The ratio is 5:1 for a 40 tooth wheel sprocket/8 tooth motor sprocket, which gives an output rpm of 678. If you were getting 5mph with 113rpm it doesn't seem right getting 8mph with 678rpm.

What is your wheel diameter? Put that and the 678rpm into here and see what speed you get.

I put in 113rpm and to get 5mph out the wheel diameter needed to be 375mm (which gave 30mph at 678rpm )