Car shakes when I shift gears to drive

With no other test results to analyze, this sounds like a typical engine misfire. Worn spark plugs are the most common suspects. The clinker is the seller would have taken care of that so the car would bring a higher price. When they do not solve such minor problems, it is usually because the solution is expensive, and they know it, or they tried to fix the problem, but gave up.

Hi CELESTIALPR,

Yes, please post as many videos/pictures as possible so that CARADIODOC can see exactly what is going on and give you his expert advice on the problem.

Thank you using 2CarPros.

Happy New Year!

Renee
Admin

Some confusion needs to be cleared up. If your car has anti-lock brakes, it is required that all four wheels rotate at the same speed when driving on a straight road. That means the outer circumference of every tire has to be the same. The tire you put on is smaller than the others, but the sidewall is 65 percent of the tread width instead of just 60 percent. That means the new tire is skinnier but could be real close to the correct outer circumference. If there is a significant difference, the ABS Computer will detect that within about a a few hundred yards to a mile. It will turn on the yellow warning light, set a diagnostic fault code, and turn the system off. If the ABS system works normally, you should be able to get by with that tire, but I'd put it on the back. It's more important for the two front tires to be matched so you don't develop a tire pull to one side, and for even braking and steering response.

You can measure the tires yourself with a tape measure when they're jacked up off the ground. Most salespeople at the tire stores also know which tires will have matching circumferences. Be aware too that some ABS systems are so picky about tire sizes, they can detect a problem caused by buying two new tires one time, then two new matching tires a month later. That is due to production tolerances at different times or between different manufacturing facilities.

There's a lot of misinformation about premium gas vs. The lower octane gas. Way too many people incorrectly think higher octane provides more power. It does not. Gasoline is gasoline. It's some of the additives that raise the octane rating, and even if those additives burn, they don't contribute to the production of power.

The availability of higher-octane gas allows the engine designers to build an engine capable of producing more power, but that usually is done by increasing the compression ratio, and that leads to spark knock. Spark knock is desirable and necessary in a diesel engine as that is what lights the fuel to start the burn. In gas engines, we don't want the burn to start until a very precise time, and that is determined by when the spark plug fires. "Pre-ignition", meaning spark knock, occurs from raising the compression ratio, but that higher compression is needed to get more power. (Indy race car engines develop five times more power from an engine not much bigger than what is in your car, but to do that, the compression ratio is REAL high. The octane rating of premium gas and aviation gas isn't high enough to overcome that spark knock. That's why they run on alcohol).

Now that the engine is designed to develop higher power through a higher compression ratio, that spark knock has to be controlled, and that is done with additives that raise the octane rating. Decades ago that was done with lead. The goal is to make the fuel harder to ignite so the heat generated by squeezing it doesn't do that. We don't want the gas to self-ignite until the spark plug fires.

To sum up this confusing description, if your engine calls for the higher octane gas, you should use it. When an engine does not require the higher octane, using it can cause problems. If you have a misfire now related to weak spark, that is going to be much worse when the gas is harder to ignite. The goal is for the gas to ignite as easily as possible, but only at the right time, and that is accomplished by using the lowest octane gas as possible. If an engine is running rough now due to a spark-related misfire, higher-octane gas will only make that worse. You'll have an undiagnosed problem, AND an additional contributing factor to poor performance. It's also possible for an engine with no defect to run poorly when high-octane gas is used when it shouldn't be. There's no easy way for your mechanic to determine that, so a lot of time will be wasted trying to solve an elusive misfire.

The tiny pressure molded on the sidewall of the tires is the maximum it can handle when it's cold. The numbers on the door sticker are what is needed in a tire of the size specified to hold the vehicle up. "PSI", pounds per square inch, means if the sticker calls for 30 psi, every square inch of tread holds up 30 pounds of car weight. If the tire is smaller than expected, it has fewer square inches of tread, so it needs more pounds of pressure in each square inch.

I can share what I found over the years as an alignment specialist, but this applied to Chrysler products in the '90s. The numbers on the door stickers have to consider any brand of tires the owner might buy in the future, and at that time there were still some tires that had a maximum pressure of 35 psi. New cars were coming with tires that were rated at 44 psi max. Since the door sticker couldn't list anything higher than 35 psi, the cars with 44 psi tires could have benefited with higher pressures. I found that setting 44 pound tires to 40 psi gave better tire life, better handling, and a comfortable ride. I also found 35 pounds in tires with a 35 psi max. Rating did not result in complaints of harsh ride, but they did feel more stable on the road. Eventually, since the tires on most cars were not the same as what they came with from the factory, and the door stickers didn't apply, I started setting every tire to 35 psi or 40 psi, and I never looked at the door stickers.

I have never seen a passenger car tire rated at 51 psi. That is a typical rating for a light truck or a larger trailer tire. Those tires have harder rubber compounds, so if you have one of those on your car, it is going to flex much differently than the other three and is going to have considerably different rolling resistance. The car can be expected to pull to one side and to have little "predictability" when braking or during rapid steering maneuvers. If the alignment was okay before, it should not be changed to accommodate the new tire. The best you can do is put the odd tire on the rear where it will have less effect on handling and pulling.

Driving through a parking lot is not an effective way to look for an alignment pull or a tire pull. It is a real good way to identify a broken belt in a tire. You'll see the steering wheel oscillate back and forth at those low speeds if the broken belt is on a front tire, and you'll feel the seat move back and forth sideways if the broken belt is on a rear tire. A tire or alignment pull will show up best at highway speeds, but you have to observe this over time on many different roads. All roads tilt to the right so rain runs off. We adjust in a slight left-hand pull during the alignment to offset that "road crown", but not all roads are the same. Two cars can be in perfect alignment but only one might pull slightly on one road and the other car might pull a little on a different road. We have to look at the average. In my case, I was real familiar with a stretch of road near the dealership where I worked. I knew, during the final test-drive, if the car went straight on that road, the customer would be satisfied.

For the octane rating, you want the gas that's easiest to ignite so it has less chance of misfiring, but the octane ratting has to be high enough to suppress spark knock. The highest octane ratings are needed with high-performance engines. Some manufacturers try to squeeze out a lot of power from a small engine, but that translates into higher stress and shorter engine life. If that's a trade-off they're willing to make, and a higher octane gas is needed, there will often be a label to that effect next to the gas cap. If you don't see that, check the owner's manual for the recommendation. If nothing points to the need for a higher octane gas, using it just wastes money. If spark knock isn't a problem, there's no benefit to using it.

91 is indeed a high-grade octane rating. It's not the highest there is. The warning is more concerned about not using leaded gas, but I don't think you can even find that anywhere today. They might use that in racing gas, but that octane rating, as I recall, is close to 100. It also costs a real lot more than pump gas. You might try a mid-grade gas, 89 octane, to save a few bucks, but pay attention to the engine. If you hear pinging, or spark knock, particularly when accelerating or lugging up a hill, switch back to the higher octane.

Different additive formulations are used in different parts of the country, and the northern climates switch to a different blend in winter that promotes easier starting. Winter blend gas, when used in summer, can cause spark knock when it fires too easily. Some cars even run into performance problems when we get an unseasonally-warm day in winter. Even the cheapest gas today has so many detergents and other beneficial additives in it, that there's no reason to go overboard when searching for the highest quality.

I'm not an expert at selecting tires by size. As an alignment specialist, my job was to set the angles to achieve the best tire wear patterns, and make sure the steering wheel was straight and the car went straight. When it came to diagnosing problems related to those things, my interest was that all four tires were the same size. I never was a tire salesman, but those are the people who know when a size can be substituted. Besides the three main angles that every alignment mechanic knows about, there are other non-adjustable geometric angles that are part of the suspension system that affect pulling and steering response. These are less-known, but one in particular changes when a wider wheel is used, or one with a deeper "offset", meaning it sits further out from the car. This angle is called "scrub radius". Tire diameter is also a factor that changes scrub radius. The first concern is it is the same on both front wheels for balanced handling, but it is just as important to lawyers that it be what was designed-in by the engineers.

You have the pressures backward. Every tire has an amount of the car's weight it has to hold up. I don't get involved with math, but when you have a smaller tire, it has fewer square inches, so it needs to pack that weight-carrying capacity into a smaller area. That means a smaller tire needs more pounds per square inch to hold up the same amount of weight.

To color that a little, as a hypothetical story, suppose you needed 20 pounds in a tire for it to do its job. The manufacturer is going to add a safety margin to include the additional weight imposed on a tire when it hits a bump or pot hole. Lets say that brings it up to 25 pounds. While it does support the car, the sidewalls will flex quite a bit with each revolution. That flexing can provide a smooth, cushioned ride that feels comfortable, but it generates a real lot of heat. That leads to the plies separating and eventually a blow-out. That is what got Ford in big trouble a few years ago with their Explorers. They specified an artificially-low pressure to make it look like their ride quality was better than on competing vehicles. I'm in the middle of Wisconsin where it has been close to 0 degrees, but after driving just ten miles, my sidewalls are wet from the melted snow that was stuck on them. The snow on the wheels, that don't flex, is still there. That shows how much heat is generated by that flexing. I don't even know what pressures are specified for my van. The tires are rated at 44 psi max, and I keep them close to that. Even at those maximum pressures, the temperature is rising at least 32 degrees in ten miles.

To get back to my sad story, the manufacturer has to weigh safety with ride quality, and they're going to lean toward comfort to sell more cars. As long as the tires don't blow out, they don't care as much about tire life. My priority is long tire life. Also, most of my driving is short trips where there isn't much time for excessive heat to build up. When I take a cross-country trip, I start out with the pressures a little lower than I normally run, but they're still higher than what's on the door sticker. The pressures are expected to rise when the tires get hot. The tire manufacturer has that calculated in when they determine the maximum pressures to rate each tire at. That's why we're told to check them when they're cold and to not readjust them when they get hot.

So now we're up to 25 pounds. As a lawyer for the car manufacturer, I'm going to insist on an additional safety margin, and recommend at least 30 pounds. Knowing that every additional pound of pressure will reduce the chance of a tire failure, I'm going to recommend an even higher pressure, until the trade-off becomes too great. Eventually an extra ounce of tire safety will cost a pound of comfort, and that will cause too big a hit to sales. We could use metal tires. Those would never blow out, regardless how we abuse them, but it sure wouldn't be a comfortable ride. What I'm not doing a good job of saying is higher tire pressures lean toward fewer failures, and lower pressures lead to more comfort. I look at the numbers on door stickers as the absolute minimum, then you can go up from there.

One other thing to keep in mind is with higher pressures, a tire becomes more round. That translates into a smaller "contact patch" where the tread hits the road surface. With less contact area, the tire will skid easier when braking and when cornering at high speeds. That can be considered a limiting factor when deciding how high to go. The bottom line is if you're involved in a crash caused by the other guy running a red light, you don't want to become party to a lawsuit. I get REAL excited when I read about raised trucks and lowered cars because I know altering ride height adversely affects stopping distances, steering response, and handling. The other guy's lawyer or insurance investigator will convince a jury that you were partly at fault for the crash because you were less able to avoid it, and they will be right. With that comment in mind, you have to consider how you would answer the question of why your tire pressures weren't as specified. This sounds stupid, but it is the type of thing mechanics think about every time they touch your car.

While the higher pressure leads to less tire touching the road surface, lower pressure leads to more sidewall flexing. That can detract from your ability to maneuver precisely around an obstacle. Try driving on an almost-flat tire and see how impossible it is to predict where the car will steer to. These two things are another trade-off. I find the slightly-reduced stopping power to be a good trade-off for better steering response. That would not be a good trade-off for people who tail-gate, and we have a bunch of them here.

Now you have even more to think about. Tire safety, comfort, and being sued by the guy who caused the crash. Insurance investigators just love to find any modification to the other guy's car that wasn't approved by the manufacturer.

We haven't discussed what you have for a spare tire. If that is a full-size tire, consider putting that one on the car and using the new one for the spare. A lot of manufacturers save a couple of dollars by including mini spare tires. Those really mess up scrub radius, AND stopping power and steering response. That's why they warn to stay below a certain speed. That isn't to protect the tire. It's to protect them from a lawsuit.