Mixing tyres question

[dekhelia]

In an attempt to get slightly taller gearing, I've bought a pair of 215/80/14 tyres for the rear axle. The fronts will be standard 185/65/14s. I'm assuming that because it is driven from the front hub, the speedo accuracy (!) will remain as it was.

Is mixing tyres like this ok?

[California Dreamin]

Standards are 185R14C not 185 65 14 (the 65 profile make then 9.5% smaller circumference)

Add that to the built in 'over read (around 10% from the factory) and your speedo will be reading nearly 20% too high.

No problems running two sizes on different axles (front/rear)

Martin

[dekhelia]

That's great, thanks. I've had a look at the fronts and they are stamped with "185 R14C" - dunno where I got the '65' from.

I don't suppose you know of a way of working out how much 'taller' the gearing will be when I put on the 215s at the back?

[marlinowner]

That's great, thanks. I've had a look at the fronts and they are stamped with "185 R14C" - dunno where I got the '65' from.

I don't suppose you know of a way of working out how much 'taller' the gearing will be when I put on the 215s at the back?

According to

http://www.etyres.co.uk/tyre-size-calculator" onclick="window.open(this.href);return false;

They will change the gearing by 7 or 8%

[California Dreamin]

Are you running a more powerful engine? changing the final drive can sometimes have the opposite effect to what you are after & actually increase fuel consumption due to spending more time in lower gears...having to change down at the slightest incline or head wind.

Martin

[dekhelia]

Standards are 185R14C not 185 65 14 (the 65 profile make then 9.5% smaller circumference)

Ok ... so what is the 'standard' profile?

[dekhelia]

Are you running a more powerful engine? changing the final drive can sometimes have the opposite effect to what you are after & actually increase fuel consumption due to spending more time in lower gears...having to change down at the slightest incline or head wind.

Martin

Running a slightly more powerful engine than the van was delivered with: it was a standard JX, but has been replaced with an AAZ with a mild modification to the injection pump - basically, to deliver more fuel.

The reason for fitting larger diameter tyres to the back axle is purely an attempt to reduce revs at cruising speed. Which is about 55MPH in my camper, as it's a hi-top.

Never mind changing down on an incline ... I have to change down to drive over a cat's eye ...

[marlinowner]

Standard is 80% (or possibly82%).

[California Dreamin]

When a tyre doesn't state an aspect ratio (profile) then it is full height (80/82)

So I suppose a 185R14C could read 185/80 R14C however, the manufactures just don't bother making the assumption that the expert already know this.

Martin

[California Dreamin]

Are you running a more powerful engine? changing the final drive can sometimes have the opposite effect to what you are after & actually increase fuel consumption due to spending more time in lower gears...having to change down at the slightest incline or head wind.

Martin

Running a slightly more powerful engine than the van was delivered with: it was a standard JX, but has been replaced with an AAZ with a mild modification to the injection pump - basically, to deliver more fuel.

The reason for fitting larger diameter tyres to the back axle is purely an attempt to reduce revs at cruising speed. Which is about 55MPH in my camper, as it's a hi-top.

Never mind changing down on an incline ... I have to change down to drive over a cat's eye ...

That's fine mate....the AAZ has more torque and will handle the 7.5% increase without issue.

Martin

[dekhelia]

Standard is 80% (or possibly82%).

Good to know, thanks.

[kit]

I have 205's on mine at the rear as they are more common and less pricey, I have noticed a difference but not sure on economy as I tend to be 4th gear a bit longer?

[dekhelia]

Update: had these tyres (Nankang 215 R14C) fitted today, and I can report that there is a quite noticeable difference in the gearing. Can't give any precise numbers, as I don't have a tachometer fitted, but I'd say that at 60MPH the engine is running at least 500RPM lower. May be even more. Van is a lot quieter, seems more relaxed at 55MPH cruise than it used to. 'Taller' gears seem most evident at the extreme low end and extreme high end - 1st gear much less of a 'crawler' gear now, and 5th seems much longer; as someone said above, you hold on to 4th gear for longer, and this is also true. So economy may be down a bit, but I reckon it's worth it for more relaxed progress.

Now to fit the same to the fronts!

[kit]

Is there anything to gained in changing the fronts?

[dekhelia]

Gearing-wise, I don't think so. But the van looks a bit strange with the 215s at the back and skinny little 185s at the front. I'm going to leave it as it is until I come across some more cheap 215s ... by which time I suppose i could have got used to the look.