my speedometer is reading 5 mph slower than I'm actually going. I have been looking for a speedo calibrator for this vehicle but can't find one. Do they make one for this car????
 

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2005 chevy trailblazer_ls
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When I ordered my PCM from PCMforLESS to go with my lift I asked to have it recalibrated for 265/75/16 tires. After getting the PCM in, my speedo still read about 5 mph off at 70. I talked to Kelly and just sent it back to have it corrected, hopefully it was just overlooked.
 

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Discussion Starter · #4 ·
Speedo calibrator

There are speedo calibrators out there. I have one in my 2002 GMC truck in the form of a 'FlashPac" by Superchips. But they don't make one for the 4.2 liter six that we have in the Envoy. I'm trying to find a company that make a tuner/speedo calibrator. Does anybody know which company makes one for the Trailblazer/Envoy 4.2 engine?????????????????
 

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There are speedo calibrators out there. I have one in my 2002 GMC truck in the form of a 'FlashPac" by Superchips. But they don't make one for the 4.2 liter six that we have in the Envoy. I'm trying to find a company that make a tuner/speedo calibrator. Does anybody know which company makes one for the Trailblazer/Envoy 4.2 engine?????????????????
There are no handhelds that work with the I6 at all. That is why everyone takes care of this via a tune. You would spend more money buying one of those generic calibrators than buying a tune which does the recalibration and more. Both EFILive and HPTuners can take care of the speedometer calibration easily you just need to make sure your tuner knows about it when they make your tune.
 

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2003 gmc envoy_slt
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Most people have it done as part of a tune, which is one of the highest bang-for-the-buck mods you can possibly get. Read up on tunes.
I just purchased some 265-70-17's and my tuned PCM from PCMforless should be here tomorrow however on the PCMforless website they specifically state no speedo corrections for 02-05 and I have an 03 envoy. What would I do to correct such?
 

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2004 chevy trailblazer_lt
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When I got my tune earlier this year from pcm4less, I was told that they couldn't calibrate the speedometer for the older models... Has something changed? My tires are only 1" larger than stock.
 

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2002 chevy trailblazer_ltz
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Calibration

I just ordered my PCM Tune. After a nice conversation with Kelly I understand that they will not correct the speedo calibration due to larger tires. It actually has something to do with the software they use, but the developer has not updated to allow for that fix, and correct, 02-05. I have an '02.

Because of my tire size now, 165/70R17, I read on a post here that the difference would be -6 MPH difference. I just plan on that. I have a harder time figuring out my actual miles driven for work and figuring out MPG's... feeling number stupid - anyone have a suggestion of how to figure the difference over a tank of gas?

Also, is it -6 MPH over the entire spectrum of speed? Or only at a certain point?

Anyway, just paid for the tune, so now will have to save up for the calibration. :(

Cheers!
 

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2004 chevy trailblazer_lt
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The information of "-6 mph" is misleading and doesn't really tell you anything. What's more important to know is the percentage difference: -11.7%. With this number on hand, you can adjust for anything.

What's your speed difference at 30mph? 30 + (30 x -0.117) = 26.49mph

When your odometer reads 300 miles, you have traveled 300 + (300 x -0.117) = 264.9 actual miles.

The same information applies when you have oversize tires. I run 265/70-16, so my adjustment is a positive 3.7%. 300 miles on my odometer is actually 300 + (300 x 0.037) = 311.1 miles.

Now with this information on hand, you can calculate the adjusted mileage you have actually driven, and use THAT number to calculate your MPG normally.
 

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This is awesome! (As I print it out)...

Soooo... how do I calculate the percentage difference?

(not feeling so number stupid anymore...)

245/65 SR17 - Original
165/70 R17 - New

Thanks!
 

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2004 chevy trailblazer_lt
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Soooo... how do I calculate the percentage difference?
This is the page I use, but there are others... http://www.miata.net/garage/tirecalc.html

At the bottom of the page, enter your original and new tires sizes, and it will give you the specific sizes of the tires, plus the percentage difference between them.

Be VERY careful with running undersized tires. If someone else drives your truck, make sure they are aware of the large difference in the speedometer, otherwise they may think they're driving 30mph in town when they are actually doing closer to 35... that's enough to get a speeding ticket in some cities, plus another ticket for a 'malfunctioning' speedometer.
 

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Great resource link!

So, I plugged in the tire sizes and the picture showed my older tires being smaller! I knew that wasnt right, so I did some thinking and realized the original tire size was 145/65 R17. So, after making the correction, the difference is a +6.9%. Reads 60, going 64.1.

I will be playing with these numbers now... may even create a spreadsheet for myself just so I don't have to think about it.

Thank you, again Shdwdrgn!:thumbsup:
 

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I assume you mean 245/65R17? :) Yeah I may have plugged in 70's instead of 65's ... but you have the idea now. The conversion is easy once you have the numbers laid out in front of you. That difference in your speedometer is certainly a lot safer to work with.
 

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Just to make sure I am stating the correct information - because I didn't initially - not sure what I was thinking.
Original: 245/65R17
New: 265/70R17.

:)

I spoke with a local GM dealership/service center manager. He explained to me that the calibration in our trucks (02-05) used to be quite easy, but at some point in the recent past the software used to update the control modules did not include that ability anymore. It now requires the service center to send in a request with the VIN of the vehicle to GM, they write the code specific to the changes necessary, send it back to the service center to then be applied. I really can't imagine this is a cheap process.

If PCMforless or any other company can come up with an affordably smart fix for this issue as we continue to do upgrades to our vehicles, I am sure there would be a fair market for it.

Thank you again for your help!
 

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Just to make sure I am stating the correct information - because I didn't initially - not sure what I was thinking.
Original: 245/65R17
New: 265/70R17.
OK now you've gone back the other way... These numbers show that your new tires are actually *larger* than the stock wheels, not smaller. If you are running larger tires, then your percentage change will be a positive number, meaning your speedometer will read higher than your actual speed. The difference between th 1 and 2 in the tire sizes is very significant!
 

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OK now you've gone back the other way... These numbers show that your new tires are actually *larger* than the stock wheels, not smaller. If you are running larger tires, then your percentage change will be a positive number, meaning your speedometer will read higher than your actual speed. The difference between th 1 and 2 in the tire sizes is very significant!
Probably an "oops" in your reply, but if you're running larger tires, your speedo reads lower than actual speed. Larger tires travel further per revolution, but your speedo doesn't know about tire switches (or worn tires either).

The percent method works for all mileage related things. If your tire diameter/circumference is 5% larger than stock, you're speed is 5% higher than indicated, you're mileage driven is 5% higher than your odometer shows, and your gas mileage is 5% more than you're calculating off the odometer, or showing on the DIC.
 
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