Inaccurate data (Autodata) used to service car

iwaddo77

https://forums.moneysavingexpert.com/discussion/comment/82022673#Comment_82022673

The tyres are 235/60 R17 102H. They are the original tyres on a new car. There is no door sticker.

iwaddo77

Might I have too much time on my hands? :-)

Based on a change of 0.07 bar per 6.5° (is the math correct?)

Air Temperature	Inflation Pressure
35	32.00
34	33.00
33	33.00
32	33.00
31	33.00
30	33.00
29	33.00
28	34.00
27	34.00
26	34.00
25	34.00
24	34.00
23	34.00
22	34.00
21	35.00
20	35.00
19	35.00
18	35.00
17	35.00
16	35.00
15	36.00
14	36.00
13	36.00
12	36.00
11	36.00
10	36.00
9	37.00
8	37.00
7	37.00
6	37.00
5	37.00
4	37.00
3	37.00
2	38.00
1	38.00
0	38.00
-1	38.00
-2	38.00
-3	38.00
-4	39.00
-5	39.00

Mildly_Miffed

It's lovely, but you've got it the wrong way round. Pressure is higher for higher temperatures.

So if you inflate to 32psi in -5degC, then they'll be 39psi in +35degC.

And that is, of course, TYRE temp not just ambient.

(If we're using antiquated parochial measures for pressure, why not be consistent and use degF, too?)

James2k

Not sure parochial means what you think it means. But psi is still widely used in many areas, whatever your personal feelings on the unit are.

Are you counting the days till we drop statute miles from all those pesky road signs? Troglodytes!

iwaddo77

https://forums.moneysavingexpert.com/discussion/comment/82022714#Comment_82022714

I thought it seemed wrong but I have found several references online to

• Winter adjustment: Add 3-4 PSI to manufacturer’s recommendation during sustained cold weather

I might have asked the question online

So if my tyre reads 32 psi at -5°C what number should I set the pump to at the garage when I check them consider the baseline is 35 psi at 20°C

If you're specifically trying to maintain the same amount of air that would give 35 psi at 20°C, then you should target about 32 psi at −5°C.

Using the temperature correction:

P−5=35×268.15293.15≈32.0 psiP−5​=35×293.15268.15​≈32.0 psi

So:

• 35 psi at 20°C ≈ 32 psi at −5°C
• If your tyre already reads 32 psi at −5°C, it contains roughly the same amount of air as a tyre reading 35 psi at 20°C.

However, this is not how vehicle tyre pressure recommendations are normally used. When a manufacturer says "35 psi cold," they generally mean the tyre should actually read 35 psi when cold, regardless of whether it's −5°C or 20°C outside.

So there are two different answers depending on your goal:

Goal	Target pressure at −5°C
Maintain the same air mass as 35 psi at 20°C	32 psi
Follow the manufacturer's recommended cold pressure	35 psi

For normal driving and tyre wear, follow the vehicle manufacturer's recommendation and set the tyre to 35 psi when cold. If you're doing a physics calculation to compare pressures at different temperatures, 32 psi is the equivalent of 35 psi at 20°C.

Arunmor

Op I think you need the RaceSense tyre pressure gauge with temperature probe to keep in top of this problem a bargain at ~£280!

James2k

Your chart IS the wrong way around

At 39 ambient you need more pressure than at -5 for the same amount of air in. Because air expands as it's heated. Thus you are fitting larger air 'volume' Inside the same sized tyre, thus more pressure

If you had the same air in both the 39 degree ambient would read considerably higher than the -5 ambient reading

But this is all academic, put the recommended amount in and forget it. Life's too short

Or fill them with nitrogen for less variation

Mildly_Miffed

https://forums.moneysavingexpert.com/discussion/comment/82023272#Comment_82023272

Oh, I'm fairly sure it does.

A unit that's in widespread use in just one country in the world, with limited and outdated use in another. That's it.

Versus a unit that's global.

Why on earth wouldn't you?

James2k

You could say the same for the UK's use of both imperial and metric.

Psi is used worldwide, still. Like it or not

LightFlare

https://forums.moneysavingexpert.com/discussion/comment/82022714#Comment_82022714

to be fair - most people know the price of fuel per litre, but then talk about mpg.