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  #16  
Old 09-21-2010, 02:19 PM
NeedForSpeed NeedForSpeed is offline
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Re: How accurate is your speedometer?

Quote:
Originally Posted by Huskymaniac View Post
Well, not for me. That would correct for about 1.8% of the error but my speedo is off closer to 5%!

So you don't get tire rub with tires that are more than 13mm larger in diameter and 10mm larger in width? That would be good to know as it would allow me another option for snows. Up until now I have only looked at 205/55/16 and 225/50/16.
Tony,

I don't understand how the speedo/odometer work. Interesting, in using this site to calculate and compare my 3.70 geared 235.40/18 tires with stock 3.545 gears and stock 225/50/16 tires, both in 0.694 4th gear (love this site)
http://www.kabamus.com/garage/gears.html

Comparing stock tires, 3.70 to 3.545 ratios, 4th gear, the calculated speed increase with 3.545 gears at 2000 rpm is 2.57mph, 2500 is 3.22mph, 3000 is 3.86mph, and 3500 is 4.51 mph (103mph vs 107.51 mph).

Now, factoring my tire change:
3.70 gear with 235/40/18 at 3000 equals: 88.29 mph
3.545 gear with 225/50/16 at 3000 equals: 90.18 mph,

so, I"m slightly less than 2 mph less in 4th at 3000 rpm, compared to 3.86mph different comparing the same gear change and stock tires.

At 2000 rpm, my big tire 3.70 are only 1.26 mph slower than stock tire 3.545 gears, 58.86mph vs. 60.12 mph.

What all this has to do with speedo/odometer factory calibration, I don't know, but the numbers are fun. We run 3.545, 3.70, 4.111 and 4.444 gears in our SVXes, so the website helps sort out gear and tire changes.

For snow tires on stock rims, you could certainly use 215/55/16, 97-spec.

Tire rub depends on the tire size and tread width (different models of same size tire have different tread width), and wheel offset. My 18" '07 Lexus wheels are 10mm closer to the fender than OEM wheels.

I can get all fingers between the TOYO VERSADO LX rear tires and the wheelwell .
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Last edited by NeedForSpeed; 09-21-2010 at 02:56 PM.
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  #17  
Old 09-21-2010, 02:30 PM
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Re: How accurate is your speedometer?

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Originally Posted by NeedForSpeed View Post
Tony,

I hope the engineers come in with their calculators, but I believe that the tire circumference is the important number. In my test, the diameter difference may be 2% more, but the rolling circumference is 4% more, if I got it right.
Circumference is pi times diameter so they are proportional which means the same 2.2% increase for your case

Quote:
Now, factoring my tire change:
3.70 gear with 235/40/18 at 3000 equals: 88.29 mph
3.545 gear with 225/50/16 at 3000 equals: 90.18 mph,
Your 2.2% diameter increase effectively turned your 3.7 into a 3.62. That is one concern about going to a bigger diameter tire. It will send the ratios in the wrong direction for tranny life. On the other side we have speedos that read too high so we don't want smaller diameter tires to exaggerate the problem more. I think they all read high because the spring loses its springiness over time. Plus, the odometer will rack up the miles faster for a smaller tire diameter. Bad for resale.....
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  #18  
Old 09-21-2010, 02:55 PM
NeedForSpeed NeedForSpeed is offline
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Re: How accurate is your speedometer?

Quote:
Originally Posted by Huskymaniac View Post
Circumference is pi times diameter so they are proportional which means the same 2.2% increase for your case

Your 2.2% diameter increase effectively turned your 3.7 into a 3.62. That is one concern about going to a bigger diameter tire. It will send the ratios in the wrong direction for tranny life. On the other side we have speedos that read too high so we don't want smaller diameter tires to exaggerate the problem more. I think they all read high because the spring loses its springiness over time. Plus, the odometer will rack up the miles faster for a smaller tire diameter. Bad for resale.....
Thanks, you are right about the formula, too many years past college. All I do is calculate area, and cubic displacement, so, I was stuck on auto pilot, and errored.

Yes, I effectively and purposely moved the gearing between 3.7 and 3.545. I do not believe this will have any effect on the life of the tranny, certainly less effect than the way I abuse it with my right foot Either way, I like this side better than the other. My son and I built the tranny in the car now, and have done many others, so, that is not a huge problem.

I'm already running a hybrid cluster in this car, so, I'm familiar with the units. Where is the 'SPRING' you refer too? I have a couple spare gauge clusters, I'd like to check it out
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  #19  
Old 09-23-2010, 06:38 PM
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Re: How accurate is your speedometer?

My 92 is consistantly 5 mph off. I checked it against gps, no matter how fast I go the speedo says I'm doing 5 mph more.
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  #20  
Old 09-23-2010, 06:58 PM
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Re: How accurate is your speedometer?

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My 92 is consistantly 5 mph off. I checked it against gps, no matter how fast I go the speedo says I'm doing 5 mph more.
225/50/16?
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  #21  
Old 09-23-2010, 07:58 PM
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My speedometer seems to be extremely accurate. It reads the same as the GPS up until 70, at least. Beyond that I won't say.
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  #22  
Old 09-23-2010, 08:35 PM
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Re: How accurate is your speedometer?

My first two claret were each 4mph under. The teal is about 2mph under. I read on here at one time that most all are around 4 mph under so 10% off the odometer and would be real close. As i said read it somewhere on here, not quoting.
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  #23  
Old 09-23-2010, 08:47 PM
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Re: How accurate is your speedometer?

Quote:
Originally Posted by Jonnycash View Post
My 92 is consistantly 5 mph off. I checked it against gps, no matter how fast I go the speedo says I'm doing 5 mph more.
I checked mine at low speed today and mine does the same. Off by the same amount. That is odd. I would have expected a consistent percentage error.
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  #24  
Old 09-24-2010, 06:23 AM
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Re: How accurate is your speedometer?

Quote:
Originally Posted by Huskymaniac View Post
I would have expected a consistent percentage error.
Step 1: Make a speedo that is spot-on.
Step 2: Meet legal spec and keep consumers out of trouble by turning the label 5mph counterclockwise.
Step 3: ?????
Step 4: Profit.
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  #25  
Old 09-24-2010, 11:13 AM
NeedForSpeed NeedForSpeed is offline
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Re: How accurate is your speedometer?

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Originally Posted by LetItSnow View Post
Step 1: Make a speedo that is spot-on.
Step 2: Meet legal spec and keep consumers out of trouble by turning the label 5mph counterclockwise.
Step 3: ?????
Step 4: Profit.
As for Step 2, some, not me, have removed the plastic cover, ran the car at known speed, pulled and repositioned the needle. That would have no effect on ECU/TCU programming though
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  #26  
Old 09-24-2010, 11:35 AM
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Re: How accurate is your speedometer?

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