Tyres

[Jimbob]

Nice one fella!

[TW166Y]

no probs dude

[316eyes]

what size rims? do you want to keep the original overall diameter (keeps speedo and miles clock correct)?

Does this mean that everyone with 15" wheels and above have incorrect speedos and if so are they + or - in their reading?

[kam-325i]

As you increase the size of the tyres, the speedo will be reading under, as the larger wheels will travel further per rev.

There is a calculator about here or on BMWE30Net that will tell you how much your speedo will be out.

With my 16" wheels, i think it was about 5-8% out. an indicated 70Mph was like a real 73-74

[kam-325i]

Heres one

http://www.paspeedo.com/calculator.htm

[316eyes]

Calculator only uses change in tyre size. What happens if you go from standard 14" wheels and stock tyres to 15" BBS and say 205/55/15 tyres?
Looks like you don't need a big % change to put you in danger of being caught by a speed camera, so I assume the guys that are running 17" wheels and fat tyres must drive round watching their speedos all the time!

[kam-325i]

Calculator only uses change in tyre size. What happens if you go from standard 14" wheels and stock tyres to 15" BBS and say 205/55/15 tyres?
Looks like you don't need a big % change to put you in danger of being caught by a speed camera, so I assume the guys that are running 17" wheels and fat tyres must drive round watching their speedos all the time!

You put in the size of your cars standard tyre size on the left, and the size that you want to put on on the right, this will then give you the % difference over standard......and put in the speed, say 70

I would put in 195/60/14's as standard, and 205/50/16 as my tyre size now. 70 on the standard would be 74 on the larger 16"'s

[Nuno318is]

Well on my car i have 9*16 and i use 225/40-16 rear and 215/40-16, no problem so far

[TW166Y]

Well on my car i have 9*16 and i use 225/40-16 rear and 215/40-16, no problem so far

you have pm dude

[Simon13]

9j's all round. The car will drive like an utter heap of sh!te. I know it's looks but don't do it!

[Nuno318is]

i didnt say for him to do it !!!!!! i sed what i have?!

[fuzzy]

your speedo is about 10% out anyway, shows 77 when your only doing 70 etc, according to gps .so that would offset any discrepancy when changing your wheels.but if you drop a profile size in tyres to keep the overall rolling circumference roughly the same then it shouldnt matter to much.

[Simon13]

your speedo is about 10% out anyway, shows 77 when your only doing 70 etc, according to gps .so that would offset any discrepancy when changing your wheels.but if you drop a profile size in tyres to keep the overall rolling circumference roughly the same then it shouldnt matter to much.

ditto, aslong as the overall size of the wheel is the same to standard then the speedo will be fine, plus 10% out from the factory

[Nuno318is]

yes I know, perhaps I must have rank 225/35 or put the same 215/40, buy the way, you know the site to make these accounts?

[TW166Y]

9j's all round. The car will drive like an utter heap of sh!te. I know it's looks but don't do it!

oooops ive gone 9j all round, purely for looks of course

[316eyes]

your speedo is about 10% out anyway, shows 77 when your only doing 70 etc, according to gps .so that would offset any discrepancy when changing your wheels.but if you drop a profile size in tyres to keep the overall rolling circumference roughly the same then it shouldnt matter to much.

Are you saying that all speedos are always giving a reading 10% over because, if that is the case, why can't BMW correct this. If they consistently fit 10% over reading speedos it would be a relatively simple job for an design engineer to correct this.

Or are you saying that speedos don't give an accurate reading up to 10%.
Why don't they give a reading up to 10% below? As the reading is mainly mechanically produced wear and tear will affect reading over time (either + or -). If this increases the over read by another 2% say, by your reasoning this adds 1.4mph to the 77mph giving 78.4mph; but if your speedo is under reading your actual speed could trigger a camera!

Not trying to knock your info Fuzzy, just trying to get my head round how you can be reasonably sure when you cruise through a fixed camera in a 40 zone your not doing 48!!!!

[kam-325i]

Manufacturer's will have the speedo's made to a certain spec, the general rule is that the accuracy of the speddo is within 10%, The more accurate the speedo, the more expensive it is to make......

doing 48 but showing 40 is 20%

[316eyes]

Manufacturer's will have the speedo's made to a certain spec, the general rule is that the accuracy of the speddo is within 10%, The more accurate the speedo, the more expensive it is to make......

doing 48 but showing 40 is 20%

Agreed. But is it always 10% over? "within 10%" means it could be under reading by 10%. Still can't understand BMW consistently fitting units with, by their normal standards, such a big manufacturing tolerance. And the E30 was never considered a cheap car. Does anyone know if the modern digital units are any better as a lot of the mechanics which can cause inaccuracy are now missing?

[kam-325i]

This "10% over reading" is an unwritten rule....some say it is to make the car appear to be going faster.......Its just not BMW that do it, it applys to all production cars....

[316eyes]

This is the sort of discussion you have in a pub about 11.05pm after plenty of throat wetting and after you've done the football, telly, work and weather and you can't think of anything else to talk about!!! How come it makes sense on here when it would prompt most people to drink up and go for a curry!!!!!!!!!!!!!!!!!

[Kos]

i disagree about the spedos, on bmw they are electrical and more accurate then most cars which are mechanical.

seen a test on it in a motoring magazine years agoe, they used an e30 sierra renault and a mitrubishi, the bmw gave the most accurate reading never being more than 3mph over than actual speed, sierra was the worst, at low speeds it over read then at high speeds under read.