Boat tail on a normal car? (3-box sedan)

[Pendragon]

Just asking about the tires because that can have an effect on the cruise RPMs.

The link says that stock rims are 6x15J and the tires are 195/65 R 15 H, but if you have the stock ones check them to make sure.

[cliffyballs]

The tires on there now say 225/40 ZR 18, 92w xl, i don't know what effect the bigger wheels will have, but i will do some tests today on my Speed to RPM ratio then swap the stock tires back on and see what happens.

[Pendragon]

According to this calculator, the 225/40x18 tires make 804 revolutions per mile as compared to 807 revolutions per mile for the 195/65x15 tires. In other words, not real difference. There are greater variations than that as tire tread wears. The 18 inchers appear to be a perfect "plus 3". As I said before, they look nice and if you decide to sell them should be worth something...or just keep them for those occasions when you want more grip for a day trip through the twisties.

The only reason I asked was that sometimes people go for some very low profile tires that wind up lowering the car and affect the speedometer as well as the cruise RPM. A friend recently had this happen by accident...it seems he dropped the car off to get some new tires and the shop accidentally put one size smaller tires on it. It corners like nobody's business, but tends to scrape on rough roads. His car gets good mileage anyway and so he did not notice it right away.

Cheers

[NachtRitter]

Keeping an eye on this one as I also have a Mk4 Jetta...

[cliffyballs]

Did I do the streamline right?

I looked at the stock tires and they are bald, where as my brother-in-law just put new tires on the aftermarket wheels, would it be cost effective to buy four new for stock wheels? What would you do?

[Pendragon]

I am hesitant to say "do this", except to suggest that you prioritize what you would like to do and compare that with your budget, both short term and long term.

The car runs right now with the rims and tires you have, so I would say that changing tires is something that can be put on hold while you sort things out. I would look into the options for low rolling resistance tires, keeping in mind that all low rolling resistance tires have made compromises which have made the tire's performance in braking and wet weather traction less than many of the tires which are not marketed as low rolling resistance. That said, most of the tire manufacturers have been working to improve the rolling resistance of their 'regular' tires...and have been pushed by the auto manufacturers because it can improve their CAFE ratings. Only you know the conditions in which you will drive and only you can best balance the qualities you want in the tires.

Frankly, there is nothing "wrong" with the tires on the car at the moment. Sure, they are wider than one would choose for all out fuel economy and are probably a bit heavier than the mileage tire, but your brother-in-law made a good choice in the tire sizing to keep the speedo, gearing and so on correct which matters a lot when dealing with the modern electronically controlled systems. It is all too easy to get a car set up where the readings of some sensor or other are out of parameters and so the system thinks something is wrong.

Do some thinking before you spend money on anything other than maintenance that you would do anyway.

As far as the streamlining goes, it looks reasonable, but look at how some other people have constructed theirs and see how you can approach it on your Jetta. For one thing, do you use the trunk much? Easy access to the trunk would be very important to me, but might be less so to you. Remember to keep an area large enough for you to mount your license plate, tail lights and so on.

[COcyclist]

Originally Posted by Pendragon

Just as an example though, swapping these transmissions out should get you down to a bit over 2,500 RPM if my calculations are close. (Obviously, do your own calculations before making any sort of decision/purchase.) There may even be other options.

You can swap out fifth gear with the tranny in the car. No need to swap transmissions unless you are going to the six speed. Lots of diesel VW guys go to a taller fifth so you should be able to buy a used diesel fifth gear set in good shape. Search tdiclub.com for lots of how-tos and info.

[Pendragon]

Originally Posted by COcyclist

You can swap out fifth gear with the tranny in the car. No need to swap transmissions unless you are going to the six speed. Lots of diesel VW guys go to a taller fifth so you should be able to buy a used diesel fifth gear set in good shape. Search tdiclub.com for lots of how-tos and info.

Exactly, if the OP can swap out 5th gear and the final reduction gear he should be able to get a substantial improvement in the car's fuel economy and comparatively inexpensively. If he does need to swap out the transmission and can get a six speed that works out that would be the way to go.

Cheers

[cliffyballs]

Thanks for all of the information guys. I think that deleting the trunk hatch might be the way to go. Plus i would have a ton of easy mounting points. Maybe put a hatch on the tail so i can still use the inside part of the trunk. I don't ever use the trunk and when i have something to put back there I usually put it in the back seat.

Does my high tech simulation seem about right or does it need to be steeper?

[euromodder]

Originally Posted by cliffyballs

I am thinking of having the "butt" of the boat tail rounded is there any aero gain to that?

No, you want clean air separation, so use a sharp transition from the sides of the boat-tail to the rear butt plate.

You could even extend the shape of the boat-tail a few inches beyond the butt plate, i.e. make the butt plate recessed.


If you look at the rear bumpers and / or taillights of ever more cars, especially eco versions and hybrids, they use a sharp edge rather than curving inward - or they use a sharp ridge somewhere near the start of the curve inward, keeping the curve for esthetics.

Side note Someone suggested getting an air splitter for the front of the car you negate a belly pan...anyone ever heard of this?

There's always going to be air underneath the car, so guiding it properly will improve drag.
An air dam and splitter reduce what goes underneath, but they don't eliminate it.