Hi, I need 2 new tyres for my 96 discovery. At the moment it has 225/75-16 on front and back, but it seems this is not such a common size (so are more expensive) also seems that this not standard option for this vehicle, which is 235/70R16 as far as I can work out. Is it OK to have 235/70R16 on the back and 225/75R16 on the front?, they have slightly different diameters (743mm / 735mm). Also anyone know what the min speed rating of the tyre should be? (300tdi) It has Q's on the back and S's on the front right now, I think Q is not high enough. Thanks Andrew
225/75R16 are not uncommon but were never an OE fit I think.
Many 225/75R16 tend to have a stronger casing than 235/70R16 - the most likely reason for the higher price. Most 235/70R16 do not to go beyond a load index of 105: I've been through several sets of 225/75R16s at 107 load index on RRC without issues.
A Q speed rating isn't an issue until you start using maximum load (for the tyre) at maximum speed (for the car) for a sustained period - not an issue for you (My RRC ones were Qs). That's way beyond the legal limit in this country and unlikely to concern you even on your forays into the 'uncivilised' world.
As far as size - specifically the important rolling radius/circumference
- is concerned there's nothing in it between 225/75R16 and 235/70R16 - something like 710 and 715 rev/mile respectively. You'll exceed that difference as the tyre wears.
Thanks Dougal, Lots of info there, I thought that for insurance purposes the speed rating had to at least match the OEM fitment regardless of what speed you actually want to do. Andrew
As I understand it from my tyre supplier whether or not you exceed the speed rating of a tyre is irrelevant. If you fit tyres that do not have the same or higher speed rating than that specified by the manufacturer you are risking your insurance cover in the event of a crash.
That is correct, an old pal of mine had a smash in a 3ltr Ford Capri many moons ago. Norwich union insurance as it was then refused to pay out after the repair inspection reported that the tyres were rated at (I don't remember exactly but something like)
95MPH when the vehicle was capable of 110MPH, despite all agreeing that the smash happened at just 30 MPH.
It's not irrelevant if, as I did, you restrict your comments to the purely technical aspects of the matter i.e. whether or not the tyre is operating within its manufacturer's limits.
An unscrupulous tyre supplier also has a vested interest in providing, at a premium, a tyre of a specification that exceeds the actual needs of his customer.
That's another aspect and one which, obviously, concerns Andrew. Nothing that a quick call/e-mail to the insurer won't sort out if necessary. Obviously don't start the conversation with "I'm wanting to fit downrated tyres"!
There is an argument, as articulated by Mike, that insurers may take the view that, regardless of maximum territory speed limits, a tyre should be rated to accomodate the capability of the vehicle.
However, the 300Tdi Disco 1 may not require an S rated tyre. The owner's manual for a 300Tdi-engined RRC of similar vintage to Andrew's Disco states that petrol-engined vehicles should only be fitted with S or T-rated tyres i.e. 300Tdi-engined vehicles can use something less. How much less will depend either on what Land Rover specifies (if they do/did) or on the maximum speed of which the vehicle is capable. Wasn't the quoted maximum speed for the 300Tdi-engined model 98mph? That's within the allowable range for a Q-rated tyre.
There is another way out of this - 225/75R16s are available with speed ratings in excess of Q. Michelin do several rated T but with a load index of only 104 and they're a bit roady. Goodrich do S-rated tyres with up to a 115 load rating.
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