[Ethereum] Change a pending transaction’s “Gas Limit” & “GWEI”

Yes, a transaction's gas limit can have an effect on the time it takes to get mined into a block.

Larger gas limits can be more enticing to miners since their potential fee is gas limit * gas price. However, this is a potential fee, since unused gas is refunded. Thus, it is possible that transactions with very high gas limits, can be prioritized lower by miners and take longer to get included in a block.

Consider the case where there's 11 transactions:

• 1 transaction T has a gas limit equal to the block gas limit.
• 10 other transactions have a gas limit that's 1/10th of the block gas limit
• All 11 transactions have equal gas price

If you're the miner, which of the transactions would you include first?

By choosing T first, you have reached the block gas limit and cannot include any other transactions in the block. If T actually only used 1/10th of the gas, you lost the opportunity to include other transactions and collect their fees. In this limited example, there is more likelihood of getting more fees by including the 10 transactions first.

That's some theory and the default behavior of clients like Geth and Parity would require examining their code. But miners can choose to modify the default behavior.

The gas limit should be as high as your transaction will consume, and no higher. The amount of gas a transaction consumes depends on what it does. (Bear in mind that if your transaction results in gas refunds, for example because it deletes storage data, you still need to account for it in the gas you supply.)

Most miners will fill blocks up to the limit, and it's OK to set a limit that takes close to or all the gas available in a block, but since miners take transactions with higher fees first, this will increase the gas level you will need to set to outbid other transactions that could fit in the block. So you will need to compensate for the higher gas limit by setting a higher gas price. See https://ethgasstation.info/ for specific data on how high a gas price you are likely to need for any given gas limit.

If you're just doing a simple send, just use 21,000.