Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[bigegg]

I have 245/45 zr 19 tyres on my omega MV6.
I understand that the standard size is 245/45 r17.
Am I correct in assuming that the outside diameter of the wheel is 19in + (245mm x 45%) = 23in approx.
compared to 21in on standard wheels?
So my speedo is going to read approx 10% slow?
ie, road speed = 30mph, indicated speed is 27mph?

[Jimbob]

I have 245/45 zr 19 tyres on my omega MV6.
I understand that the standard size is 245/45 r17.
Am I correct in assuming that the outside diameter of the wheel is 19in + (245mm x 45%) = 23in approx.
compared to 21in on standard wheels?
So my speedo is going to read approx 10% slow?
ie, road speed = 30mph, indicated speed is 27mph?

they should be 235 45 17...

[Agemo]

Close enough. Compared with the standard 235 45 17 your tyres would make the speedo show 7.25% greater than indicated. i.e. Speedo 30mph - actual 32.3mph.
We are assuming your speedo is calibrated, but most are way out, they will read high anyway. Best to compare with a TomTom.

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

[bigegg]

I meant 235/45 

Thanks - I don't have to worry about re-calibrating the speedo then?

[hotel21]

There is a wheel/tyre size comparitor further down the following page....

http://www.carbibles.com/tyre_bible_pg4.html

[bigegg]

That's interesting - I've just read it - ta.

next question is, how does one re-calibrate the speedo?
I'm assuming by using the tech2 to reprogram the abs ecu?

Not a certified calibration, btw, just for my piece of mind.

[2woody]

I have 245/45 zr 19 tyres on my omega MV6.
I understand that the standard size is 245/45 r17.
Am I correct in assuming that the outside diameter of the wheel is 19in + (245mm x 45%) = 23in approx.
compared to 21in on standard wheels?
So my speedo is going to read approx 10% slow?
ie, road speed = 30mph, indicated speed is 27mph?

no. you're not correct.

the only way to do this is to compare the published wheel revs per mile figure from the tyre manufacturer. doing a static calculation will be wrong, as will be the on-line jobbie that you found.

the standard car is "expecting" 817 wheel revs per mile. Your new tyres have 753 wrpm - so that's an induced speedo error of 8% - a whopping great error.

there's a further point - your new setup places the hub 25mm further away from the ground (hub height 12.7inch vs. 11.7inch) this WILL make the car handle significantly worse.

sorry !

[bigegg]

Don't be sorry!
I bought the car with 19" wheels on it - I'm investigating the options of returning to 17" standard wheels.

Looking at the tyre guide above, it would appear that 235/35 r 19 are within 0.5% of 235/45 r17, so I may go for those.

I'm assuming that this will make the ride much less comfortable tho?

is a "swap" advert classed as a "for sale" or "wanted" ?
just in case I decide to go back to 17's

[2woody]

it's even closer than that - 235/35 x 19 is 817 wrpm, so it'll be identical to standard.

most of the calculators on the web, etc only take into account the nominal details of the tyre (the 19" + 234 x 45%, etc.). The whole purpose of using wrpm is that it takes account of slip and deformation that you get as the tyre is rolling and is much more accurate.

regarding the actual tyre size for a minute - generally good handling and comfort rely on having a tall sidewall to move around a bit. A very low sidewall results in a very firm ride and poor handling, but good levels of grip. Just think of handling and grip as two opposites.

in my humble non-expert opinion, 235 is far too wide a tyre for this chassis, optimum handling occurs at around 215 width, but even 195 has proved to be great fun in the past.

you could always double check that your speedo still over-reads against a sat-nav. at 62.5mph on the sat-nav, the speedo should show between 62.5mph and 71.25mph

[Agemo]

Not sure what a "comparitor" is, but the tyre size size calculator I put up seemed simple, but if it is wrong then Mods please feel free to delete.