For grip, playing with the wheel blueprint can help heaps with this. As for a single gear vehicle, you would want to look at the ratio of the single gear you are using, and adjust the value to make sure it’s not too low. Also you will probably need to play with the final ratio as well. The engine torque curve will probably need to be adjusted, so your power doesn’t come on too late in the rev range (wheels spinning at high speeds)
I’d try setting the max rpm to something like 8000, then adjusting the ratios and torque curve to get the desired speed. Keep in mind that the gear and engine setup in unreals vehicle physics can be a bit complicated to get a simple effect, so you might spend a fair bit of time playing with all these values.