Twister

Let's face it, there is a lot of mis-informationabout what a rear gear is and the ratios...A rear gear converts the revolutions of the driveshaft into revolutions of the rear wheel/tire. In other words, a 4.10:1 rear gear turns 4.1 turns of the driveshaft into one turn of the rear tire. The larger the first number is, the lower the gear ratio is, and vice versa.

A low gear ratio is for acceleration, while a high gear ratio is for a top speed. Drag racer's usually want to have as low of a gear as posible, since they are only worried about getting to the finish loe that is 1320 feet away, so a 4.10:1 is usually ideal for them. If you are driving your bone stock 1987 5.0 5 speed 200 miles on the freeway, the stock 2.73:1 rear gear ratio is better suited for you since your speed will be higher at the same RPM than a car with 4.10:1's.

No matter what, there are some constant things when it comes to cars, and they are all tied into math. Let us take that bone stock 1987 5.0 5 speed mentioned before and look at what it will do in 4th gear with a set of 225/45R17 tires. Now, we are also going to set another guideline before chaging the rear gear ratios, this car is going to be running at a constant 3000 RPMs. I chose 4th gear since it is a 1:1 gear, meaning that each time the flywheel of the engine turns, the driveshaft will turn once also.

The new 225/45R17-sized tire has a calculated diameter of 24.97", a circumference of 78.44" and will roll 835 times each mile, no mater what gear you are in, what your gear ratio is, or your RPM that the engine turns.

2.73:1=81.67 mph
3.08:1=72.39 mph
3.27:1=68.18 mph
3.55:1=62.80 mph
3.73:1=59.77 mph
4.10:1=54.38 mph

now, lets mix it up a little, and see what RPM that you are running with the same car, tranny gear and tire size with different gear ratios at 65mph....

2.73:1=2388rpm
3.08:1=2694rpm
3.27:1=2860rpm
3.55:1=3105rpm
3.73:1=3263rpm
4.10:1=3587rpm

dan

and now you now...

the truth....... about gear ratios

Tim37

wow i didn't know 4th on a t5 was 1:1.

JD1969

I have tried before to clear this up , I also used basic math like you did. People here STILL did not beleive me and to this day say that 4.10's will kill your milage and such. The truth is, as you and I both have proved, that they only increase your rpm about 200 rpm at a givin speed over the 3.73. I have givin up on this subject a long time ago, most of these people don't want to learn from those of us who have done this stuff before. FWIW I hope you can get through to them, but I would'nt hold my breath.

Twister

so why don't you sticky this and use it as a point of refferance?

Tim37

all of this information is on the site and a lot of it is in stickies through out diffent forums ist just that nobody wants to hear that they want to aske the same old questions agian and agian.

Tim37

https://mustangforums.com/m_866326/tm.htm
https://mustangforums.com/m_600073/tm.htm
https://mustangforums.com/m_87748/tm.htm

Twister

just re-thought it, actually, with the numbers above, at 65mph, the 3.55 will save a ton of fuel over the 4.10 since the 3.55 would be out of open loop

ToplessGT

well after seeing this, Im going to stick w/ 373's. Im one that worries about gas mileage a little and think that the rpms w/ 355's are crappy, so I'll push it one more notch to 373's. Since the difference isnt really that much between 373's and 410's. I know its only 200 and some odd rpms, but I like my stock 2300 rpms the best.... but I have to get a little quicker out of the hole!!

This thread does help, but fortunatly I have a buddy w/ 373's in his and I test drove it.... Will work great for me. For others, maybe not enough gear, I am never going to the track and hardly race as it is, but 273's are just ridiculously slow....

JD1969

People who are running the 4.10's don't reprt a huge differance in mileage, in fact some claim that they get better town mpg. I would'nt know, personally I don't give a rats about mpg (I have a 4cyl beater for that). To me it all boils down to one simple question. Do you want to be as fast as you can, or not?