tire size and highway driving

[twobone]

I have a set of 185/70/13 tires on my Caterham. The speedometer already reads 10MPH over.

Am I correct that many folks run a 175/50/13 high performance tire on their cars?

Not only will that throw off the speedo, but won't I be running a very high motorway RPM?

[NuisanceFactor]

Yes, and yes. Also, depending on the age of your car, that sidewall depth may well have been part of the suspension design process. Reducing the depth reduces the ride height and it also reduces the compliance at that end of the suspension and you may find that the ride suffers adversely as a consequence.

[BertBert]

I don't think that 175/50 is a standard caterham tyre size. The CR500s for 6/13" wheels are 175/55. The ZZRs are 185/55, CR322s are 185/70, ZV3 185/60!

I remember asking in the very early days of CR500s about the size as the diameter would be small and was told (by Lenny U I think), that the CR500 175 was actually the same size as a 185 (strange).

Bert

[Incorrigible]

You may have the wrong speedo drive, they are different depending on your diff ratio

[downsman]

Have you tried different 185/70/13 tyres. A021r Yokohamas are very reasonably priced in that size, and are supposed to be excellent tyres

[PEGJY]

May be you have a wrong speedo setting.
Om my 2010 R300 with 13" I had to correct the factor a lot until the speedo reading was equal to my GPS reading.

[Risky]

50 profile tyres on 13 inch wheels will give you ground clearance problems for normal motoring.
What type of speedo is it? Electric or Cable. Electric can be adjusted on the back and cable can be fitted with a different drive.

[Matt UK]

BertBert said:

I remember asking in the very early days of CR500s about the size as the diameter would be small and was told (by Lenny U I think), that the CR500 175 was actually the same size as a 185 (strange).

Bert

Yes, I was chatting to the chaps up at Dartford the other day and he mentioned this. Apparantly the 185 comes up fractionally under that figure, so they have to mark them as the next size down. Strange.