Car wheel/tyre sizes

What does the eg 155/70 mean on a car tyre? i think its width x height as a percentage of that width, am i right?

so 155 mm wide and a sidewall height of 108.5 mm?

on 13 inch rims this gives 330mm wheel diameter plus (108.5 x 2) = 547 mm diameter. circumference = 1719mm

on 15 inch rims 195/50 tyres should give

381 wheel diameter plus (97.5*2) = 576mm diameter. circumference = 1828 mm

if im right, then the low profile tyres go 109mm further per revolution, and that is a 6% difference.

so, at an indicated 70 mph is the car then doing 70x.94 = 65.8mph
70*1.06 = 74.2 mph?

3 questions.

is any of the above correct?
what speed is the car actually doing at an indicated 70mph
how hard/expensive is it to get the speedo to show the correct speed?

the reason being, i need to change all 4 tyres soon, and my hubcaps are knackered.
halfords do a tyre/alloy combo at 230quid and my tyres will cost at least 140+20 for caps so for an extra 70 squid i thought i may as well look into it

not particularly after performance, but the weight difference can only help. and anyway alloys look better :smiley:

Regarding the size stuff: no idea.

How far out the speedo is: borrow a GPS navigation unit from someone. They usually have the option to show how fast you’re going. Then compare indicated with actual.

Looks right to me :nod:

Yes, and no :slight_smile:

Speedometers tend to read anywhere between 1 and 10 percent of correct speed, check one with a GPS and you will see this. I know the Discovery reads 6% low as standard. I believe also that the speedo itself has a tendency to creep as speed increases and so the error is not linear - hence digital speedos are becoming more common on cars.

The only reason to be concerned about the above is when buying a high-mileage car - mine has covered 128000 miles, which is actually 120030 :slight_smile: On a different make this could be 140000 :eek:

Roadspeed? The men with hairdryers and cameras hiding in the shrubbery will tell you the only roadspeed which matters under law.

You can get speedo gearing units which retrofit to your vehicle and will correct the reading, assuming you have tested with a GPS and know what the error is. I doubt personally whether for most vehicles the cost and effort would be worth the result - certainly not for 6% which is well within the motor manufacturers allowed margin of error which is set at 10%.


In general the numbers mean this:

For a 335/35 Z18

335 is the width in CM

35 is the profile or height of the tyre wall

Z is the rating of the tyre, usually it’s maximum rated speed

18 is the diameter of the wheel rim in inches etc

That’s some seriously fat rubber :smiley:

I remember finding a tyre this width at my mates farm, it was off his Dad’s Lambo. Very fat rubber indeed :slight_smile:

If you dont want to affect speedo reading you could go for bigger rims and lower profile tyres.

Theres a tyre size calulator here to allow comparison between your current tyres and various other combinations.

Was more refering to the cm part - that’s 10ft wide rubber taken litterally :smiley: Thinks you ment mm.

35 is the profile or height of the tyre wall

so thats the ACTUAL height, not a % of the width?

if so then my 6% discrepancy become 2.3%, and that much less likely to result in plods hairdryer attacking m license.

cheers folks

Original tyre 155/70 on a 13 inch rim, circumference 1719.08

195/45 on a 15 inch rim, circumference 1748.29

Difference of 1.7% have 195/45 15 tyres at £46.60 (155/70 15’s are £21.90)

Symplistic but demonstrates the principle.