buying 18'' alloys - best tyre size - advice please!

[Scott & Gillian]

We are looking to buy a set of 18'' alloys and have a choice of tyre sizes -
225/40/18 and 215/35/18

In your opinion, what would be the better choice, and what effect on handling and ride comfort would be had from these tyres?

Cheers!

[francophile1947]

Personally I'd go for the 225/40s - should give a softer ride and, being bigger, you'd be using less revs for the same speed.

[ebygum32]

don't forget to get your speedo recalibrated as the bongo is set to run on 15" rims, when i was looking for all terain tyres spoke to couple of tyre companys and they advised me to get 205/70/r15 all round instead of 215/65/r15

[trevd01]

The 18 in 225/40s will make your speedo 3.5% fast and the 215/35s 8% fast compared to 215/65 x15s. 225/45s are the same.

Play with this tyre calculator:

http://www.miata.net/garage/tirecalc.html

[francophile1947]

That's a useful little site Trev.

[Scott & Gillian]

Cheers for the info!
I think i'll go for the 225's.

Any idea how much getting the speedo recalibrated might cost?
Is it stupid money or not much to worry about?

Also, are there any issues with running the same tyre size front and back on a 2wd?

[Peg leg Pete]

Run ours on same size front and back 2w.d.

[madmile]

Hi Scott and sophie - recalibrating your speedo is just a case of prising off the needle and moving it a bit so that it reads correct - its about a half hour job assuming you have access to a gps/sat nav.
I would suggest you get the wheels on then check your speed with a gps - as most speedo's are rarely accurate, you may find that you dont have to touch it at all.
If you get stuck then let me know.

[Scott & Gillian]

Thanks again for the replies!
As the calculator shows - with the 225/40/18's we'd be travelling at 57.9mph if the needle showed 60mph.
Handy me thinks when coming up against speed cameras!

I've got a handheld gps unit, so as madmile says, we should be able to tweek the needle if thats all that's needed.

Now, this might be a dumb question (it almost bedtime though, so please forgive me!) -- does the wheel/tyre size affect the mileage clock?

Cheers again everyone!

[madmile]

Yes is the short answer - if you fit bigger wheels then you will travel further for each turn of the driveshaft, and this(more or less) is what tells tha car how fast you are going (or how far).
As you point out, as long as you are not going faster than you think you are its no problem, but its better to get it within 5% if you can.

[trevd01]

Remember this wheel/tyre [225/40 18] combo is actually smaller, not bigger. There is more metal (wheel) and less rubber (tyre) than on a standard wheel [215/65 15 or 195/70 15, which are both the same diameter as each other, BTW]. But the overall diameter is less, hence the wheel turns more times for a given number of revs causing the speedo to over estimate.

As most car speedos are deliberately up to 10% optimistic (over estimating), 3.5% further change could make it more up to 13.5% out. (I do know you can't add percentage changes together, but its not far wrong).

Of course in theory, you will use 3.5% more fuel - maybe 1 mpg less? But in practice who can say?

Have I got this right? When I started out writing this I was crystal clear, I'm now beginning to doubt myself!

[Vanmanerik]

Is it safe to fit non-standard wheels and tyres?

Page 202 of the Mazda Bongo owners handbook.
Different sizes of tyres are fitted to front and rear wheels. Do not swap front and rear tyres as this will affect the balance of the vehicle. Always fit the correct size of tyres.

So you take your life and others in your own hands by going against Mazdas advice.

If you do go ahead please notify your insurers you have modified your vehicle, don't wait until you need to claim and find you are not insured.

This has been discussed before, loads of info at:
http://www.igmaynard.co.uk/bongo/forum/ ... =tyre+size

[bigdaddycain]

Hi scott and sophie,
I had 15"( 195/70/15 ) alloys on my bongo when i bought it and the speedo used to overead i.e. actual 65 M.P.H would read 72 M.P.H. on my speedometer.
I have now fitted 18" alloys, shod with 245/40/18 tyres, and now my speedometer is accurate according to my G.P.S. i.e. 65 M.P.H is actually 65M.P.H.

The choice of tyres can be down to personal preference.
I would find 35 section tyres a bit too low profile for my tastes, the ride would be slightly harsher,crashing into potholes would be less forgiving,however,the smaller sidewalls would theoretically improve cornering ability, and should give a more responsive feel through the wheel. (its a bongo though,not a ferrari 360!)

The 40 section tyres would be a little more forgiving when introduced to a pothole/ speed bump etc. And would offer a little more protection for the wheels,with regards to accidental kerbing etc.

If your bongo has lowered suspension, or if you are considering lowering the suspension, then go for the 35's(more clearance) if on standard springs, go for the 40's(fill the arches)

A final point, the reference to the section of tyre i.e. the depth, 35 doesn,t mean 35mm deep, it is a percentage reference to the width of that particular tyre, i.e. with 225/40/18 tyres, the sidewall is 40% depth of the 225 mm width.

[andyfb78]

the reason the tyres are different front to rear is to ensure that the car will understeer rather than oversteer at the limit.

Most people react correctly to understeer (back off the throttle), but wrongly to oversteer. Understeer will self correct when you lift off, oversteer is unstable and so requires deliberate driver intervention to control.

Don't be too alarmed though, fitting the same size tyres front and rear won't suddenly make the bongo dangerous.

Lower profile tyre will generally loose ride comfort, but gain responsiveness and slightly increase grip limits.. however a lower profile (therefore stiffer tyre) will reach it's limit more suddenly so the breakway characteristics are less forgiving.

In general, if you fit non standard tyres just drive the car carefully unitl you fully understand the effects on the handling.

As said above it is not a ferrari so it rolls alot which is unnerving, and so the drivers limits are usually lower than the car.

[Scott & Gillian]

Ok, now I'm getting a bit confused!

Will 225/40/18's be the same size as my 215/65/15's as trevd01 says

Or, will they fill the arches as bigdaddycain mentioned?

I'd like to fill the arches a little and thought that 18'' alloys would help with this. I hadnt considered lowering the Bongo, hoping that the wheels would fill the space enough.

Anyone with any further comments please!!??