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Chuck_Sorensen's avatar
Chuck_Sorensen
Explorer
Dec 12, 2014

Larger Diameter Tires - Odometer / Speedometer

When we bought our van, its stock tires resulted in the speedometer reading high. After replacing the tires with the next size larger tire, the speed indication was dead on. I assumed that the odometer was then accurate along with the resulting MPG calculations from the Ford computer.

Later I observed that the Ford odometer mileage to one of my 100 mile auto club towing radius points was 5% short. I verified the my decades old 100 mile point was still correct using two on-line road trip sites.

Using the 5% odometer correction, the Ford V-8 gas mileage now moves up to an overall 14 MPG. But only if the fuel measuring system and Ford computer calculations are correct.

I wanted to know the gas mileage of my first vehicle, a 1953 Chevy pick-up. I filled a one gallon gas can and carried it in the truck’s bed until I ran out of gas. Then, after noting the mileage on the odometer, I put the gallon of gas into the truck. Using the truck until it ran out of gas, including refilling the gas can, the odometer reading let me calculate the MPG. Even at 23 cents per gallon for gas, economics prevented a full gas tank.

This is another example of marvelous teenage thinking. Like using a hot soldering iron to melt tread into bald tires.

Chuck Sorensen
Buellton, California
2012 PW Excel TS
Class B
  • PSW's avatar
    PSW
    Explorer
    Dec 12, 2014
    Chuck Sorensen wrote:

    This is another example of marvelous teenage thinking. Like using a hot soldering iron to melt tread into bald tires.

    You, too eh? It sounded like a good idea at the time didn't it?
  • bsinmich's avatar
    bsinmich
    Explorer
    Dec 12, 2014
    I like having my GPS. It gives more accurate speeds and is easily switch to kilometers when I go to Canada. A normal speedometer will vary a few mph over normal tire wear. The circumference of any given tire will be less when the tread is worn down than when it was new. GPS doesn't count tire rotation to determine anything.
  • naturist's avatar
    naturist
    Nomad
    Dec 12, 2014
    Over the course of half a century owning cars, I've noted that some vehicles, notably those of German manufacture, have odometers that are spot on, while the speedo reads 3 mph fast. I have been told this is to insure that folks do not get speeding tickets unless they richly deserve them. Needless to say, fitting a different tire size in order to correct the speedo will cause the odo reading to be off. Perhaps this is an implementation of that ancient saw: you can't win.

    So when we bought our newest Tow Vehicle, a BMW X5, I was annoyed but not surprised to see that indeed the owner's manual even mentioned that it was set to read 63 when actually doing 60. And sure enough, it does. Even though it was built in Spartanburg, SC, right here in the US of A.
  • Oldme's avatar
    Oldme
    Explorer
    Dec 12, 2014
    Very few speedometers are spot on.

    Even having the speedometer re-certified on
    PD cars we often had to change tires or have
    the speedometer head reworked.

    Your spot on will change as the tires wear also.
  • Davydd's avatar
    Davydd
    Explorer
    Dec 12, 2014
    In our two Sprinters I noticed the speedometer was always faster than the actual speed but the odometer reading was accurate verified against GPS readings and timing marks on the highway. So I am not sure what you accomplish with larger diameter tires. You could just now be getting inaccurate readings.
  • booster's avatar
    booster
    Explorer
    Dec 12, 2014
    As others have said, the odometers and speedometers rarely are both correct. The explanation I got from an engineer who worked for Ford was that they usually were setup for the Speedo to be about 1-2mph faster than actual to help prevent tickets, and the possible lawsuits to pay them. The odometers tend to be a bit closer to actual, but both of the readings will change with tire wear. They have been this way as long as I have been driving back into the sixties in 50's cars.
  • Gene_in_NE's avatar
    Gene_in_NE
    Explorer II
    Dec 12, 2014
    I keep track of all my odometer readings on all of my vehicles. I really had not given much thought to smaller diameter wheels when worn. That would explain why my 1995 Geo Tracker was slowly getting better MPG figures the more I drove it.

    The log book I keep in every one of my vehicles is handy to see when the last brake job, tire replacement, oil change, or something else was done. I also thought I had an accurate MPG figure as I could add all the gallons and divide that into the total miles travelled.

    Sample below - date, odometer, gallons, tank fill mpg, overall since purchased.
    02/12/12 117,491 7.6 22.4 23.7
    02/19/12 117,652 6.3 25.6 23.7
    03/02/12 117,801 6.6 22.6 23.7
    03/13/12 117,959 6.4 24.7 23.7
    03/23/12 118,135 7.0 25.1 23.8
    03/31/12 118,324 7.1 26.6 23.8
    04/10/12 118,492 7.0 24.0 23.8
    04/17/12 118,665 Oil Change 23.8
    04/17/12 118,667 7.1 24.6 23.8
    04/27/12 118,789 4.9 24.9 23.8
    05/10/12 118,962 7.0 24.7 23.8
    05/25/12 119,155 7.7 25.1 23.8
    05/31/12 119,314 6.5 24.5 23.8
    06/11/12 119,512 7.8 25.4 23.8
    06/17/12 119,693 7.3 24.8 23.8
    07/01/12 119,870 7.4 23.9 23.8
  • Sebtown's avatar
    Sebtown
    Explorer
    Dec 12, 2014
    Thanks for the confirmation of speedometer vs. Odometer. The speedometer on my sprinter can be as much as 4mph off the speed as calculated by the GPS but the odometer is spot on.
  • Bob_N_'s avatar
    Bob_N_
    Explorer
    Dec 13, 2014
    Hi,
    It's very easy to get a pretty accurate calibration of both odometer and speedometer by driving a few miles on the interstate. The mile markers posted on the side of the road are usually quite accurate in their placement, so you can calibrate the odometer by recording the odometer reading at a milepost, then driving 10 or so miles by the posts numbers and recheck the odometer reading. From this you can calculate the percentage of error. Speedometer calibration involves driving at a constant speed for a mile or two and recording the time between mileposts with a stopwatch. At 60 mph it takes 60 seconds to drive a mile, so divide 3600 (seconds per hour) by the number of seconds it takes to drive a mile and the result is your true speed in mph for any one indicated speed. Accuracy will improve if you check the time for 2 or 3 miles.
    Bob N.

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