how much does it cost to turn off the ignition to save gas?

Hybrid cars already do that

Heck, golf carts have done that for over 40 years. Gas golf carts would start when you pressed the gas pedal.

The only difference is hybrid cars are using the fairly large, direct-drive traction generator to spin the engine, electronically reversing it to make it a motor, with this reversal being done by large semiconductor controls acting as a Variable Frequency Drive (VFD). Which lasts forever, until it doesn't.

Take the 2004 Chevy Silverado pickup hybrid, please... That's pretty much all it did, unless you count "create 120V at 30A to power your worksite all day". By shutting down the prime mover at stops, they gained about 10% fuel economy.

So instead of a big solenoid (or a Bendix drive) that engages the starting gear to the flywheel, it's just always engaged - you aren't wearing out the Bendix or chipping flywheel teeth, there are no brushes or commutator to wear down. It's always mechanically connected and is switched by a VFD.

So yeah, if you're willing to endure the starter wear, have a field day. It's not that bad, especially if you know how to pull a starter on your car, and who to send it out to for rehab.

The starter is probably one of the most robust and durable parts on any car and usually has no problem starting the car regularly. Although they are known to wear out from time to time it's usually a pretty easy fix and new ones can be found from china for dirt cheap if it does end up needing to be replaced. Also it takes about 10-30 seconds of idling to use the same fuel as starting with newer cars more than likely being closer to the 10 second figure. So there will be a savings on fuel especially if you spend alot of time street driving and idling at red lights. Basically it depends on your driving style but more than likely it will end up saving you money in the long run just with the added risk of getting stranded when the stater does give out at some point, which to be fair is always a possibility regardless... Personally I wouldn't do it because I'm too lazy and paranoid to go through turning off and on the engine everytime but if you go strictly by the numbers I'm almost positive it will end up saving you money in the long run.

This is available in Europe and Japan on some non-hybrid models (including all shapes and sizes of car from Bentleys to Fiat 500s), with the engine stop/start function linked to the clutch and gearstick operation in manual transmissions, so there is no need to manually stop/start the engine. The driver can override the engine stop/start by keeping the clutch pedal depressed when the car is stationary.

For long life, engines need some new design features for this, for example electric-powered coolant pumps and fans which operate when the engine is off, and dry lubricant coatings on the crankshaft bearings to reduce wear on every engine start until the working oil pressure is reached. Fuel consumption is typically improved by about 10% for stop-start driving.

Some non-hybrid models use regenerative braking to recharge the cranking battery in addition to the alternator, to further reduce fuel consumption.

There are several aspects of turning off the ignition to save gas.

Firstly, if you don't disengage the clutch, turning off the ignition when engine braking doesn't help at all. The car ignition and fuel injection computer does already that -- detects you're engine braking and turns off the gas. It would only help on a carburetor car, not available as new anymore.

So what could help are:

• Turning off the engine when stopped
• Turning off the engine AND disengaging the clutch when slowing down

An modern compact idling engine uses about half liter of gasoline per hour. An American V8 could obviously use more than that. So if your average journey of 15 km has 4 minutes waiting at stoplights, it saves 0.03 liters per 15km or 0.2 liters per 100km. Not much.

One problem is that if the car has not been designed as a stop/start car, the extra wear and tear on the starter motor could cause an early failure. Most cars have been designed to last 300 000 km for average use. If you stop and start the engine once per 2 km, you probably are increasing the starter motor use by nearly one order of magnitude (10x). It will fail early. Also the battery might not like it -- stop/start cars have at least enhanced flooded batteries with more sturdy plates and in some cases even absorbed glass mat batteries. A traditional flooded lead-acid battery wouldn't work long in a stop/start car.

If you turn off the engine not only when stopped but also when slowing down, you can increase the amount of time your engine is off. For example a Toyota hybrid car does exactly that. A Toyota Corolla hybrid with 2-liter engine uses 4.6 l / 100 km gasoline, whereas a 1.2-liter turbocharged non-hybrid Corolla uses 5.9 l / 100 km. So the savings potential could be 1.3 liters / 100 km. However, do note only some of this savings potential is achieved in hybrid cars through stopping the engine when coasting. Most of it actually comes from regenerative braking and operating on battery when driving with low power, and if the battery becomes flat, charging it with high power so the engine is operating at its most efficient. So because of that I'd say a system that turns off the engine when coasting could save perhaps 0.4 liters / 100km. Not 1.3 liters / 100km however. You also need to learn a new way of slowing down -- slowing down without engine braking, which would annoy other drivers who slow down with engine braking.

Now should you turn off the engine when slowing down? No! Absolutely not! You'll turn off the power for power brakes and power steering in most cars if doing that. You could also by accident turn the ignition key too far, activating the steering lock (VERY DANGEROUS!). The power brakes will operate for a while using the stored vacuum, but if you deplete that, you'll be very surprised... and crash to the rear of the car in front of you.

However, some modern automatic transmission cars automatically turn off the engine when slowing down, perfectly safely. They have been engineered to be safe. Don't emulate this behaviour on other cars!