I've just recently done the math on this because we bought a home closer to where I work, and take possesion at the end of April. It took some convincing, but my wife has agreed we should sell my truck, and I'll become a full time bike commuter (we'll keep her [our] car). The numbers are a bit big because we're getting rid of a 1 ton truck we bought for a slide-in camper we're also selling.
My annual savings estimate is:
Fuel (Just going to work): $1492
Routine Maintenance: $130
Insurance: $2200 (gotta love Vancouver)
Car Washes: $90
Wipers and wiper fluid: $75
Tires $200 ($1000 per set averaged over 5 years)
Total: $4187
If we disciplined enough to take those savings and apply them to the Mortgage, it would more than double your savings.
I didn't factor in financing costs outside the mortgage pre-payment because the truck is paid for. I also didn't factor in any kind of savings besed on what we do with the proceeds of the sale of the truck. My math shows that for every time I bike to work after selling the truck, I would effectively be paying myself a little over $42/day.