I currently run 34 front with a 11-36 rear, (10 speed), but an thinking about upgrading to 11 speed, mainly to give me more downhill speed, not to make climbing easier, if I go to a 42 rear, what would I need up front to give a gear the same as I currently have,thanks

Either 38 or 40

Low gear? Up it, how often are you spun out on a 34?

More downhill speed? Just change the front ring for a bigger one – sticking a 42 on the back isn’t going to make the 11 any smaller!

Aye, but if he changes the front to a bigger one and keeps the same cassette he’ll lose some low gears, hence the desire to calculate a new front ring size.

Unless your pedalling down a fire Road the thing that makes you faster dh is generally less braking, smoothness and carrying momentum.

Scotroutes has summed it up, my climbing gear is perfect at 34f x 36r, but I will have a 42r so can afford to have a larger chainring to compensate, but also gaining a better downhill gear