Gear ratios

Wanda

Authorised Trader
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Location
Kent
Car
Renault Megane 2 cc
Might be a silly question but bare with me, if you up size your alloy rims from say 17" to 18" or 19" does that indirectly affect your ratio? I was thinking if you are increasing the circumference of the wheel meaning one hole revolution would take you slightly further would that mean the revs of your car would go down at a steady speed?
 
Interesting question! I know when I went up in wheel sizes from the original 17" through to the current 19" size; I had to have the garage reprogram the ECU to say so, then the speedometer was automatically calibrated accordingly.
 
The idea is to keep the overall diameter the same as the original rim and tyre combination. This is done by fitting tyres of a lower aspect ratio and as such the speedometer will remain correct. What is the current tyre fitment to your car?
 
I currently have 205/50 17" standard tyres, I was thinking of upping to 225/40 18" just to cope with pot holes.

As a result the circumference should go up slightly, therefore would it mean the car would rev slightly lower at speed?
 
The idea is to keep the overall diameter the same as the original rim and tyre combination. This is done by fitting tyres of a lower aspect ratio and as such the speedometer will remain correct. What is the current tyre fitment to your car?

I thought the same thing HDi, the tyre profile was always reduced each time but the garage insisted on doing the reprogramming.
 
They are technically correct if you want the speedo to be totally accurate this is the only way.

Working out the overall diameter is pretty easy - I have a little spreadsheet I put together years ago for this purpose - anyone who wants it please let me know. A couple of mm difference in diameter is not worth worrying about, tread wear will account for 16-18mm difference in the life of a tyre anyway.

I have 225/55/16 on my car, which is the factory fitment. I quite like the idea of fitting 17" rims and 245/45 tyres. They'd look nice but the cost is prohibitive and it will make no difference at all to the handling of the car, which is very good anyway.
 
I currently have 205/50 17" standard tyres, I was thinking of upping to 225/40 18" just to cope with pot holes.

As a result the circumference should go up slightly, therefore would it mean the car would rev slightly lower at speed?

The 18s will be more uncomfortable as there is a smaller profile. You will also find the car more prone to tramlining. Not to mention would 225 tyres even fit? In theory bigger wheels do 'gear up' the car, but you won't notice any difference.

I have 225/55/16 on my car, which is the factory fitment. I quite like the idea of fitting 17" rims and 245/45 tyres. They'd look nice but the cost is prohibitive and it will make no difference at all to the handling of the car, which is very good anyway.

Do you run staggered or square mate?
 
The 18s will be more uncomfortable as there is a smaller profile. You will also find the car more prone to tramlining. Not to mention would 225 tyres even fit? In theory bigger wheels do 'gear up' the car, but you won't notice any difference.



Do you run staggered or square mate?

Same all round. I know some of the Sport models had different setups front and rear but mine is an SE model which is basically means more kit than the 'base' model.

I will be fair, the handling is superb; BMW seems to have managed to disguise a large sporty saloon as a luxury car. I've driven loads of E39s in the past and this one is as good as any. It was a panic purchase last year, probably not my first choice of car, but it presented itself so well I that bought it.
 
In answer to the OP question it will affect your gear ratio. Acceleration will be slower but your top speed will be increased.
 
True but the OP had clearly said he was planning to increase the circumference of the wheel ;)

:) I interpreted wheel to mean the metal portion which (certainly here) is more usually referred to as the rim.

Additionally I think it's better to stick with the OEM overall diameter.
 
if you want to check the change in rolling circumference then there are ready made calculators on the interwebby which will do this for you.

A few points

It will only change the ratios if the drive wheels RC is different
It it is 4WD/AWD any change can upset the system
 
"It will only change the ratios if the drive wheels RC is different"

Well spotted! It's obvious now you mention it. Having different rolling diameters is likely to cause ABS and stability systems an almightly headache.
 
Working out the overall diameter is pretty easy - I have a little spreadsheet I put together years ago for this purpose

There's also this site which is pretty useful: http://wheels.corinthian.se/

I used it when I decided to go from 235 to 265 but still wanted to maintain the same rolling diameter.
wheels-calc.png
 
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