90 Dakota speedo correction- How to?

OK I discovered that according to the little mileage signs posted on the Interstate, the odometer on my 90 Dakota chassis reads 7.5 miles when the little signs tell me I've traveled 10 miles. Thinking the first ones I checked was a fluke, I checked several more times at different locations, all were consistent.

This is a 90 Dakota with what I believe is the AX15 5 speed manual trans (the trans and bell are two pieces and it came with a 3.9 V6).

Now that I know its off, I'm sure I will start collecting tickets.

How do I go about correcting this? Gene

Change the speedo gear.

Do you know what gear I will need? Where do I get speedo gears?

You need to measure the height of your tires and know your rearend ratio. Then go to the chart in the tech archives here and match that info to get the correct gear tooth count. Then try Ebay or put an ad in the parts wanted section here. If you try a google search here for the way to measure your tires you will get the info you need.

Have you put really tall rear tires on it? It won't get you tickets if reading real slow.

Actually, I've put shorter tires on it. Current tires are 225 x 75 x 15 on 6" wide wheels, the old tires were 235 x 75 x 15. When I originally bought the truck it had 4 different tires on it, ranging between 205 70 15 and 235 75 15. According to the door sticker on my 93 Dakota 4x4, a 235 x 75 x 15 was an optional tire, there was no tire sticker anywhere on the 90.

I'll get tickets much faster if its reading slow, If I'm actually driving 60, but the speedo says I'm doing 50, I will speed up until the speedo says I'm doing 60 and I will actually be doing over 70. Even if I know 50 is actually 60, I will speed up, I just can't refrain from driving what the speedo says is under the limit. I got to get this fixed. Gene

Okay, gotcha.

A shorter tire will make the speedo read faster. Ron

Last your I put shorter tires on it then it had before. Its going to be a a few years before I'm changing the tires again.

I'm going to have to dig into my speedo reading. The other day I went past one of those city speed limit things that tells you what the limit is, and how fast you are actually going, according to the radar inside the box. According to it, I was traveling 30 and my speedo said I was going 30. If my speedo is accurate at 30, it can't be far off at 55, can it?

The next time my wife and I are on a stretch of road traveling at a steady speed, I will have her pull up the car's speed on GPS on her phone. Is there a simple location she can get that info from, like an app she needs to download? Gene

If you have a smart phone get the speedo app. That or WAZE.