This is one of the longest quests in the game. Out of the 95 vehicles, it is not uncommon to get the first 40-50 during the natural course of the game. The next 20-30 will require a watchful eye but aren't difficult. Getting the last 10 is usually a time-draining process.
Note that when the achievement says "Drive every vehicle in the city", literally getting into the car and turning it on and then getting straight back out counts. If it's a new vehicle, you will see the name of the vehicle and your progress (x/95) in the HUD.
One thing to keep in mind is that there is no guide which can tell you exactly where every vehicle will be every single time. This is because there is almost always a random factor involving where the different cars spawn, however, it is possible to make out in which area or in which location a certain vehicle often appears, and if the vehicle in question isn't where it usually is, just drive around and return to the location and there's a chance it will have spawned.
- Tip for finding vehicles:  Note: this trick doesn't work every time.
- 1. Go to the Vehicle Showroom and select the missing vehicle you wish to collect.
- 2. Press the Color button (X on 360, square on PS3, Spacebar on PC) then leave.
- 3. Select The Streets of L.A. (Free Roam) from any desk in the Cases menu and the vehicle you did the trick on should now be found driving or parked near you when you start.
- Print a list of vehicles - you can use the Vehicles article as a template. Mark off each vehicle as you obtain them. If you've already started, check the list of obtained vehicles on your checklist on the Rockstar Social Club (also useful to track other 'collectibles' such as Street Crimes and Landmarks).
- Throughout the course of the game you will be taken to a vast amount of locations. Use these opportunities to get into various cars you see parked on the streets, driveways, parking lots, car dealerships, etc. You will not get penalised (in terms of 5-star performance) under most circumstances if you wander off and check cars before proceeding with the case (exceptions being chases, tailing people, and if you're required to immediately check a scene for clues). So if you spot a parking lot or an unfamiliar vehicle, even if you're en route to an investigation, pull over.
- Pulling people out of vehicles won't harm your case score or ranking, should you see an unfamiliar vehicle driving on the street.
- Note that some vehicles will be locked (International Police Wagons being the most common), but you can find unlocked ones later in the game or during Free Roam.
- Some vehicles will appear more often in cases & street crimes than they will in Free Roam, however it is still possible to obtain all of the cars in Free Roam (with the exception of vehicles such as Jack Kelso's car, which you will drive as a natural part of the game).
- The 'Hidden' vehicles are easy to find as long as you're levelling up through the course of the game as they are marked as "?" on the game map (usually 5 at a time), and are parked in identical-looking garages.
- Look for unusual looking vehicles to quickly tick off - most of the Service-class vehicles stand out in the traffic.
- Driveways can sometimes yield rarer vehicles.
- Don't panic if don't have 100% of vehicles until late in the game - you literally cannot get all of the vehicles until you're well into the Arson chapter.
- The last 5 to 10 vehicles are the hardest to find: there is no explicit list of rare cars, but some are harder to find than others (e.g. Cadillac Series 61, Nash La Fayette Convertible, etc.). Look at the photos of the cars in question (either on this wiki or other websites) and look for unique characteristics; e.g. grill, lights & indicator layout, top, side, etc. They will become easier to identify if you know what you're looking out for. Drive slowly through busy areas - if you go too fast you won't see sufficient detail.
- With the exception of the hidden vehicles, regular vehicles are usually placed in random locations. Meaning if you unlock a vehicle parked at a certain location during a case and you restart the case or quit the game and restart later, there is no guarantee that vehicle will still be parked in the same location.
- Keep in mind that some vehicles cannot be entered and therefore don't count. For example, the delivery trucks outside Ramez Removals seen in The Black Caesar, as well as buses and streetcars.
Further information on vehicles and their locations can be found here.