* Both shows ran for a short period of time.
Sonic ran for two seasons (1993-1995) and 26 episodes, while Road Rovers ran for one season (1996-1997) and 13 episodes.
* Both shows were very popular in their initial run.
But see the next item for more on this point...
* Both shows were cancelled due to ratings concerns.
Sonic was cancelled because, according to the Lost 3rd Season section of Fans United for SatAM, its ratings had fallen in the second season due to the (then brand new) FOX show Mighty Morphin' Power Rangers. One of the more popular rumors I've heard on a couple newsgroups over time is that FOX considered Sonic as their chief competition and when ABC moved Sonic to a diferent time slot, FOX moved Power Rangers to the same time slot to continue draining its ratings.
Road Rovers, on the other hand, was cancelled for what virtually everyone sees as a very stupid reason; according to Save Our Road Rovers, Bob Doucette (producer of Road Rovers) stated in 1997 that although it was garnering high ratings similar to (or even better than!) the big-name show Superman, Road Rovers was still cancelled because WB execs didn't consider it a smash hit.
This brings us right up to the next point;
* Both shows were replaced with very, very lousy cartoons.
Sonic was replaced by Free Willy, based off the then-popular movie. In this version, the whale talked and fought against evil submarines and tuna boats. It lasted roughly one season (though I can't tell if it was a full season, or a few episodes repeated constantly until the next fall season came around.)
The replacement for Road Rovers, the ill-fated Calamity Jane, turned out to be a true calamity and lasted only three episodes before getting the ax. I don't even remember exactly what it was about to be honest, but there was something involving a woman with a shotgun in the Wild West. Possibly. The plot was awful muddy, one of the reasons it stank so much.
* Both shows retain a large and still growing fanbase.
Well, admittedly it's easier for Sonic considering he's the flagship character for Sega and all. But nonetheless, it's very evident that both shows have a large number of active fans to this day, even though no new episodes have been made for either shows for quite some time now.
* At least two people have done voice acting for both Road Rovers and Sonic:
-> Jim Cummings --- Dr. Robotnik (Sonic, main character); General Parvo (Rovers, main character)
-> Rob Paulsen --- Antoine (Sonic, main character); Wolf King, Katzenstok Ambassador & Gas Station attendant (Rovers)
* The main villains of both shows, Dr. Robotnik and General Parvo, share some physical similarities.
Take particular note of:
A. The large, goofy-looking mustache;
B. The cybernetic limbs;
C. The deranged, evil look in the eyes, and
D. The absolute lack of fashion sense.
* They both sound very similar as well.
Robotnik: "Blue hedgehog? Hmmmmm... ah, nothing to fret about."
Parvo: "Soon, all will call me MASTER!"
(This is mostly attributed to the fact that the same person did their voices, but I felt it was still worth noting.)
FUS: Fans United for SatAM (fact checking)
The Sonic Foundation (fact checking, Robotnik picture)
Team Artail (Robotnik sound clip)
Steven Today's Unofficial Road Rovers Site (fact checking, Parvo picture, Parvo sound clip)
Steven Today's Save Our Road Rovers (fact checking)
R.U.F.F. - Rovers Under Fiendish Fire (fact checking)