R

RubyRR

Member
So I've done a lot of searching here, there and everywhere but it's not something I can find.

My P38 has 265/60 R18 (not 255/55 R18) but the spare doesn't fit in the wheel well. I like the 265/60 look and ride but I want the boot back!

My plan is to get a 255/55 spare to carry (call it a space saver) and have the spare 265/60 in the shed to swap once I'm home. But will the diff lock up with the difference in size?

I make the 265/60 5% longer circumference so 60mph = 63mph

I appreciate your thoughts..
 
1. What diff?
2. A Limited Slip Differential (LSD) by definition 'slips...' and does not lock.

That said, for a short journey you'd probably get away running a smaller spare. Although the car will sit oddly and lean. Also there is some risk of being pulled. If you travel long journeys, or touring, then you may want to think of another solution.
 
265/60/18 is 247.78 cm circumference. 255/55/18 is 243.54 circumference. So with one of each size on the same axle one wheel will travel 4.24 cm further per revolution than the other. This will give your planet gears problems by loading them. Not a particularly good idea to do it. Fit some proper tyres.
 
So what happens when you drive round a round-about or corners? I'm betting those outside wheels have to travel further.
 
Within the 'plan' is the stipulation that if I'm travelling further than the next county then I would throw the spare spare in the boot. It's only a few times a year with this fuel consumption.

RE getting pulled; Isn't there a claus in the legislations to allow odd-sized wheels for short distances in case of failure? ie. space saver wheels. I better check..

Roundabouts. If you go round a 40meter diameter rndabout with a track of 1.5m and a wheel dia of 243cm, the inside wheel turns 517 times and the outside wheel turns 536 times. That's one extra turn per 27 turns of the inner wheel, or 9cm more per turn. Although that doesn't have much effect on the center diff.

Taking the 4cm difference in wheel circumference though, once it goes through the axle diff it'll be equivalent to 2cm difference through the propshaft and at the center diff.
 
300bhp/ton said:
So what happens when you drive round a round-about or corners? I'm betting those outside wheels have to travel further.
Click to expand...

Indeed they do, that is what the differential is there for. But they are not for driving a 70 MPH on a motorway with different sized tyres on each side. Something will fail before it's time.
 
I have larger tyres on my P38 so have made up a swing away spare wheel on the rear and can also fit a spare tyre in the compartment without a rim and carry a lot of spares in the empty tyre but then I do high milages in out of the way places.
 
If used for a relatively short time I see it being no more wear/working of the diff than say driving a winding section of road, the diff remains "open" and functioning normally. Prolonged use with unequal rolling radius tyres could cause the diff clutches and ramps to wear at a higher rate than normal.
 
ovalandrover said:
I have larger tyres on my P38 so have made up a swing away spare wheel on the rear and can also fit a spare tyre in the compartment without a rim and carry a lot of spares in the empty tyre but then I do high milages in out of the way places.
Click to expand...

Do you have any details/pictures of your spare wheel carrier?
How big a tyre can you fit in the compartment without the rim?
 
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