I'm a motorcyclist and an accountant, so the Motorcycle Action Group's (MAG) campaign to increase the approved motorcycle mileage allowance caught my attention from two directions.

The allowance has been 24p per mile since April 2002. That's nearly a quarter of a century without an increase, despite everything that's happened to the cost of fuel, servicing, insurance and generally keeping a vehicle on the road.

Meanwhile, the approved rate for cars and vans increased from 45p to 55p for the first 10,000 business miles earlier this year. The motorcycle rate stayed exactly where it was.

Now, I don't think motorcycles and cars should necessarily attract the same mileage allowance. They have different running costs, and it's reasonable for the tax system to recognise that. But there's a difference between recognising those differences and leaving one rate untouched for 24 years while another is reviewed and increased.

MAG is asking the Treasury to increase the motorcycle allowance to 45p per mile in the Autumn Budget on 28 October. Having looked at its arguments, I think MAG has made a reasonable case for the increase, and it's a campaign I'm happy to support.

I should probably declare my interest here. I use my motorcycle for some business journeys in my accountancy practice, so I benefit from the mileage rules myself. But I don't cover particularly high business mileage, and the financial difference to me personally would be fairly modest. My interest is more in whether the system makes sense and treats different forms of transport consistently.

There's also a distinction worth making between a mileage allowance and the amount of money somebody actually receives. An employer can reimburse an employee for qualifying business mileage at HMRC's approved rate without that payment normally being subject to tax. Where an employer pays less, the employee may be able to claim tax relief on the difference, which isn't quite the same as receiving the missing mileage allowance.

For a self-employed person using the simplified mileage method, the allowance reduces taxable profit rather than providing a reimbursement. So an increase from 24p to 45p wouldn't automatically put another 21p per mile into everybody's pocket.

What I particularly like about MAG's campaign is that it hasn't simply demanded the same rate as cars. Instead, MAG has put together research and modelling to support its proposal, including estimates of how much the change might cost the Treasury.

Its modelling suggests that increasing the rate to 45p could cost the Treasury no more than £34 million a year, even under its higher-cost scenario. That's MAG's estimate, rather than an independently verified Treasury costing, but at least it gives us something specific to examine and challenge.

And I think that's a worthwhile approach. If we're going to argue that an allowance is unfair, we should be prepared to explain what a fairer allowance might look like and what changing it could cost.

There's another aspect of the policy that interests me. We tend to think of mileage rates as if somebody at the Treasury has worked out the average cost of running each type of vehicle and arrived at a carefully calculated figure. In reality, the Treasury considers a range of factors when setting these rates, rather than applying one fixed cost formula.

That seems entirely reasonable. Different vehicles have different costs, and there are wider questions about what forms of transport the government wants to encourage.

Motorcycles generally occupy less road space than cars, particularly when parked, and can offer advantages in congested areas. They're not without environmental or other drawbacks, and I certainly wouldn't suggest that every motorcycle is especially economical to run. A large touring motorcycle and a small commuter bike are rather different propositions.

But if wider policy considerations form part of the decision, it seems reasonable to ask whether motorcycles' potential contribution to reducing congestion and demand for road space is being given appropriate consideration.

There's an interesting comparison with bicycles, too. Their approved mileage rate is 20p, just 4p less than motorcycles. I'm certainly not suggesting that the bicycle allowance should be reduced, and encouraging cycling is a perfectly legitimate policy objective. But it does raise the question of whether we're applying a consistent approach to different forms of transport, particularly if these allowances reflect wider policy considerations as well as running costs.

Ultimately, I'd like to see mileage allowances based on current evidence, with a clear explanation of the reasoning behind them. If that produces different rates for different vehicles, fine. What is harder to justify is a motorcycle rate that hasn't changed since 2002, particularly when the car rate has recently been increased.

MAG has made its submission to the Treasury, and the decision now rests with the government. Whether the Chancellor accepts the proposed 45p rate remains to be seen.

I hope the campaign succeeds. Not because I expect to make a fortune from claiming a few extra pence on my own business journeys, but because I think there's a good case for reviewing an allowance that has been overlooked for far too long.

You can read MAG's campaign and the reasoning behind its proposal here:

https://mag-uk.org/a-pay-rise-for-motorcyclists-its-down-to-the-budget-now/

With the Budget due on 28 October, we shouldn't have too long to wait to see whether the Treasury agrees.