Bigger tires = faster speedometer?

9

97GreenMeanie

Well-known member
Mar 24, 2014
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0
ny
Hey all..


I upgraded to 235/85's on my 97 Disco I and my speedometer is off obviously now.
But the question is this; My Ultra Gauge is telling me that I'm going slower than my speedometer. i.e. Ultragauge says 36.7mph and speedometer says 40+/-.


Is either one actually correct?
Where do the signals come from and are they affected by a tire change?


I thought my speedometer was supposed to run slow when running larger tires like every single other vehicle I've had?


I haven't tried a GPS to determine actual speed yet.
Can do that tonight if need be.


Any input is appreciated.


Thanks
 
stu454

stu454

Well-known member
Dec 15, 2004
5,407
61
Atlanta, GA
Try the GPS. My '96 D1 speedometer reads correct with 235/85's.

I want to say that the speedo reading comes from the Vehicle Speed Sensor (VSS) on the transfer case.
 
R

robertf

Well-known member
Jan 22, 2006
4,801
366
-
speedometer and odometer are around 10% fast from the factory for gems and 14cux. The ecm speed is correct with factory tires. 235.85r16 will make the speedometer read almost perfect but your ecm will be 10% off


maybe this was done to get out of warranties 10% faster? who knows

both get the reading from the same place
 
p m

p m

Administrator
Staff member
Apr 19, 2004
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La Jolla, CA
www.3rj.org
As a point of reference:
All my older trucks (2x GEMS, 3x 14CUX) had speedometers about 7% too high with stock-sized tires (e.g., showing 75 mph instead of actual GPS-indicated 70 mph), and odometers dead-on, within a small fraction of one percent.
 
Tugela

Tugela

Well-known member
May 21, 2007
4,766
566
Seattle
Can you calibrate your ultragauge for different tire sizes? I haven't fully explored the settings (or the instructions) on my code reader but I wouldn't be surprised if the tool came with this capability. Mine has a VSS readout as part of the live data.
 
9

97GreenMeanie

Well-known member
Mar 24, 2014
48
0
ny
Tugela said:
Can you calibrate your ultragauge for different tire sizes? I haven't fully explored the settings (or the instructions) on my code reader but I wouldn't be surprised if the tool came with this capability. Mine has a VSS readout as part of the live data.
Click to expand...

That is a good question.
I'll look into that tonight.

ETA: Looks like the Ultragauge can calibrate for larger/smaller tires, gearing, etc..
 

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B

bri

Well-known member
Apr 20, 2004
6,184
155
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So are any of you running 265/75R16 or 235/85R16 and 4.11?


Whats the speedo show for in that case?
 
B

bri

Well-known member
Apr 20, 2004
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No shit. If the original speedometer is 7% too fast, then I might expect 235 or 265 to help even that out. But less so with 4.11s. I have 4.11, but have not gotten the chance to upgrade tires yet, so I am curious how close I might get through practical experience. From what I guess and calculate, I am still going to be pretty far off.
 
Buddy

Buddy

Well-known member
Nov 6, 2006
2,839
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Central NC
bri said:
No shit. If the original speedometer is 7% too fast, then I might expect 235 or 265 to help even that out. But less so with 4.11s. I have 4.11, but have not gotten the chance to upgrade tires yet, so I am curious how close I might get through practical experience. From what I guess and calculate, I am still going to be pretty far off.
Click to expand...

Honestly there are so many variables in that. It does not really matter what mine reads. But if you must know it reads 2-8 mph lower than with stock gears depending on speed. But if your tires a slightly bigger or smaller and all brands measure differently. That number will change. It also depends on how accruate your speedometer was before you made the change. Going purely by the numbers assuming that your 235/85 actually measured 31.7" and your stock tire was 28.9" (which they probably were not). Then your speedo would read -6% with 4.11 gears over stock with stock gears. So if the speedo said 65 you'd be going 61. Now considering your stock speedo probably already read 2-4mph slow. You'll now be reading even slower.

Edit:

Now factor in that your 235/85 probably only measures out to around 30.9=31.3" and that throws it off by another % or two. Then how worn out were your old tires? Did they measure 28.9" Probably not. So that will throw you back the other direction. It's all just messy. Personally I think the 255/85's are a pretty good match for the 4.11 gears. Small is going to give you more power but effect your highway driving.
 
B

bri

Well-known member
Apr 20, 2004
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Buddy said:
Honestly there are so many variables in that. It does not really matter what mine reads. But if you must know it reads 2-8 mph lower than with stock gears depending on speed. But if your tires a slightly bigger or smaller and all brands measure differently. That number will change. It also depends on how accruate your speedometer was before you made the change. Going purely by the numbers assuming that your 235/85 actually measured 31.7" and your stock tire was 28.9" (which they probably were not). Then your speedo would read -6% with 4.11 gears over stock with stock gears. So if the speedo said 65 you'd be going 61. Now considering your stock speedo probably already read 2-4mph slow. You'll now be reading even slower.

Edit:

Now factor in that your 235/85 probably only measures out to around 30.9=31.3" and that throws it off by another % or two. Then how worn out were your old tires? Did they measure 28.9" Probably not. So that will throw you back the other direction. It's all just messy. Personally I think the 255/85's are a pretty good match for the 4.11 gears. Small is going to give you more power but effect your highway driving.
Click to expand...


I can do the math (which you cannot, BTW) and if you could read you will see that I asked for practical experience with specific tires and gears. I'll know soon enough so no big deal.

You got a couple of numbers right. The speedo does not read slower than actual, you are actually going slower than the speedo, your last statement being assbackwards.

I'll restate.

If someone is running 235/85R16 or 265/75R16 and 4.11 what's your particular vehicle speedo read vs GPS?


I am just curious.
 
Bannon88

Bannon88

Well-known member
Nov 3, 2004
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Columbia, IL
I can't speed in my truck unless I really try, all this talk seems silly. Just drive as fast as most the cars around you and then you won't stand out nearly as much.

BTW mine reads about 4mph off...but I never did trust any of the gauges anyway.
 
B

bri

Well-known member
Apr 20, 2004
6,184
155
US
@Bannon88 stock tires and gears??

I'm not wanting to speed. Just curious about how far off people are with 4.11's and a certain sized tire.

However, my D1 has no problem keeping 80-85 with stock tires and gears. At present with 4.11 and small tires, it is really easy, but RPM are really high. It is going to get better with 235/85 I am certain, but IMO, I think I will still be reading much faster than I am going.

Usually I tend to go slower than surrounding traffic, especially if they are already speeding.
 
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