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Old 12-26-2016, 10:58 PM   #1
46stude
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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?
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Old 12-27-2016, 12:09 AM   #2
izzy_Britton
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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
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Old 12-27-2016, 08:24 PM   #3
46stude
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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.
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Old 01-22-2017, 11:44 AM   #4
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Re: Q- speedo off & bad MPG

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Old 01-22-2017, 12:51 PM   #5
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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.
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Old 01-22-2017, 08:34 PM   #6
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Re: Q- speedo off & bad MPG

Hmm, that's worth looking in to. Thanks.
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Old 01-23-2017, 12:18 AM   #7
izzy_Britton
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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
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Old 01-23-2017, 10:24 AM   #8
46stude
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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.
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Old 02-01-2017, 12:17 AM   #9
Chris H
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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.
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Old 02-16-2017, 03:38 PM   #10
Twin Turbo Skylark
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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.
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