speedometer/tire size ?

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Somebody please help me.

drawason said he got a gear with one less tooth to correct a slow speedo (I assume slow because he went from 235/85 to a 255/85 which is a larger tire). Would'nt you need more teeth to correct a slow speedo. IE to trun the speedo faster with the same number of revolutions of the wheels.

 

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Iceman,you won't have to prove anything in court as long as the judge believes you when you say your oversized tires caused the problem. I got pulled over for 70 in a 55 told the cop my speedo said 60 and it must be the oversized tires. He wrote the ticket for 65. Went to court and said my speedo said 55 and it must have been the oversized tires. The woman judge dropped the ticket and gave a little speech to the court room about how oversized tires change the speedo and that 60 really wasn't much over the limit anyways. Thought I saved some money until I went to leave the court and found a parking ticket on my truck that was more than the speeding ticket. Instant karma for a double lie I guess.

 

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drawson,so your saying our trucks have the gear? what did it cost?

 

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I believe 94-97 have conventional speedo gears in the transfer case hooked to an A/C generator. My truck was off by 6 MPH at 60 when I put the 255s on. From what I recall I had a 31 tooth gear and went to a 30.

 

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I also run 255/85R16s on my '97 5 spd. I went down 2 teeth on the speedo gear, I think from a 31 to a 29 tooth. I used the mile marker method, I noted the odo reading as I passed a mile marker, then again about 120 miles down the road, then compared the readings. The gear cost $12 at the dealer, special order. The problem with the cruise control method is that the cruise is not all that consistent and there is a greater chance for error.

As for the "flat spot" issue, air pressure does make a difference. I run 30 psi in the Swampers on my Blazer on the street. Off road I run 6-7 psi. When I hit the pavement with the tires aired down, the speedo reads noticably slower. However, the measured loaded radius of the tire is still not the best way to calculate the speedo (although it is much better than just using the advertised height or measured height), since the tire will still "tuck in" a bit and get slightly taller at speed (ever watch a top fuel car leave the line?). However this probably does not have a very notable effect in a properly inflated LT tire.

------------------
'97 2500 CC 4X4 5spd Sport
'83 K5 Blazer 350 TBI (ex 6. 2), 39. 5 TSLs, 3" lift, Dana 60/GM 14 blt, Detroits

 

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Please Help. I'm still confused.

pcorssmit stated that with the tires aired down the speedo reads noticeably slower. Can this be correct?

Here is my logic. Say my speedo is accurate, with a 31" tire and I replace the 31" tire with a 29" tire (this could represent airing down a tire). The smaller tire will make more revs/mile causing the speedo to read faster. Right?

I'm still lost on the teeth on the gear. Lets say I go from a 31" tire to a 33" tire. The tires are now making less revs per mile so the speedo will read slow. To get the speedo to read faster wouldn't you need to add teeth to the gear?

Maybe I just don't understand how the gear controls the function of the speedo. Help.

Ryan

 

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Ryan,it's the opposite,the smaller the gear the faster it spins. As far as a tire with less air reading different on the speedo,think of this-most modern tires are steel belted do you think that the amount of air can stretch/shrink the steel belts?No,the outside circumference will remain the same,you will go the same distance per revolution no matter what pressure is in the tire.

 

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I thought I would share something I noticed on a long trip to Florida last year travling down Interstate 95. I wanted to see how accurate my odometer in my 96 3500 was so I set the cruise at 60 and traveled ten miles at that speed. When I passed my first mile marker I reset the trip meter. When I went by the next mile marker the odometer rolled past 0 right on the money. I thought great, this thing is right on the money. Well I passed the next marker and the odometer said two tenths. Now I'm thinking what the heck. The next marker was back at 0. I then had one to come up at 8 tenths.

Nowadays I don't put alot of faith in the accurace of mile markers. What did the state use to measure the distance between the markers anyway? It would be my guess they used the trucks in which they hauled the mile markers. So now you are relying on the accuracy of this truck and the crew that installed the mile markers. Do these guys take any pride in their work? I don't know, but you are trying to get very technical with this while also making some large assumptions regarding the accuracy of the mile markers.

Just my . 02 cents worth

Wayne

 

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illflem,
thanks. i understand a smaller gear spins faster. i'm not sure how i was originally thinking about this, but i'm sure i made it way more complicated than i needed to. i feel kinda dumb now. what's new.

 

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Originally posted by Cooker:
Please Help. I'm still confused.

pcorssmit stated that with the tires aired down the speedo reads noticeably slower. Can this be correct?

Sorry Ryan, that was my brain fart. The speedo reads faster with the tires aired down. Not the speedo reading really matters with 6 psi, as the truck starts shaking apart at 35 mph.

When going to a larger tire you need a speedo gear with less teeth, since you are replacing the driven (as opposed to the driving) gear. Less teeth on the driven gear will cause it to spin faster, compenstating for the oversize tires. If you were to replace the drive gear instead, then you would need one with more teeth.

Pete

 

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Originally posted by Cooker:
Please Help. I'm still confused.

pcorssmit stated that with the tires aired down the speedo reads noticeably slower. Can this be correct?

Sorry Ryan, that was my brain fart. The speedo reads faster with the tires aired down. Not the speedo reading really matters with 6 psi, as the truck starts shaking apart at 35 mph.

When going to a larger tire you need a speedo gear with less teeth, since you are replacing the driven (as opposed to the driving) gear. Less teeth on the driven gear will cause it to spin faster, compenstating for the oversize tires. If you were to replace the drive gear instead, then you would need one with more teeth.

Pete

 

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Thanks Pete,

Now I don't feel so dumb about how I was thinking.

Ryan

 

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Guys. . you might want to look at the new Peterson's 4 wheel & off road. . page 107 upper right shows a new product from Superlift... called Superlift TruSpeed www.superlift.com
800-551-4955
this devise will recalibrate the spedo for '92 and up. . all you need to know is the original delivered tire size and the new size. .

 

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Here's a formula I've used for buying tires and calculating speed.

rear diff ratio x overdrive ratio
[3. 55 x . 69(auto)= 2. 45

MPH formula
(RPM divide by rearendratio2. 45)x31"tire)
(divided) 336


(1586 Div. 2. 45)x 31"
336

20067. 76
336

59. 8Mph