The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 99-06 GMT800 Chevy and GMC Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 12-26-2016, 10:58 PM   #1
46stude
Registered User
 
Join Date: Dec 2014
Location: Rockport, Texas
Posts: 192
Q- speedo off & bad MPG

We have an '02 Suburban 1500, 2wd, 5.3L. It's got somewhere around 230K+ on it, & we've had it since 198K. It's stock, with 265/75/16s, which are just a touch taller than the factory 70s.

It's always got poor fuel mileage, & I discovered a long time ago that the speedometer doesn't read correctly. According to a couple GPS speedo apps I've tried on my phone, it's off by about 6-7 MPH at highway speeds, less as speed decreases. Rough guess of MPG (since the odometer isn't accurate) is the thing averages 10-12. Oh, & no SES lights.

I've tried lots of searches on the web, but everything goes to the dreaded cluster failures. That's not my problem, as far as I can tell.

The only thing I can figure is maybe the PCM needs a factory reflash. No idea how or why it would be outta whack, but there's nothing else I can think of that'd kick the speedo out like that. Any other ideas, or similar problems somebody knows a fix for?
46stude is offline   Reply With Quote
Old 12-27-2016, 12:09 AM   #2
izzy_Britton
Registered User
 
izzy_Britton's Avatar
 
Join Date: Apr 2013
Location: Nor-Cal
Posts: 632
Re: Q- speedo off & bad MPG

you COULD have the PCM/ECM adjusted to accommodate the different tire sizes.

there are 2 ways that I know of to do this, the easiest would be to find someone with a newer scan tool, like the snap-on brand. SOME of these have an option for tire size adjustment/re-calibration.

the other option would require you to remove the PCM/ECM from your truck and send it to someone who can re-program it for the correct gearing and tire size. there is a guy on ebay that does re-programming for 65$ i think it was.

-Izzy
__________________
WHO_DCTC - Wifes 05 Z-71 Build - Now defunct as she decided she wants an Escalade too
Cream Puff - LS'd Square - Slow going on the back burner
Tripple Threat - 3 Wrecks for the price of one build thread
Project Delongdium - Step-Sons Plastic Clad 2003 Avalanche - DONE and Delivered.
izzy_Britton is offline   Reply With Quote
Old 12-27-2016, 08:24 PM   #3
46stude
Registered User
 
Join Date: Dec 2014
Location: Rockport, Texas
Posts: 192
Re: Q- speedo off & bad MPG

Well, there's maybe an inch of height difference, which really isn't enuff to kick the speedo out like that.

And, it read the same with the 70s that were on the Burb when we got it. Forgot to put that in my original post.
46stude is offline   Reply With Quote
Old 01-22-2017, 11:44 AM   #4
NGN
Registered User
 
NGN's Avatar
 
Join Date: Nov 2001
Location: Old Bridge,NJ
Posts: 810
Re: Q- speedo off & bad MPG

Posted via Mobile Device
NGN is offline   Reply With Quote
Old 01-22-2017, 12:51 PM   #5
NGN
Registered User
 
NGN's Avatar
 
Join Date: Nov 2001
Location: Old Bridge,NJ
Posts: 810
Re: Q- speedo off & bad MPG

Ever since I replaced my broke rear in my 06 Silverado at 86k , now 162k with replacement, the speedometer has been off similar to yours , man at the bone yard said it was a match , o well it rolls. So maybe the rears been replaced in the pass with different gears causing the problem.
Nick
Posted via Mobile Device
NGN is offline   Reply With Quote
Old 01-22-2017, 08:34 PM   #6
46stude
Registered User
 
Join Date: Dec 2014
Location: Rockport, Texas
Posts: 192
Re: Q- speedo off & bad MPG

Hmm, that's worth looking in to. Thanks.
46stude is offline   Reply With Quote
Old 01-23-2017, 12:18 AM   #7
izzy_Britton
Registered User
 
izzy_Britton's Avatar
 
Join Date: Apr 2013
Location: Nor-Cal
Posts: 632
Re: Q- speedo off & bad MPG

Quote:
Originally Posted by 46stude View Post
Well, there's maybe an inch of height difference, which really isn't enuff to kick the speedo out like that.

And, it read the same with the 70s that were on the Burb when we got it. Forgot to put that in my original post.
not to argue, but just wanted to split some hairs... for information purposes, ya know?

245/75/16 = 95.72 in circ. 30.47 in height

265/70/16 = 96.15 in circ. 30.61 in height

265/75/16 = 99.43 in circ. 31.65 in height

What does this mean and how does this effect us?

the difference between a 245/75/16 and 265/70/16 is not noteworthy BUT...

If you change from 265/70/16 to a 265/75/16 that means that if your speedo reads 60mph, your actually moving at 62.05 mph, meaning a 2.05 mph difference.

If you change from 245/75/16 to a 265/75/16 that means that if your speedo reads 60 mph, your actually moving at 62.33 mph, meaning a 2.33 mph difference.

I am not a gear ratio kinda guy but I do know that changing the tire size changes your rear gear ratio slightly and that slight amount will undoubtedly change the way your ecm reads your tire rotation vs rpm, which would change your shift points and your speedometer reading. i dont know what the fullest extent of this all means just because i am simply thinking out loud but, i would assume that this means your fuel economy would change.

this would be the exact same as changing the rear axle for a different gear ratio, everything would change, drastically. 4x4 people would know a heck of a lot more than i could ever begin to understand.

-Izzy
__________________
WHO_DCTC - Wifes 05 Z-71 Build - Now defunct as she decided she wants an Escalade too
Cream Puff - LS'd Square - Slow going on the back burner
Tripple Threat - 3 Wrecks for the price of one build thread
Project Delongdium - Step-Sons Plastic Clad 2003 Avalanche - DONE and Delivered.
izzy_Britton is offline   Reply With Quote
Old 01-23-2017, 10:24 AM   #8
46stude
Registered User
 
Join Date: Dec 2014
Location: Rockport, Texas
Posts: 192
Re: Q- speedo off & bad MPG

When I bought it, it had 265/70/16s on factory wheels. About 7-8 mph off by GPS.

Swapped to 265/70/16s on wider aftermarket mags. About 7-8 mph off by GPS.

Swapped to 265/75/16s on factory wheels. About 7-8 mph off by GPS.

That's what I'm going by. Internet calculators are great & all, but the real world experience w/ this particular scenario shows otherwise.
46stude is offline   Reply With Quote
Old 02-01-2017, 12:17 AM   #9
Chris H
72 C30 Trailer Queen
 
Chris H's Avatar
 
Join Date: Dec 1999
Location: SE MN, USA
Posts: 794
Re: Q- speedo off & bad MPG

On a 2002, you can use a handheld tuner (hypertech, etc) to adjust your speedometer. I used one on my 2005 when I put bigger tires on.
I think that would be the first step if you really wanted to start diagnosing poor mpg. I just like the speedo reading accurately so I dont have to constantly figure out how fast I am really going.
__________________
Chris H

72 C30 133"
72 C30 157" flat-dump
69 C10 short fleet
71 ElCamino
2000 K2500 crewcab shortbed (sold, sorry old friend)
2005 GMC 2500HD shortcrew Duramax
2006 K1500 RCSB
and a couple other projects/parts
Chris H is offline   Reply With Quote
Old 02-16-2017, 03:38 PM   #10
Twin Turbo Skylark
Registered User
 
Join Date: Nov 2015
Location: fort saskatchewan
Posts: 73
Re: Q- speedo off & bad MPG

I agree, use a programmer to adjust the tire size till the spedo reads correctly.. regarding the poor fuel mileage how bad is it? I went with a Blackbear tune and I have WAY better power and also better fuel mileage now.
Twin Turbo Skylark is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 02:25 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com