Speedometer Question

[theslawdawg]

Hello Everyone,

Here is my question for the day. It appears my speedometer is way off. I normally don't drive 85 down the highway and get passed by every vehicle. At first, I thought it was the small tires that were on it when I first purchased the vehicle. However, after replacing my tires to a reasonable size (275/60/15) I noticed the speed is still way off. At low speeds, say 35 mph, it is accurate. Just not at the higher speeds (past 55mph). Can someone please tell me how to correct or calibrate? Thanks in advance.

[71blksuper4x4]

You could easily swap out the speedometer. Trevor

[sameyrasmea72]

If it is way off it is probably a bad speedo, if is is say 10 mph off at highway speed you could change the gear where the speedometer attaches to the tranny.

[Longhorn Man]

Ask your local inspection station, the last I heard, Cali will tell you how far it is off for no charge. That'd the first step in this.
After that, you'll need to figure what speedo gears you'll need. There's planty of online charts that'll tell you what gears you need. (you'll need to know the height of your rear tires for the formula) Someone is sure to post a link, I think there was a nova site with a good chart, offers part numbers, gear colors, and an easy to use chart.

[425HP409]

If the speedo reads ok at 35 but is way off after 55, the problem is probably the speedometer head itself. If the gears were the problem, it would read off all the way up but be proportionally off as you got faster. Such as if it read 30 and you were going 25, it would read 60 when you were going 50, and read 90 when you were going 75, etc

[Wrenchbender Ret]

First compare the odometer reading with the mile markers on the freeway. Like when you pass a marker look at the 10th's. If it's like 5.3 watch it as you pass the next 2. If it's like 7.5 then you are 10 per cent fast. Change the gears like the previous posts. If the mileage is correct & your speed is off, then the speedo head needs to come out. If you are not near a freeway mark the distance with a vehicle you know is right. Like the tree in front of the Smiths house to the pole at the Joneses.

[theslawdawg]

Thanks all, for the information. I really appreciate it.

[piecesparts]

Do you know what you rear end gear ratio is? What transmission do you have? Do you have a tach in the truck? If so, do you know what RPMs you are running at what you think the truck is going at 60 MPH or 70 MPH? With that information, we can calculate the speedo's differentail. I already have your tire size and that gives me a basic start.

A basic way is to take a look at your speedo and clock off a mile on a highway. Do this at various mile markers to ensure that you have a combination of 2 or 3, that way you can get an average. Take a look at your Odometer and see haw many tenths are registering for that mile.

If a speedo is off any amount such as 2%, the difference will grow with the speed that you go, just due to the design of the process. So you could be accurate at 35 MPH, but off at 70 MPH. I usually try to calibrate my speedos at what I would drive on the highway, so that I am close for the moments of meeting "Johnny Law".

The speedo may just fine, but the speedo gears in the tranny may need changing, to match what you are using for gearing or tires. A good tranny shop or even a search on the internet will give you sources of gear numbers and desriptions for solving the dirve or driven gear needs.