We were in Teotihuacan and Guanajuato / San Miguel de Allende in March and did rent a car in Mexico City and drive. Our trip took us from MC to Teotihuacanm Valle de Bravo and then to Guanajuato / San Miguel, so I cannot comment on what you want to do directly. You can probably do the drive in 2 to 3 hours each way. I would recommend overnighting at the ruins as they are best seen early in the day and in the evening when tourists, light and heat are less. It also cannot really be seen and appreciated (it is 4 kms long) entirely in one day. The road signage in this part of Mexico is terrible, especially coming out of Mexico City. It seems that road planners figure everyone knows where they are going, or should be on a bus. You should have a clear idea of what highways you want and what the distances are between exchanges. Most maps we vfound we out of date or had the wrong hwy #s. The toll roads are great, but watch out for the 50 kms 40 year old Datsuns on your right and the new Porsches going 200 kms on your left. Many of the secondary highways are also good, particularly those that parallel the toll roads. Follow the trafic to avoid the tolls (approx $15 each way on the route your trip will probably take). I believe it was high #57 that took us most of the was south from San Miguel towards MC. There is a highway that cuts east from it towards Teotihuacan about 40 kms north of MC.
AT Mexico City airport we paid $7 a day for a Jetta for 2 weeks, but had to take out minimum insurance at $25USD a day as well (credit card ins. is not accepted at Dollar Rent a Car). It would probably be in your best interest nerve and $ wise to take the bus to San Miguel and arrange for a rental over the www there (in advance as www rates are better). Each state has different policies on insurance and cc use. We have avoided paying there excessive rates by way of cc benefits in more touristy destinations on the west coast. We stayed in a moderate hotel called Posada Sol & Luna in San Juan Teotihuacan for $40 a night. It is clean and quiet and about 2 kms from Teotihuacan gate #2. They have a poor www site, but will eventually answer email. I will try and post a review as there is not one now.