mounting a spare tyre on the roof

[casperfromholland]

Hi!
I am going in two weeks doing some off-road driving in Piemonte Italy. My car has a LPG tank mounted inplace of the spare tyre and a small special sparetyre mounted under the car just before the rear bumper. In Holland on tarmac thats fine, it can bring you home. But it is limiting offroading while the spare tyre easely touches the ground when you drive upon a hill or so.

In Italy I want to take a full size tyre with me and leave the small one at home, but seeing all the stuff that my wife wants to take with us there will by no room for that tyre in the car. So I am thinking to mount it on the roof. Has anybody experience what that do with your milage? I have to drive 1300kms to get there, and with nowaday fuelprices I would not like to double the fuel consumption. Or wouldn't it be that bad?

Casper

 

[subyroo]

Hi there cozy bro', Casper from Australia here. :biggrin: :biggrin:

If you were put a rack/cage type carrier on the roof it will decrease you mpg or L/100k no doubt about it, but it will also depend on your speed and any winds you may encounter on the trip.

Something like this without anything at all in it will even effect your economy to a degree as do bare roof bars.

 

[stilson]

I had better mileage with my roof top tent on the highway than I do around town. Without tent 610km, with tent 510km, around town without tent 490km per tank.

I'd expect a full size spare on the roof to reduce your range by 7-15% depending on what speed you're cruising at.

 

[Ratbag]

Gidday Casper

In Italy I want to take a full size tyre with me and leave the small one at home, but seeing all the stuff that my wife wants to take with us there will by no room for that tyre in the car. So I am thinking to mount it on the roof. Has anybody experience what that do with your milage? I have to drive 1300kms to get there, and with nowaday fuelprices I would not like to double the fuel consumption. Or wouldn't it be that bad?

Casper

Even towing a fully loaded 7x4 foot box trailer will not halve your fuel economy, mate. Knock it around a bit, depending on the roads, speeds, wind, etc; but not halve it!

A roof rack has more drag than a moderately loaded trailer however, as it wrecks the aerodynamics of the car (so do the exterior mirrors!). It interferes with the laminar air flow over the roof, causing it to be turbulent flow instead.

Therefore, the more aerodynamic you can make it, the better.

Regardless of any of that, you will spend far more money on the roof bars and baskets than any increase in fuel costs ... :iconwink: :lol:.

Buy aerodynamic roof bars. They don't have much effect on the fuel economy vs cheaper square or round section bars. They are also reasonably quiet if the basket isn't fitted.

Make sure the basket has a wind deflector like the one Peter posted a photo of. This helps with both the wind noise, and lowers the drag.

I would tend to put a spare tyre at the rear of the basket, as less interference with the aerodynamics of the vehicle. Air flow towards the rear of the roof is already becoming turbulent rather than laminar, so should have less effect.

Have a wonderful safe trip!! :ebiggrin:

 

[casperfromholland]

I managed to borrow a roofbox and will put the spare wheel inside the car. I put the lightweight stuff in the box.
More than halve the distance i drive trough germany were there are no speed limits on the highway. I think i will drive a 140 / 160 km p/h. Curious how the fozzie will handle at that speed and the millage will be. The other counties have a speed limit of 130 km p/h.

 

[dirtyRU]

I managed to borrow a roofbox and will put the spare wheel inside the car. I put the lightweight stuff in the box.
More than halve the distance i drive trough germany were there are no speed limits on the highway. I think i will drive a 140 / 160 km p/h. Curious how the fozzie will handle at that speed and the millage will be. The other counties have a speed limit of 130 km p/h.

At 140kmh it should do okay. Speed limits out in west Texas are 85mph and my 99 2.5L did it okay loaded down. 160 kmh might be pushin it though in a 2.0

 

[taza]

At 140kmh it should do okay. Speed limits out in west Texas are 85mph and my 99 2.5L did it okay loaded down. 160 kmh might be pushin it though in a 2.0

Far out, My 2.0l Foz struggles to do 120km/hr loaded up let alone sitting on 160km/hr. Fuel economy will go out the roof if your sitting on speeds like that in a 2.0l as you will be doing about 4400rpm! I did a highway trip with the car stock, nothing on the roof and sat on 140km/hr the whole time. I was doing 4000rpm and averaged 10-11l per 100km. I got just over 450km to the tank..

 

[dirtyRU]

Far out, My 2.0l Foz struggles to do 120km/hr loaded up let alone sitting on 160km/hr. Fuel economy will go out the roof if your sitting on speeds like that in a 2.0l as you will be doing about 4400rpm! I did a highway trip with the car stock, nothing on the roof and sat on 140km/hr the whole time. I was doing 4000rpm and averaged 10-11l per 100km. I got just over 450km to the tank..

Haha, yeah gas milage pretty much went down the shi**** at that speed. I believe at 80-85mph the 2.5 was sitting around 3500-3700rpm.Not too shabby for something as aerodynamic as a brick in the wind!

 

[Cañon Carver]

Wow, you guys are lucky. Rusty rolls along at 65 mph in 5th @ 4300rpm At least I still pull between 38-45 mpg at that speed though, lol.

-Jamie

 

[casperfromholland]

Far out, My 2.0l Foz struggles to do 120km/hr loaded up let alone sitting on 160km/hr. Fuel economy will go out the roof if your sitting on speeds like that in a 2.0l as you will be doing about 4400rpm! I did a highway trip with the car stock, nothing on the roof and sat on 140km/hr the whole time. I was doing 4000rpm and averaged 10-11l per 100km. I got just over 450km to the tank..

Well taza my 2.0 forester had no problem with 160km/ph. It did a 4200rpm than. I only did it withovertaking other cars so not for long periods. For the rest I drove 140/145 on the cruisecontrole in all comfort with the AC on and using lpg. It used around 12 / 13 liters per 100 km. Taza, aren't your tires to big so that your gearing is too long?