Rear End Gear Ratio Question

[SCFFR]

While I wait for my MkIV kit to arrive in a few weeks, have been looking for rear end to get ready to install. Have a few people looking for a 94-98 unit and will get an axle kit from Richard at North Racecars to get to the proper width. I will be using either a T5 or TKO500 trans so trying to figure out the best rear end gear ratio. Based on a few articles I’ve read in Hot Rod/Car Craft, the ideal situation is one in which the first gear ratio times the rear end ratio should be a number of 9.5 or less. The theory being if this number is higher, it makes first gear pretty much useless. That said, it looks like a good rear gear ratio with a T5 with a 2.95 first gear would be a 3.27 (used that number since it is available in the rear end I’m looking for). However, in reading the info on members cars in their posts, I see where the biggest majority are using a 3.55 or 3.73 rear end gear. I was thinking that if I found a good unit with a 3.27 gear, I wouldn’t have to change it but now not so sure.

Would appreciate some feedback. If I should change the gear to a 3.55 or 3.73, would like to do that up front and not have to deal with it after the car is on the road.

Many thanks in advance

[Grampiescobra]

I am running a 3:27 with a T-5 in my MK-4 and I love it. My power source is a 347 stroker with Al. heads , roller rockers , EDELBROCK MANIFOLD AND CARB. and it is a perfect match.

[rponfick]

I think the reason for the predominance of 3.55 and 3.73 is that is what was supplied with the complete kit from FFR. I got mine with a 3.73 just after the 3.55 became no longer available from Ford. With my 347" and T-5, is wish I had the 3.55, or lower numerically. I find 4th gear too low and 5th gear too high for my engine.

Ralph

[E-Ticket Ride]

Gear ratio selection kind of depends on your objective. To replicate the "feel" of the originals they used a top loader (2.32, 1.69, 1.29, 1), and I think 3.73 gears. When coupled with the torque of the motor, size and traction of the tires, you ended up with a final drive ratio that yielded a speed of about 53 at 6000 in 1stand 73 at 6000 in 2nd.

My "match" to that is I am planning on a Coyote, a TKO600, and 3.55 rear gears. This will get me 48 in 1st, and 73 in 2nd.

The trick is not overpowering your rear tires, as that is the limiting factor in acceleration. I have a spread sheet and an article I can forward if you PM me that goes through the analysis. Not my work - I stole it fair and square, and happy to share!:)

[Scott Zackowski]

Oh boy! Now you all have me second guessing myself. Give me your thoughts!

I have the FFR/Moser axle 3.31 rear gears (3-link set up) - was planning on marrying that with TKO 500 and 351W based engine with about 400 hp and just a little over 400 ft-lb of torque.

Mk4 Complete Kit
SWZ

[riptide motorsport]

The 3:27's are perfect for your car, you'll love the performance and somewhat reasonable MPG......steven

[Bob Cowan]

This question comes up a lot. There is no such thing as the "perfect" gear, unless the car is dedicated to a single purpose.

Figure out abut where the your engine power band will be - roughly. Then figure out where you'll be driving the most; back country 2 lanes, high speed 4 lanes, etc. From that, calculate the gear ratio that puts your car at the right cruising speed. Pay attention to the 5th gear, and the lower 4 will take care of themselves.

[vnmsss]

Scott....Lots of good advice here, and my $0.02 worth is to not over think this....Your 3.31's will be awesome, and guess what, if you really, really want to twist your motor tighter or want to run out of top end on the freeway sooner, you can always change them out for 3.73's later....The Challenge Series cars have been racing with 2.73's for 11 years, and guess what, they hit 120+ on track, and are a ton of fun on the street...Granted, you're not going to beat folks from stop light to stop light, but the motor will last and last, and the top end is amazing.......I'm running 3.27's in my Coupe. The acceleration is impressive, the mileage is good, and I've topped 170 mph on the high banks at Cal Speedway in 5th.....I'm building a new Coupe, and I've already bought the 3.31 Moser solid axle for it 'cause I know it's gonna be awesome! Good luck with your build! K

[Ray]

I used this site (http://www.wallaceracing.com/Calculators.htm) that has a bunch of different automotive calculators when I had to change the rear gears. Ended up going for 3:08's to 3:55's which solved a couple of different problems I was having.

Good luck,

Ray

[SCFFR]

Thanks for all of the feedback and info. I intend to spend most of my time cruising the back roads so looks like I will keep looking for a good unit with the 3.27 gear. I looked at my collection of old road tests of the original Cobras and they were between 3.31 and 4.11 but then again they were using a four speed trans with first gear ratios of about 2.3.

If I can find a good unit with 3.27 gears, will stick with that and see how the car drives. If I can't find anything decent used, will look at the Moser unit with the 3.31 gears.

Again, thanks for all of your replies and have a Happy Thanksgiving

[AC Bill]

I find cruising the country roads around here, even with the 3.55 rear, I rarely even get into fourth gear.

[Bob Cowan]

I intend to spend most of my time cruising the back roads so looks like I will keep looking for a good unit with the 3.27 gear.


Back roads driving usually means speeds of around 60-65'ish. A 3.27 gear puts you at 1700-1900 rpm's. For a mostly stock to mild performance engine, that works out pretty well. If your engine is a bit radical, you could be lugging the engine too much.

[Richard Oben]

Avoid the over cam over carb engine combination that will avoid the lugging. We have used 355 more than any other. HTH, Richard.

[KC Wildcat]

I agree with a lot of the logic, but I see 1 huge factor nobody has brought up.... What engine - what HP & TQ are you planning? I think the reason you see FFR shipping 3.55 & 3.73 is because those are great with a stock/mild 302. Gearing is all about torque multiplication. If you are going with a big block, a large stroker or something in the neighborhood of 500 HP/TQ, then IMHO you really do not need the "extra" gearing - unless you plan to do serious drag racing. You see lots of those guys going with 3.08 & 3.27. Use the gear calculators/spreadsheets - you plug in your rear ration and transmission gears, you then can find the rpms for a given speed in a given gear. Look at the speeds you anticipate driving, what gear you would typically be in, what rpm you will be turning (consider the noise factor too) and where you will be in your TQ curve. Remember, TQ is what gives you the pushed back in your seat feel when you hit the go pedal. There is no one perfect choice, it is all about compromises.