How accurate is your speedometer?

fr7 · Jan 21, 2018

Hey guys, nothing special, I'm just curious about how true your speedometer is compared to a satellite GPS.
My 98 ranger 4wd manual transmission seems to be way off.
When I had 255/70/16 tires it was reading 2 miles under what the GPS said. A month ago I put new tires on it, this time I went back to the factory size, which the door sticker says 235/70/16, after that my speedometer reads 5 miles under, which to me is quite too much.
I've compared it to 2 different GPS apps on my phone and a Garmin GPS thing, they all read the same speed but my truck still reading 5 miles under, no matter how fast I go, if it reads 65 I'm actually doing 60 according to the GPS. I also watch the speed trackers on the side of the street that tell you how fast you're going and it still 5 miles off compared to that.
Please comment with your readings from your ranger/explorer, I'm just curious to see if it's just me or this trucks are ridiculously off. Thanks..

Spott · Jan 21, 2018

Unless the needle has been moved on the gauge spindle, your speedometer is not 5MPH off; instead, it's 8% off. The faster you go, the greater the difference, and the shower you go, the smaller the difference, until you come to a complete stop and it's perfectly accurate, pointing at 0.

When I bought my '97 used in July, the moron PO had just put a lot of money into some fairly nice new AT tires that were about three sizes too small. I was pretty disappointed in that, but the used car dealership had to sell it for what it was worth. It had 215/75R15 tires on it, and the door sticker specified 265/75R15 tires.

As expected, since the PO never had the speedometer corrected for the smaller tires, the speedometer read about 11% higher than my actual ground speed by GPS.

In November, I got a set of Toyo Open Country AT II tires installed, in the correct size. In addition to giving me more room to work under it, and better terrain/snow handling, now my speedometer is quite accurate. At 80 on the gauge, I'm moving about 81 down the highway, which is about 1% error.

Contrast my older '88 Ranger, which was put together from random donor vehicle parts by the PO, who installed completely wrong gearing and then larger tires. The speedometer was way off, and the speedo cable drive gear wore out within the first year I had it. I did a little research and replaced the cable drive and driven gear with the best combination for the tire size and axle gearing, which was still about 3% high.

You should assume that your error is caused by having the wrong speedometer gearing at the transmission output or the wrong programming in the computer, depending on the age of your truck. It's probably caused by something the PO did, or didn't do, when changing tires or gearing or replacing parts.

With a little math, you can determine what to adjust in order to get it as accurate as possible. You should be able to get within 3% at worst. If you need help, let me know, I've played this game before.

fr7 · Jan 22, 2018

I've never messed with it, I can't tell you the previous owners didn't but I doubt it. It's nothing that bothers me too much, I have other problems to fix on it, it's just weird that it's that far off with stock tires. I might eventually get it fixed and get a hold you, in the meantime I'm waiting for winter to be over, right now I'm snowed in by over 12" of snow and up to 3' drifts behind my truck, can't even make it out of the driveway. There's no way I'm going anywhere in the ranger for a least a day lol.

Elutheros · Jan 22, 2018

Oddly enough, my 1992 Extra Cab pushing 33 inch tires with 3:73 gears is the most accurate of my three vehicle's speedometers. I changed the speedo gear to the white version and checked it against my GPS app and its on the money. I was very happy and surprised since I figured my combination of tires and gearing, trans and engine would never result in an accurate speedometer.

How accurate is your speedometer?

fr7

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fr7

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