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The ultimate guide to inching up your tires.

When I browsed through tire forums, a recurring topic always appears. A guy with an OE size 17″ wants to move to 18″ and ask what tire size he should buy? So in this article I would like to dive deep into this topic and give a technical perspective in selecting the right tire size when inching up.

Motivation

Most of the time, people like to inch up because of the coolness. Having a bigger rim would make the car looks somehow muscular or more aggressive looking. If you look at the higher specification of a similar car, you will also see it is also offered in different rim sizes. Why is it so?

OE manufacturers does a lot of tire size testing before moving on to the tire specification development. Comfort, traction and handling are the main factors an OE manufacturer has to account for. Making a higher inch size available in the market allow the OE manufacturer a bigger leeway to charge a higher price and also to make it justifiable with other addon features.

Grip

The wider you go the wider your contact area resulting in a better traction or braking performance. You do however have to consider whether there is enough room for your extra wide tire.

Comfort

Imagine the sidewall as a flexible cushion which absorbs your weight. This is exactly what the sidewall in the tires does. More often as you go higher in rim sizes, you have to reduce your sidewall height to keep the same outer diameter. This has a pronounce negative effect on car’s comfort.

Looks

A 20″ tire being fitted onto an original 18″ Hilux.

A higher inch tire definitely looks much bigger and aggressive looking. Sometimes it also enhances the value of the car as it is being perceive at a much higher status.

Price

Higher inch tires usually also means higher cost. Some owners tend to think the other way by inching down to save money on tires. This is however very rare and most of them like going up.

How to find out your inch up tire size

The tire diameter is one of the key attribute which allows the car to determine its speed. To avoid needing to change this constant in the car’s computer, we need to determine what kind of error tolerance that we can live with.

How to calculate the outer diameter , OD ?

OD= Tire width * AR /100* 2 + Rim Diameter

For example tire size 175/65 R14:
OD = 175*65/100*2 + 14*25.4
= 583.1 mm

Let’s say you would like to inch up your myvi from 14″ to 15″. So you ought to look for a similar outer diameter tire size in the 15″ category. With some quick clever scanning, you found out a potential candidate

Tire size 185/55R15:
OD = 185*55/100*2 + 15*25.4
= 584.5 mm

Bam ! 185/55 R15 is your choice here for an upgrade. How about the differences in the outer diameter? How would this be accounted for? By calculating the ratio between the old and new OD, you will be able to know your speedometer error.

Speedometer error = New OD/ Old OD * speed

Ex: Travelling at 80km/h

Speedometer error = 584.5/583.1 * 80
= 80.2 km/h

As the differences are really small here the speedometer error is negligible. Hence it is crucial to find a outer diameter which is as close as possible to your original OE tire size.

Try out the effects of your inch up tire

A lot of people had investigated the inch up scenarios and to get a feeling of what to expect, please have a look at the 2 links below. Definitely worth a look in my opinion.

The differences between 17, 18 and 19 inch tyres

The differences between 16, 17 and 18 inch WINTER tyres tested and explained!

As in life, there is always a trade off. It is rarely that you get a clear cut answer on which tires are the best.

Inch up option table.

To save everyone some time, I made up a list of possible inch up options for your tire size. I took a plus minus difference of 1% rather than the industry standard of -3% +4% as this value really opens up the range. My target is to find the best possible inch up sizes in relation to your outer diameter and not just outlying every single possibility.

OE size123456
155/70R12
145/80R13155/65R14
175/70R13165/65R14185/60R14195/50R15
155/65R14175/50R15
165/55R14
165/60R14175/50R15
165/65R14
175/65R14185/55R15195/45R16
185/60R14195/50R15195/45R16
185/70R14175/65R15195/60R15185/55R16
215/70R14205/60R16
175/50R15
175/60R15
175/65R15185/55R16
185/55R15195/45R16
185/60R15195/50R16
185/65R15195/55R16205/45R17
195/50R15
195/55R15195/50R16215/45R16205/40R17
195/60R15185/55R16195/55R16205/45R17
195/65R15185/60R16195/60R16205/55R16205/50R17
195/70R15205/60R16205/55R17
195/80R15215/60R17
205/65R15205/60R16215/55R16215/50R17
205/70R15205/65R16215/60R16215/55R17
235/75R15235/65R17
255/70R15
185/55R16
185/60R16
195/45R16
195/50R16205/40R17215/40R17
195/55R16205/45R17215/45R17205/40R18
195/60R16205/50R17215/50R17
205/55R16205/50R17225/45R17215/40R18225/40R18
205/60R16205/55R17215/50R17225/50R17225/50R17215/45R18
205/65R16215/55R17225/55R17225/55R17215/50R18
215/45R16205/40R17215/40R17
215/55R16205/50R17215/50R17235/45R17225/40R18235/40R18
215/60R16215/55R17235/50R17225/45R18235/45R18235/45R18
215/65R16215/60R17225/55R17225/55R17235/55R17225/50R18235/50R18
215/70R16215/65R17225/60R17225/55R18
215/75R16225/65R17225/60R18
235/85R16245/75R17
245/70R16245/65R17
245/75R16265/65R17265/60R18
255/70R16255/65R17255/60R18
205/40R17
205/45R17205/40R18225/35R18
205/50R17225/40R18225/35R19
205/55R17225/45R18225/40R19
215/40R17
215/45R17205/40R18215/40R18
215/50R17215/45R18235/40R18235/35R19
215/55R17215/50R18235/45R18235/45R18225/40R19235/40R19
215/60R17215/55R18235/50R18225/45R19235/45R19
215/65R17225/55R18235/55R18235/50R19
225/45R17215/40R18225/40R18245/35R18225/35R19
225/50R17215/45R18225/45R18245/40R18225/40R19245/35R19
225/50R17215/45R18225/45R18245/40R18225/40R19245/35R19
225/55R17225/50R18245/45R18225/45R19245/40R19
225/55R17225/50R18245/45R18225/45R19245/40R19
225/60R17225/55R18245/50R18245/45R19
225/65R17225/60R18245/55R18225/55R19235/50R19245/50R19
235/45R17225/40R18235/40R18225/35R19235/35R19
235/50R17235/45R18235/45R18255/40R18225/40R19235/40R19255/35R19
235/55R17235/50R18225/45R19235/45R19255/40R19
235/65R17235/60R18255/55R18225/55R19235/55R19255/50R19
245/45R17245/40R18235/35R19245/35R19
245/65R17265/50R19
245/75R17265/65R18
255/65R17255/60R18255/55R19
265/65R17265/60R18
285/70R17
205/40R18
215/40R18
215/45R18235/35R19
215/50R18235/40R19
215/55R18235/45R19225/40R20235/40R20
225/35R18
225/40R18225/35R19
225/45R18225/40R19245/35R19245/30R20
225/50R18225/45R19245/40R19225/40R20
225/55R18245/45R19245/40R20
225/60R18225/55R19245/50R19225/50R20245/45R20
235/40R18225/35R19235/35R19
235/45R18225/40R19235/40R19
235/45R18225/40R19235/40R19
235/50R18235/45R19255/40R19225/40R20235/40R20
235/55R18235/50R19255/45R19255/40R20
235/60R18235/55R19255/50R19225/50R20235/50R20255/45R20
235/65R18255/55R19235/55R20255/50R20
245/30R18
245/35R18
245/40R18235/35R19245/35R19245/30R20
245/45R18245/40R19
245/50R18245/45R19235/40R20245/40R20
245/55R18245/50R19245/45R20
255/30R18
255/35R18255/30R19
255/40R18255/35R19245/30R20
255/55R18255/50R19255/45R20
255/60R18255/55R19255/50R20
255/70R18
265/31R18
265/35R18
265/60R18265/50R20
265/65R18
275/40R18275/35R19275/30R20
285/35R18285/30R19295/30R19
225/35R19
225/40R19
225/45R19225/40R20
225/55R19225/50R20245/45R20
235/35R19
235/40R19
235/45R19225/40R20235/40R20
235/50R19255/40R20
235/55R19235/50R20255/45R20
245/35R19245/30R20
245/40R19
245/45R19245/40R20
245/50R19245/45R20
255/30R19
255/35R19245/30R20
255/40R19
255/45R19255/40R20
255/50R19255/45R20
255/55R19255/50R20275/45R20
265/35R19275/30R20
265/40R19275/35R20
265/50R19265/45R20265/40R21
275/35R19275/30R20
275/40R19275/35R20285/35R20
285/30R19
285/45R19285/40R20285/35R21295/35R21
295/30R19
295/35R19295/30R20305/30R20
195/50R20
225/40R20
225/50R20
235/40R20
235/50R20
235/55R20
245/30R20
245/40R20
245/45R20
245/50R20
255/40R20
255/45R20
255/50R20
255/55R20275/45R21
265/45R20265/40R21
265/50R20265/45R21
275/30R20
275/35R20
275/40R20275/35R21285/35R21
275/45R20
275/50R20275/45R21285/45R21
285/35R20
285/40R20285/35R21295/35R21
285/45R20295/40R21
295/30R20
295/45R20295/40R21
305/30R20
305/40R20315/35R21
315/35R20315/30R21325/30R21
325/30R20
265/40R21
265/45R21
275/35R21
275/45R21
275/50R21
285/35R21
285/45R21295/40R22
295/35R21
295/40R21
315/30R21
315/35R21
315/40R21325/35R22
325/30R21
275/50R22
295/40R22
325/35R22

Summary

Inching up your tires is not a complicated task. It does however takes some consideration and should not be rush into. There are certain benefits on doing so and you have to decide yourself whether there is enough value for you to inch up 🙂

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