Fuel and Emissions
Tire-Size-Corrected MPG Calculator
Correct MPG when a tire-diameter change alters the odometer distance. This method assumes the original speedometer calibration was accurate.
Enter one vehicle measurement set
Keep units and reference points consistent before comparing the output with a specification or earlier measurement.
Before interpreting the answer
Correct MPG when a tire-diameter change alters the odometer distance — the comparison is meaningful only after its reference case is fixed.
This method assumes the original speedometer calibration was accurate — that condition defines when corrected fuel economy is comparable with another result.
After recording this output, use the Trailer Fuel Economy to estimate fuel economy and fuel volume while towing a trailer.
Measurement basis and units
For Displayed fuel economy, use the quantity described as fuel economy calculated from the uncorrected odometer — in the vehicle record, use one clearly defined operating interval rather than combining records from different periods.
Original tire diameter is defined here as overall diameter used by the speedometer calibration — keeping that definition intact requires you to measure from the stated reference points and note whether the vehicle or component is loaded.
Installed tire diameter. Measured or specified diameter of the replacement tire — for this measurement, measure from the stated reference points and note whether the vehicle or component is loaded.
Keep this result separate from the task to compare annual fuel volume using gasoline and diesel economy estimates, which is available in the Diesel vs Gasoline Efficiency.
How the quantities interact
In “corrected MPG = displayed MPG × new tire diameter ÷ original tire diameter,” the calculation does not infer a missing vehicle measurement.
No term beyond displayed fuel economy, original tire diameter, and installed tire diameter is introduced in “corrected MPG = displayed MPG × new tire diameter ÷ original tire diameter.”
Putting the number in context
Corrected fuel economy answers “Correct MPG when a tire-diameter change alters the odometer distance.” The additional displays, Distance correction and Corrected miles per 100 shown, are a different view of the same entered measurements.
Use loaded rolling diameter when it differs materially from nominal size — when that condition changes, compare separate calculator runs instead of blending the inputs.
Because this method assumes the original speedometer calibration was accurate, a disagreement between corrected fuel economy and an outside reference should trigger a review of displayed fuel economy and installed tire diameter.
When you need to estimate how much fuel is needed from the gauge reading and tank capacity, avoid adding an improvised field here and open the Fuel Gauge Refill.
What requires independent verification
A change in original tire diameter is not isolated if another vehicle condition changed between the two cases — for original tire diameter, the page specifically expects overall diameter used by the speedometer calibration.
The Driving Speed Fuel Economy Comparison complements this result by calculating how to compare fuel use over the same distance at two measured or modeled cruising economies.
Clarifying the calculation
What measurement source fits Displayed fuel economy when it represents fuel economy calculated from the uncorrected odometer?
Because displayed fuel economy represents fuel economy calculated from the uncorrected odometer, use a source tied to the exact vehicle, component, and operating period described by the other fields.
How does the warning “This method assumes the original speedometer calibration was accurate” affect Corrected fuel economy?
The condition “This method assumes the original speedometer calibration was accurate” is not corrected automatically by the numeric inputs, so create a separate tire-size-corrected mpg case when it changes.
What assumption is expressed by “corrected MPG = displayed MPG × new tire diameter ÷ original tire diameter”?
In “corrected MPG = displayed MPG × new tire diameter ÷ original tire diameter,” displayed fuel economy and original tire diameter are treated as parts of one vehicle case.