Speedometer 4mph off after tire/wheel size change

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Chrismb5.7

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Put new and taller tires on my 04 1500 5.7 yesterday and she's reading 4 mph too slow. I'm pretty sure the 20" dodge chrome clads that are on it arent stock... My door says there are 17s on it. Rear diff gear is 3.55. I went from 285/45R20 to 275/60R20. It feels just a tad bit more sluggish in 1st. Still breaks the tires loose with no problem though. Also, will this change affect my mpg? If so, will it be significant? Any help is appreciated, thanks!
 

Marley

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Taller tires changes everything but a simple fix is to buy a tuner like a Superchips or similar.....sluggishness will remain though.
 

Mike Murphy

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Put new and taller tires on my 04 1500 5.7 yesterday and she's reading 4 mph too slow. I'm pretty sure the 20" dodge chrome clads that are on it arent stock... My door says there are 17s on it. Rear diff gear is 3.55. I went from 285/45R20 to 275/60R20. It feels just a tad bit more sluggish in 1st. Still breaks the tires loose with no problem though. Also, will this change affect my mpg? If so, will it be significant? Any help is appreciated, thanks!

Hello, I am going through the exact same thing at the very moment. After getting passed by a few cars on the freeway while doing 65 mph with cruise on I figured something may be wrong. I broke out the cell phone with a gps mph app and I was 5 mph slower. I then went to my work place and used a snap on scanner to check my wheel size in the ABS module. The truck used to have 35's but I have installed smaller tires. The tires are now 30 +- inches. The scan tool showed the tire size as 606 mm. After doing some math I ended up with 724mm. You might catch on here... I drove the truck yesterday and the speedometer was off 10-15 mph. I had someone follow me and my speedometer said 55 mph. According to them the truck was going 90 mph when they were following. Ouch... :mfr_omg::confused: My phone verified the off speedometer. Having 3.55's the truck did the speed with relative ease(1600-1800ish rpm). I realized I may have went the wrong way with the numbers on the scanner or the scanner really is not mm. I will input a smaller number into my ABS module later this morning and see if I can get a GPS mph - speedometer match.
 
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Chrismb5.7

Chrismb5.7

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I put mid length headers on it a while back so I've needed a tuner. I might do that soon. But for now, is there a cheaper fix? I've read about the gear on the other end of the cable to the speedometer but I can't find the name of the gear or the gear itself online anywhere...
Taller tires changes everything but a simple fix is to buy a tuner like a Superchips or similar.....sluggishness will remain though.
 

JOHRY

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Look up the AlfaOBD thread. You can use it to change your tire size to correct the speedo.
 

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The speedo gear is on the rear differential ring gear. That way the diff gears can be changed and not affect the speedo. The tire size does affect the speedo because it is after the ring gear. You most likely need a software change in the EBCM with a scan tool. In the EBCM you can change the revolutions per mile of your tire diameter.
 

Mike Murphy

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On my truck I found out the scan tool is revolutions per mile instead of the milimeters it shows. I set my tire size revolution to 617 mm and the speedometer is right on.
 

ST-8

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You went from a 30.1” tire to a 33” tire of corse it’s gonna read different....
 

dhay13

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My daughter put 4.88's and 35" tires in her Wrangler and bought a Superchips to re-calibrate her speedometer. I actually did it using the speed on the GPS to verify accuracy.
 
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Chrismb5.7

Chrismb5.7

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You went from a 30.1” tire to a 33” tire of corse it’s gonna read different....
I expected the change... I came here for help to fix the issue. These forums are not for people who feel they should say things like that to somebody asking for help. I do not need your help. Thank you.
 
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Chrismb5.7

Chrismb5.7

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My daughter put 4.88's and 35" tires in her Wrangler and bought a Superchips to re-calibrate her speedometer. I actually did it using the speed on the GPS to verify accuracy.
Thanks for the verification... I am definitely going to get a tuner soon.
 

dhay13

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IIRC she bought the cheaper one that only had the speedo tuning ability and not performance changes
 

Jason G Slabaugh

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Fyi my truck originally was to have the 17" tires to, but an option was the 20" tires 275/60/20 which is pretty similar in diameter over all. I bought my truck used with factory 20s. However the 20s of course are a little heavier
 

HemiLonestar

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Pinion factor change, Superchips can do it by tire size, factory style software uses overall diameter/tire height

I expected the change... I came here for help to fix the issue. These forums are not for people who feel they should say things like that to somebody asking for help. I do not need your help. Thank you.

Isn't up to you to decide what these forums are for, especially since you made a change without thinking about what might be affected; it's called common sense.
 
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Chrismb5.7

Chrismb5.7

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Pinion factor change, Superchips can do it by tire size, factory style software uses overall diameter/tire height



Isn't up to you to decide what these forums are for, especially since you made a change without thinking about what might be affected; it's called common sense.
I knew what would happen, but I did not know how to resolve it. I came here for help, not to be harassed for needing it. I got the help I needed. Your comment was unnecessary.
 

HemiLonestar

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If you feel "harassed" after someone pointing something out...in a public forum....then the internet might not be the place for you and you might wanna go crawl back in your safe space...
 
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corneileous

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Put new and taller tires on my 04 1500 5.7 yesterday and she's reading 4 mph too slow. I'm pretty sure the 20" dodge chrome clads that are on it arent stock... My door says there are 17s on it. Rear diff gear is 3.55. I went from 285/45R20 to 275/60R20. It feels just a tad bit more sluggish in 1st. Still breaks the tires loose with no problem though. Also, will this change affect my mpg? If so, will it be significant? Any help is appreciated, thanks!

First and foremost, those 285/45’s you had were just under two inches shorter that even the largest of the stock 17’s your truck came with. What is your original stock tire size?There’s like three different OE 17’s for your truck; 245’s, 265’s and 275’s. How long have you had that truck? We’re those itty-bitty little rubber band tires on it when you bought it? Have you ever driven the truck with the stock size tires on it?

As far as guesstimating how much your mileage will be affected, we kinda need to know what your original tire size is and whether or not your truck has been recalibrated to run those 285’s or not, bein if that was what was on it when you bought it or, as far as what’s been on it since you’ve owned it.

Those 275/60/20’s you put on are the stock 20’s for our 1500’s; but compared to those 285’s you had, you went up three inches in height from those. And even compared to the largest 17’s that your truck could’ve came with factory, your new 20’s are a just a touch over 2 inches taller than those.

Another thing to consider- or at least what I found on the old 98 1500 I had back in the day; it’s stock size was a 245/75/16. When I bought it, it had already had a set of worn out BFG/AT’s on it that were 285/75/16. That’s 2.3 inches taller than the stockers. Granted, those BFG’s were in fact 8-ply LT tires, and even though they were also only an inch and a half wider than the stockers, they were still a much heavier tire just because of them being 8-ply but still, my mileage and the overall performance of the truck went up considerably after I recalibrated my speedometer to my actual road speed. I had read that having your computer think you’re going a lot slower than you actually are affects fuel delivery, timing, and air/fuel ratio. My fuel mileage came up quite a bit too because I was all the sudden racking up more miles to a tank than I was before.

And even though your gears are 3:55’s.... that was with your stock tire size. Your new 20’s are gonna make your 3:55’s feel even shorter. In other words, your new gear ratio now is a 3:46. You would need 3:64’s to bring your gear ratio back up to the stock 3:55’s with them 33” 20’s.




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corneileous

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Isn't up to you to decide what these forums are for, especially since you made a change without thinking about what might be affected; it's called common sense.

For the most part you’re right, but still, if someone can’t help in fixing the issue and would rather just point out the problem that the OP already most likely probably knows about, then why post? It’s just like when somebody just has to comment negatively on another new tire recommendation thread that pops up when there’s already so many, just because they can. Yeah, again, that person would be right but then again, if that’s all the contribution you’re gonna make then not just pass the thread on up?

Just a little common decency to go along with your common sense.


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HemiLonestar

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For the most part you’re right, but still, if someone can’t help in fixing the issue and would rather just point out the problem that the OP already most likely probably knows about, then why post? It’s just like when somebody just has to comment negatively on another new tire recommendation thread that pops up when there’s already so many, just because they can. Yeah, again, that person would be right but then again, if that’s all the contribution you’re gonna make then not just pass the thread on up?

Just a little common decency to go along with your common sense.

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:)

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