I did the math

 This last weekend we did a bit of shuffling with our cars. I've stopped driving the small car and I'm now driving the big 4WD. So I thought "wouldn't a motorbike be a cheaper option"?

First, I need to lay down a few assumptions:

• I already have the big 4WD, so I'm not comparing purchase cost of one vs the other, I'm comparing potential savings on running costs.
• I travel roughly 200km per week for work. I'll just use this figure for my math, as most other travel I do is with the family so we'd be in one of the cars, not on the bike.
• The 4WD is listed as using 9.5L/100km but that's when it was new. Due to the accessories we've added and the age of the car, I'll assume 10L/100km (though it is probably more).
• A learner approved bike (under 250cc) is generally listed as having an average consumption of 3L/100km - I'd probably buy an older bike, but I tend to be a cautious driver so we'll go with this figure.
• We'll assume an initial purchase price for the bike of $2,000 - learner bikes vary by price, but odds are I'd buy one second hand and even if I got one cheaper I'd still need to do some repairs, rego transfer, insurance, etc.
• We'll assume safety gear is $1,000 - can save money by buying second hand (except for the helmet) and any money saved on the gear would probably go on the bike anyway.
• Cost of fuel is currently $1.20/L we'll assume it'll stay at or near that price (lol).
• And final assumption - we'll ignore all other costs such as servicing. The bike service might be more expensive, but cars can have things go wrong too, so it's much of a sameness.

On the above assumptions, per 100km the 4WD is costing $11.52 in fuel, and the bike would be $3.60 - so over an average 200km week, the net savings for riding the bike over the 4WD are $15.84 in fuel.

That means that (assuming $3,000 purchase cost on bike and gear) it would take 189 weeks (or 47 months or just shy of 4 years) to break even. That's assuming always and only using the bike to get to work and back, assuming the cost of fuel remains the same and assuming no unexpected expenses (like getting knocked off the bike by some idiot in a car).

The more likely scenario that I can see would be using the 4WD on Mondays and Fridays, and only using the bike on Tue / Wed / Thu to get to work and back. In that scenario, the savings drastically drop to $4.94 per week - and to break even at that value it comes out to 12 years!

And the savings will be even less than that, because in the 4WD only scenario I'd be using a bit more fuel, but in the 4WD + motorbike scenario I'd be saving a bit in fuel but having to pay registration (and insurance) on two vehicles.

So the math says no, no motorbike. Also, my wife said no - and she has the final say anyway.