You May Have Underclaimed Your Mileage Since April 2026: Here's the Simple Fix

If you've been keeping a mileage log since the start of the tax year, you probably had one specific thought when you heard about the rate change: I've been logging all of this at 45p.

Six weeks of journeys, diligently recorded at the old rate, and now the rate has moved. It's a reasonable thing to feel mildly irritated about. The new 55p rate applies from 6 April 2026, but HMRC didn't announce it until 21 May 2026. That gap is not your fault, and it doesn't create a problem, because the fix is genuinely straightforward.

You don't need to amend a previously filed return. You don't need to contact HMRC. You don't need to do anything right now other than update your records. When you file your 2026/27 Self Assessment, you claim 55p per mile for the whole year, including April and May. That's it.

Why This Happened and Why It's Fine

HMRC backdated the increase to 6 April 2026 when Chancellor Rachel Reeves announced it on 21 May. The decision to backdate to the start of the tax year was deliberate, so sole traders would get the full benefit for 2026/27 rather than a partial year. The unintended consequence is that anyone who started logging mileage from April was working with the wrong rate for six weeks.

The reason there's nothing to panic about is that the 2026/27 tax return isn't due until 31 January 2028. No return has been filed at the wrong rate. The only thing that's happened is that some people have entries in their mileage log that say 45p when they should say 55p. You have until January 2028 to correct that, and the correction is a recalculation, not a formal amendment.

What the Underclaim Is Worth

The shortfall is 10p per mile for however many qualifying business miles you drove between 6 April and 21 May 2026. Here's what that looks like at different mileage levels for the six-week period.

The individual amounts won't change anyone's life. But they're yours, they're claimable, and the correction takes about five minutes. If you're a higher mileage driver who covers 1,000-plus business miles a month, the six-week shortfall at 10p per mile is meaningful enough to be worth correcting properly.

What You Actually Need to Do

Go back to your mileage log and find all the business journeys recorded between 6 April and 21 May 2026. If your log shows the calculated expense amount rather than just the miles, update those entries to 55p. If it only shows miles, the recalculation happens once, when you total everything up for your Self Assessment.

When you file your 2026/27 Self Assessment, enter your full-year mileage claim calculated at 55p throughout. You don't need to separate the pre-announcement and post-announcement periods on the return. The total figure goes in as a single business expense in the vehicle and travel costs section of the SA103.

What If Your Records for April and May Are Patchy

If you weren't logging consistently before the announcement and your April and May records are incomplete, don't leave those miles out. Reconstruct what you can from diary entries, invoices, client emails and any other evidence of where you were and why. A reasonable good-faith estimate based on your typical business travel pattern is defensible. A blank record followed by a large annual claim is not.

The earlier in the year you reconstruct incomplete records, the more accurate the reconstruction will be. Diary entries from April are more useful now than they will be in January 2028.

The Mileage Allowance Relief Angle for Employed Sole Traders

If you have a day job alongside your self-employment and your employer reimbursed you for any business mileage in April or May at the old 45p rate, you may also be able to claim Mileage Allowance Relief on the 10p shortfall. This is a separate claim, made in the employment section of your Self Assessment return rather than the self-employment pages, and it covers the gap between what your employer paid and the approved 55p rate. It's not a lot of money per mile, but if your employer regularly reimburses you for substantial business travel it's worth including.

One More Thing Worth Checking

A bigger issue than the April and May correction for many sole traders is whether they've been claiming mileage at all. The rate change has prompted a lot of people to look at their records and realise they have months of qualifying business journeys they haven't been tracking. Regular post office runs. Sourcing trips. Client visits. Each one a legitimate claim at 55p that disappears if it's never logged.

If that's you, start the log now. An imperfect log from June onwards is significantly better than no log at all, and the tax saving on a full year of properly tracked mileage is worth considerably more than the April and May correction.

Our full guide to the 55p mileage rate covers what qualifies, how to log it, and how to claim it on your Self Assessment. And if the mileage adjustment has you looking more carefully at your overall tax position for the year, our guide to saving the right amount covers how expense claims like this affect your monthly set-aside figure.

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