How to decide the right gear ratio for your car

How to determine the right gear ratio or your auto is easy once yous take a few things into consideration. Rear end gears are EXTREMELY of import for how a car performs. In fact, they are as equally important as the engine itself! If you don't have the right gear ratio, fifty-fifty with a super powerful engine, your car can end-up being a complete turd.

When selecting a set up of rear end gears, it's always dainty to know, (or at least closely estimate), what RPM you'll be at for any given speed yous'll exist traveling. If yous desire your machine to be "quick" and launch hard with a super quick 0 - threescore MPH fourth dimension, y'all'll need low (brusk) gears. If you want your car to exist "fast" and have a loftier peak-end speed, you'll need high (alpine) gears. The numbers tin be deceiving in that a high gear ratio has a depression number, and a lower gear ratio has a college number. You have to remember, they are RATIOS, so the numbers tin fool you. For example; a really low set of gears would exist something like 4.56's. Information technology's a"depression" gear ratio but it has a loftier number. A really alpine set of gears would be something like iii.00's.

Let me give you some examples: Firstly, ever recollect this, rear cease gears are no unlike than the 5 sprockets you lot have on any given 10 speed wheel. To do wheelies and be able to climb steep hills, you demand low gears. You certainly tin can't do a wheelie in annihilation higher than 4th gear on a typical 10 speed bike considering the gear ratio is simply likewise tall, no matter how string your legs are. In retrospect though, you tin't go fast down the route if you are in 1st, 2nd or 3rd gear. Put the bike in 5th gear and endeavor to do a wheelie. You lot won't be able to because the gear ratio is too tall. On the flip side, put the wheel in 1st or 2nd gear and try to ride 30 MPH downwardly the road. You won't be able to because your legs can't spin the pedals fast enough. Y'all'll run out of RPM and volition demand to shift to a higher gear. Rear end gears on cars are no unlike, and having the wrong gear ratio is exactly like trying to practice a wheelie in fifth gear, or trying to go down the road at 30 MPH in 1st. Neither one is going to happen because the gear ratios are completely wrong for what y'all are trying to do

Drag cars need depression gearing so they can launch hard and get to the stop line as "apace" as possible, where something like a Bonneville car needs tall gears to be able to reach the high pinnacle speeds that they go. When guys want to make their cars "perform" better, (and don't know whatever ameliorate), the only matter they wait at and consider is the engine. The engine is literally just One-half of the battle. The gearing is the other one-half.

A machine at the drag strip with four.11 gears and a 300 HP engine will launch harder and will be quicker than a motorcar that has 500 HP and gears that are fashion besides tall. eighty% of the race is the first 80 feet! If you can't launch and GET to your speed quickly, you won't run a good elapse time. It also means you'll get your doors diddled off in a street race from stop calorie-free to stop light. You can brag all you want that your engine has 500 HP when your buddy only has 300, but if he has the correct gearing, his piddling 300 HP machine is going to send you home crying to mommy when you lose a race from a dead stop to 75 MPH or so, which is where 99% of street races happen.

The best fashion to make up one's mind what gear ratio you demand is to determine how you'll be driving the car. If you are only wanting to bulldoze around town and do effortless burnouts, be "quick", and look neat at the Wed nighttime drags, so you'll want a set of gears that best suits that style of driving, which volition be low gears. The thing yous need to practise is determine what MPH you think your machine is capable of going, or what your max RPM is based on your cam profile, valve springs and where your engine "noses-over' on the power bend.

If your engine peaks-out or "noses-over" at say, 6,400 RPM, then there'south no reason to make information technology rev beyond that point or you're just going to go slower. Too many guys like to brag about how high their engine tin rev, which only shows how much they DON'T know nigh cars, engines, and performance. Sure, LOTS of engines can rev way past where they nose-over, only information technology'southward completely counter productive to run it past that bespeak. You ever go by where information technology peaks out on ability... nothing higher in RPM than that!

Then using the 6,400 RPM instance I gave in the past paragraph, yous won't want to go through the stop line higher than that or the car is "nosing-over" and isn't pulling equally hard as information technology was when it was nether that summit RPM.

The next affair you need to practice is measure the diameter of the rear tires you lot're running. I'one thousand going to utilise 28" for this base line. To determine how fast I can become at my 6,400 RPM limit with 28" tall tires, I can practice some simple math to calculate it with any gear ratio. The math is: RPM 10 tire diameter, so 6400 x 28" = 179200. Now accept the gear ratio you lot desire to check against and times it past 336. I'grand going to apply 4.eleven's as a baseline, so 4.11 x 336 = 1380.96. Now divide the 179200 into the 1380.96 and we become 129.79. That means our top speed at 6,400 RPM with a 28" alpine rear tire will be right at 130 MPH.

Keep in mind, and this is VERY of import, torque converter slip! Even though 6,400 RPM is much higher than what hot street cars run for stall converters, (which are usually around three,000 RPM or so), they still accept slippage of 200 or 300 RPM or so up at high RPM, so yous demand to exist aware of this when calculating any of these equations. This is especially true when trying to determine what RPM you'll exist at when traveling downwardly the freeway at 60 MPH considering at that low of an RPM, your three,000 RPM stall converter is slipping several hundred RPM which will throw yous a curve brawl when trying to calculate these numbers. In other words, the math might say you'll be at say, 2,700 RPM at 60 MPH, merely if you have a slippy converter, such every bit a three,000 - 3,500 stall, you can easily take 500 or more RPM of slippage which volition add to that cruise RPM you calculated. So the reality is, you might THINK y'all'll be at ii,700 RPM but with the converter slippage you could actually be at something more than like 3,200 RPM or more than!

And then looking at that 129.79 (130) MPH number nosotros calculated, that's within the quarter mile time of a typical low to mid x second car. If you think your car tin can't run x's so you lot tin adjust the gear ratio up or downward to accommodate what speed or RPM yous want to become through the stop line at. Let me give yous an example; swapping those 4.xi gears out for a set of iv.56'south and using the same numbers, you'll exist going through the stop line at 116.96 (117) MPH at half dozen,400 RPM. Not equally "fast" mile per hour wise, but you'll launch harder and will get at that place quicker.

Let'south accept this same set-upward and see what it does with a typical fix of stock street gears of 3.25's. 6,400 RPM ten 28" tall tires = 179200. 3.25 gears ten 336 = 1092. Divide 179200 by 1092 and yous become 164.10 MPH. In other words, with a typical street car that has 400 - 450 HP you'll NEVER be able to get to 164 MPH in a quarter mile, let solitary 1 full mile because most cars volition float and lose control at that speed without break modifications, ground effects, etc, so that gear ratio is completely useless for drag or street racing, although it's a groovy ratio for running downward the highway at a decent RPM for long distances.

So let's accept a look at running down the freeway using this aforementioned car ready-up and numbers. If you aren't into elevate or street racing and y'all live in an surface area where you lot generally do freeway driving for extended periods of fourth dimension, yous won't want (or even intendance nigh) a low gear ratio for jack rabbit starts. You'll instead want something that'll go you meliorate gas mileage and cruise down the highway at a decent RPM. So let's wait at a couple of equations for determining what speed and RPM y'all'll be at for a given gear ratio and tire bore. I'm going to use 70 MPH as our cruise speed with the aforementioned 28" tire bore we used before. This is a slightly different equation so pay attending to the numbers. 70 MPH 10 the axle ratio you want to see about x 336. So 70 ten let's attempt a gear set of 3.50'southward, x 336 = 82320. At present divide that number into the tire bore which will give us the RPM at 70 MPH.  82320 28" = 2,940 then at lxx MPH with iii.50 gears and 28" tall tires yous'll be at 2,940 RPM.

Again, you MUST accept into consideration torque converter sideslip! If you take a slippy converter of say, three,000 RPM, then you are definitely going to run across 300 or so more RPM than what the math only gave yous unless you have a lock-up converter OR y'all are running a manual transmission. ANOTHER very important affair to consider is what type of tranny you are running. In other words; does it have an overdrive? All of this math is for a 1:1 high gear ratio which is what all TH-350, Thursday-400, C-iv, C-6, FMX automatics take for loftier gear, besides as pretty much all three and four speed manual transmissions. If you accept an overdrive tranny and so you can summate the number by dividing the overdrive percentage into the final number. If you have a .80 overdrive (xx% overdrive), then remove 20% from that RPM number and y'all'll be in the ball park for what your 70 MPH cruise RPM will be.

So let's await at that same scenario with iii.00 gears just so we "get' the math better. 70 MPH x 3.00 gears x 336 = 70560 Divide that by the 28" alpine tire bore that I chose which = 2,520. So with 3.00 gears that same car at lxx MPH dropped from 2,940 RPM to 2,520. That means in that location is a 420 RPM deviation between three.00 gears and 3.fifty gears on that auto.

Let'due south say you already take an thought of how fast you desire your car to be and at what RPM the engine peaks out at and yous desire to know what gear ratio to go with then the engine'south RPM maxes out at the top speed y'all want to go. This is another unproblematic equation. So allow's first make up one's mind the base line numbers. Let's say it's a 66 Mustang with a hot little 331 stroker in it that dyno'd at 410 HP at 5,670 RPM, and then we'll telephone call information technology five,700 RPM max. The car is going to be used on the street and maybe some weekend drag racing just for some fun. It isn't realistic to think a 331 with 410 HP volition propel a car similar that to 150, or even 120 MPH on a drag strip, so let's say 110 MPH is a reasonable acme speed for that car for either at the rails or for running downwardly the open back roads. A Mustang usually has a pretty brusk tire diameter of like 25 inches, so we'll utilize 25" as our baseline. So we don't desire to go through the finish line at any higher RPM than 5,700 RPM and we want the top speed to be 110 so the math is: 5,700 x the tire diameter (25) which = 142500. Now we have the 110 MPH and x information technology by 336 which = 36960. Now take 142500 and divide it by 36960 to give u.s. the gear ratio of 3.86. In that location are some 3.lxxx gears out at that place just for most gear manufacturers only offer 3.73'south and 4.10's. Occasionally you'll discover 3.89's or 3.91's, but but to go on it simple, let's say your choices are either 3.73's or 4.10's. This is an easy 1 to figure out. If you went with the iii.73'due south, at 110 MPH you'd be spinning 5,514 RPM, which y'all might think is a little low for what you want, BUT if you're running an automated transmission with a stall converter of say 2,600 RPM information technology will accept a little bit of slippage, even at that RPM, so figure on about another 200 - 300 RPM which would make that RPM more than like 5,700 - 5,800 which is pretty much right on the money for where you want to be at.

Just for giggles, if yous chose to run the four.ten gears, at 110 MPH you'd be spinning half dozen,061 RPM. Now add the torque converter slippage of a couple of hundred RPM and you lot're upwards into the 6,300 RPM area which is just too loftier for an engine that noses-over in power at 5,700 RPM. In near all cases information technology's amend to go less in RPM than too much. It's likewise easier on the engine and components.

So here's the math equations again all by themselves.

To determine what RPM you'll be at for whatsoever given gear ratio and tire bore:

MPH x Axle Ratio x 336  -- Divide that number by the Tire Diameter

To determine what speed y'all'll be going with a given tire diameter and gear ratio:

RPM x Tire Diameter  -- Divide that number by the Beam Ratio x 336

To determine what beam ratio y'all'll need for a given speed and tire bore:

RPM x Tire Diameter -- divide that number past the MPH ten 336

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