Why gas money is almost always underpaid
Most carpools settle on a round number — twenty dollars a week, a tank a month — and the driver quietly loses money. The reason is that riders price the trip on fuel alone, and fuel is roughly a third of what a mile actually costs. Tyres wear, brakes wear, oil changes arrive on a mileage schedule, and the car's resale value drops with every mile on the odometer. None of that shows up at the pump, so none of it gets split.
The correction is a per-mile allowance on top of fuel. The most defensible anchor is the IRS standard mileage rate, which the Internal Revenue Service publishes each year as the deductible cost of operating a car for business. For 2025 that rate was 70 cents per mile, and the IRS resets it annually by notice — check the current figure before you quote it. That rate is designed to cover fuel, maintenance, tyres, insurance and depreciation together, so if your fuel alone costs about 12 cents a mile, the non-fuel remainder is a sensible ceiling for the wear allowance, and something in the 15 to 25 cent range is what most private carpools settle on. The calculator's default of 20 cents sits deliberately in the middle of that band.
The second thing this calculator fixes is who is in the pot. There are two honest conventions and they give different answers. If the driver takes an equal share, the cost divides by riders plus one and the driver ends up paying exactly one share out of pocket — the same as everyone else, which is fair when the group rotates drivers. If the riders cover the whole cost, the cost divides by the riders only, the driver pays nothing, and the arrangement compensates the driver for supplying the vehicle. Pick one deliberately and state it, because the difference between them is large: with three riders, the per-rider figure changes by a third.
The formula, term by term
Start with fuel. Divide the round-trip distance by fuel economy to get gallons, then multiply by the pump price. Use the real economy from your trip computer, not the window sticker: a car rated at 32 mpg combined that runs a short suburban commute in winter frequently returns 26. That gap is 20% of the fuel bill.
Add wear, which is distance multiplied by your per-mile allowance. This term is linear in miles and completely independent of how many people are aboard, which is why long carpools are where wear matters most. On the 44-mile default round trip, a 20-cent allowance is $8.80 — more than the fuel.
Add tolls and parking as flat per-trip amounts. They are not per-mile, so do not roll them into the wear rate; keeping them separate lets you see how much of a rider's share is fixed cost that would not fall even if the car got twice the mileage.
Divide the total by N, the number of people sharing. This is the only place the arrangement enters the math. The driver's remaining cost follows as an identity: trip cost minus riders × share. When the driver takes a share, that remainder is exactly one share, because the trip cost is divided into riders + 1 equal parts and the riders return all but one of them. When the driver does not take a share, the riders return the entire trip cost and the remainder is zero. Both cases fall out of the same expression; there is no separate rule.
Monthly figures are just the per-trip share multiplied by trips per month. Five days a week works out to about 20 to 22 round trips, and if the group takes turns driving, run the calculator once for each driver's car and average the monthly figures.
Worked example: 40-mile round trip, three riders, one toll
Four colleagues share a 40-mile round trip. The car returns 25 mpg in real conditions, fuel is $3.50 a gallon, the wear allowance is 20 cents a mile, and there is a $2.00 bridge toll each way that only applies inbound, so $2.00 per round trip. Everyone rides 20 times a month and the driver takes an equal share.
- Fuel used. 40 ÷ 25 = 1.6 gallons.
- Fuel cost. 1.6 × $3.50 = $5.60.
- Wear. 40 × $0.20 = $8.00.
- Tolls and parking. $2.00 + $0.00 = $2.00.
- Total trip cost. $5.60 + $8.00 + $2.00 = $15.60.
- People sharing. 3 riders + 1 driver = 4.
- Share per person per trip. $15.60 ÷ 4 = $3.90.
- Per rider per month. $3.90 × 20 = $78.00.
- Driver's remaining cost. $15.60 − (3 × $3.90) = $3.90 per trip, or $78.00 a month — one share, exactly like everyone else.
- Cost per mile per person. $3.90 ÷ 40 = $0.0975.
Now change one thing. If the riders cover the whole cost instead, the same $15.60 divides by 3 rather than 4: each rider owes $5.20 a trip and $104 a month, and the driver's out-of-pocket cost falls to zero. And notice the wear line: it is $8.00, larger than the $5.60 of fuel. A carpool that splits only gas is splitting 36% of what the trip costs.
How to read the result
Compare the cost per mile per person against solo driving. A person driving themselves pays the full per-mile cost — fuel plus wear, roughly 32 cents a mile in the worked example above. In a four-person carpool that same person pays 9.75 cents. The ratio is simply 1 ÷ N, so a three-person carpool cuts each person's driving cost to a third and a four-person carpool to a quarter. That ratio, not the dollar figure, is the argument for carpooling at all.
Check the driver's remaining cost against your intent. If the group agreed the driver should be compensated for supplying the car, that figure should be zero or negative-in-spirit — and if it is showing one full share, the driver-shares box is ticked when the group did not intend it. If the group rotates drivers, one full share is exactly right, because over a month everyone drives some of the time and the arrangement is symmetric.
Sanity-check the monthly figure against a transit fare. If a rider's monthly share lands near or above a local monthly transit pass, the carpool is not saving them money and you should look at the wear rate and the trip count for an error before you look at the pass.
Treat the per-mile allowance as the honest lever, not the fudge factor. Setting wear to zero does not make the cost disappear — it moves the whole of it onto the driver. If riders object to paying for wear, the productive conversation is about the rate, not about whether wear exists.
Monthly cost per person for a 40-mile round trip
| Fuel economy | Trip cost | 2 occupants | 3 occupants | 4 occupants |
|---|---|---|---|---|
| 20 mpg | $15.00 | $150.00 | $100.00 | $75.00 |
| 25 mpg | $13.60 | $136.00 | $90.67 | $68.00 |
| 30 mpg | $12.67 | $126.67 | $84.44 | $63.33 |
| 40 mpg | $11.50 | $115.00 | $76.67 | $57.50 |
Read across and the effect of fuel economy is modest, because the $8.00 wear component does not change with mpg. Read down a column and adding one more occupant does far more for each person's cost than doubling the car's fuel economy.
Mistakes that make a carpool split unfair
- Splitting fuel only. Fuel is typically a third to a half of a mile's true cost. Everything else lands on the driver's future repair bills.
- Forgetting the pickup detour. Collecting riders often adds 10 to 20% to the distance. Enter the distance the car actually travels, not the driver's own commute.
- Using the window-sticker mpg. Short commutes, cold starts, roof boxes and winter fuel blends all push real economy below the rating.
- Charging the same when the group shrinks. If one rider drops out, the per-person share rises for everyone left, because the same trip cost divides by fewer people. Re-run the calculator rather than keeping the old number.
- Ignoring parking that only the driver pays. If the driver pays to park and the riders do not, that cost belongs in the trip total and should be split like any other.
- Letting the arrangement drift. Decide up front whether the driver takes a share, write it down, and re-check it whenever the rotation changes.
Money changing hands can have tax and insurance consequences
Casual cost-sharing among colleagues is not a business, and reimbursement at or below actual cost is generally treated as sharing an expense rather than earning income. But two things are worth checking before a carpool scales up. First, most personal auto policies cover ordinary cost-sharing and exclude carrying passengers for hire — read the exclusions if riders are paying more than their share of costs. Second, formal vanpools and employer-run programmes have their own rules, and employer-provided commuter benefits are governed by Internal Revenue Code section 132(f). If your employer subsidises the carpool, ask how the subsidy is delivered before you set the rider price.
When a different calculation is the right one
This calculator prices a carpool you have already decided to run. Three neighbouring questions need different math. If you are deciding whether to drive at all on a given day, the rideshare versus driving cost calculator puts a fare against your all-in driving cost including parking. If the real comparison is a year of commuting rather than one trip, the transit pass versus driving commute calculator annualises both sides and adds the pre-tax commuter benefit. And if parking is the dominant cost, the monthly parking pass break-even calculator finds the number of days that justify a monthly garage pass — a pass that a carpool can often share.
For occasional long trips rather than a daily commute, the same per-mile logic applies, but the timing question usually matters more than the money: the road trip drive time calculator handles stops, traffic and overnight legs. Whatever the trip, check the tyres before a cold-weather drive — the tire pressure temperature change calculator shows how much pressure a temperature swing removes, and underinflated tyres cost fuel on every mile you are about to split.
Know what this model leaves out. It uses an average number of riders rather than tracking who rode which day, so a group with wildly irregular attendance should run it per rider using that rider's own trip count. It does not price the driver's time, the value of an HOV lane time saving, or the cost of a second car sitting unused. And it assumes the wear rate you enter is right — that number is an estimate about your specific car's maintenance and depreciation, and no calculator can derive it for you.