Speedometer Calibration

Okay - with the new 32" tires on Lou, I'm sure the speedometer is off a little. Normally, it wouldn't matter much to me, but I use the odometer as a fuel gauge , so I would like to get an idea of how much it's off.

With the larger tires, the speedometer will read slower than I'm actually traveling, correct? Hypothetically, if the speedo says 50 MPH, I would be traveling at 55 or 60? So, the mileage will be slowed down a little on the odometer? If I got 15 MPG before, I might only get 14 MPG right now...

Anyone have a link to a formula to determine how much it's off? I know it'll be a percentage, but I would like to know what my top speed is, without having to get my wife to follow me around and make hand signals as to our current speed...

 

Try the calculators on this site:

http://www.4crawler.com/4x4/4LoCalc.shtml

Stan

 

Thanks Stan!!!

Anyone know the original size of the tires on a '71 CJ5 (non-Renegade)?

 

Tim you can always do what I did, I was coming home one night and saw a speed trap sitting on the side of the road up ahead and pulled off, went around this store and came out on the back side of the cop car, got out walked up and tapped on the passenger side glass, scared the living crap out of the poor cop , told him my speedometer wasn't calibrated and wanted to be clocked for a comparison to change the speedo gear, drove by him in my Jeep with the speedo at 50mph, turns out I was actually doing 54mph, just calculated the % and use it. When I redo my dash and get mine painted I intend to put in an electronic speedometer that I can calibrate.

 

I'm running about 4-5 mph faster than I see on my doohickey up there on the dash thingey.

 

If you have a friend with a hand held GPS unit, use that and you can get an exact reading of speed. When I put an electronic speedo in my M715, we used the GPS to set it very accurately.

Stan

 

Thanks guys - All are good suggestions!!!

Using that calculator, I guessed at the original tire size being 28". If so, when the speedometer says I'm doing 50, I'm really doing 57. That matches up pretty well to what you said - considering I have tires 1" larger than yours.

Thanks to everyone!!!!

 

This is also backed by our last trip. Worth is running 31's and while following me in my turd burner showed to be 'bout right.

 

and it goes faster when you push harder on the slanty pedal? R)

 

If you have an interstate highway nearby, you can drive to what you feel is about 60 mph. ANd the mile marker post should be a minute apart. If they come up quicker than 60 seconds you are going over 60. And if they come up after 60 seconds you are going slower than 60. I have done that a few times over the years just to check my speedometer.

 

As I suspected the difference in changing tire sizes is not exactly a linear relationship. It seems that the difference in the actual speed and the speedo reading is greater the faster you go(on bigger tires). All this is to say that we can't just say, "oh my speedo is about 5 MPH off" because how far off depends on how fast. What led me to post this was many of us do use our odometer as our fuel gauge. Without correcting the gear for the speedo there is no telling what the miles actually are and how much fuel we've used.... Food for thought. Check me on the conversion site. MM :v6:

 

Or you can just run your odometer against 10 miles worth of mile markers and see how far off you are at the end of the 10 miles. Should be good for your purposes since you are interested in distance traveled before your tank runs out of fumes.