Juggler. Depends on size tires, I run the small, narrow tires with a 15-6/48-50 gears,
Another racer run a tall tire .400 wide tire and gears his at 10/60. with some glue.....
Also glue comes into play, I run mine with no glue on the track or on the tires.

Try different combos and see what works for you.
  DVan

I had two slip joints at one time and fastened the bodies by holding the nose down with Velcro .the rear mount was a piece of 1/4" brass tubing squashed to an oval shape then pin tubing put inside it , allowed the chassis to flex very freely ..biggest draw back was the short pins ,easy to loose- impossiable to find after you dropped them or they end up inside the motor
 DVans gearing sounds right and if you look at the dragsters the Detroit guys run in bracket all use small , narrow tires an are extreamly consistant(does not hurt they cut .00# lights either) small adjustments to the guide and hinge point weight can also make a diffrence on how the slip joint hooks up
the best part is the fun of testing an fiquring it out

Picture of double pin tubbing for pins.

DVan

Hi guys, just a FYI on my slipjoint project... I used as many parts out of my used parts box as possible, so while maybe not ideal, they were "free", so to speak. I used new Pro Track 1 1/16 x .300 rears, 10T pinion, 56T crown, and a roughly 20 year old Mura grp12 left over from my road race days. The motor could use a re-zapping, but has powered the car to many 1.080 passes. Seems fairly consistant in this trim, so I may leave it this way for awhile as I pursue other projects. It does have roughly 12gr of lead spread over guide shoe area and around the hinge. I still have to cut the body down (heighth-wise) because I left it full heighth while I was adjusting the weights and guide wires. The car uses little to no glue, and is probably my favorite car to run. I'd post pics, but don't have any at the moment.