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Discussion Starter · #1 ·
When you have 33 inch tires on a standard stock 88 rc how much is your speedometer going to be off?
 

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new tire height divided by stock tire height minus 1 = percentage the speedo will be off.

33" / 28" = 1.18 - 1 = 18 %

in this case, the speedo will read 18% low, so when you were doing 60 on the speedo, you are really doing 70
 

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Discussion Starter · #3 ·
Thanks RamChargerMan!
 

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So how do you fix this. I have an 82 with 3/4 ton axles out of an old power wagon. And my current gear ratio is 4.10 and I run 35's. My speedo is way off and I don't know how to fix that.
 

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The way to fix it is to replace the speedometer gear in your transfer case. You will need a few numbers to calculate it properly.
  You need to know what gear you have now. Pull the gear iut where the cable goes in. It will have a number stanped on it.
  The you need to figure how far off your speedometer is. If you can drive a set distance and see how far off your odometer is or use a gps ( I use one on my phone) and see how far off your speed is.
  Google "dodge speedometer gear" and you will find a wide variety for sale from speedometer shops.
 

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[smg id=2056]
 

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Try to figure out where you're at before you change anything.

My speedometer is closer to correct with true 33" tires than it was with stock 235/85R16s. Gf's Jeep got closer to correct when I upped the tire size too.
 

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u2slow said:
Try to figure out where you're at before you change anything.

My speedometer is closer to correct with true 33" tires than it was with stock 235/85R16s. Gf's Jeep got closer to correct when I upped the tire size too.
Mine was just about dead-on when working. If they had to pick between low or high they chose high so larger tires might not throw it all off. That and most of these trucks have had ~32" tires for many years, it used to be much cheaper and easier to change the speedo gear for tall gear/tire selections than it is now.
 

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Thanks for the info, I have some home work. My problem is though it's never a set speed off. It varies and the faster I go the larger the gap between what my gps says vs what my speedo says. And my speedo read fast, I thought it was suppose to be the other way around?
 

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Thats the way speedos work. They are all very accurate at zero MPH, very close at 5, and as the speed increases, they get further off.

Think of them like a graph, and all the lines start at the right bottom corner, but have a different angle as they go to the right.
 

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Mudrunner said:
Thanks for the info, I have some home work. My problem is though it's never a set speed off. It varies and the faster I go the larger the gap between what my gps says vs what my speedo says. And my speedo read fast, I thought it was suppose to be the other way around?
Steeper gears/shorter tires make speedo read faster than you're going.

Taller gears/taller tires make speedo read slower than you're going.
 

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And just to clarify I have 4.10's and 35's, so in theory I should be going faster then my speedo says ya? And since it's not does that mean I have another issue? Thanks again guys for the replies, not trying to drag this out just want to learn all I can.
 

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In addition to what 'burban' said above about the error......another way to think about it is your speedo will be off a percentage of actual speed, not off by a fixed amount (i.e. 10% at 70 mph is more of an error than 10% of 35 mph).
 
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