gbpackers4

I've never really noticed it until yesterday when I was on the highway with my 20's (chrome), and I was trying to go past 90... I know its not a good idea because the rims are at a higher risk of being damaged, but when I was at 90, my RPMS stayed between 5/6....... Not only was it so high, but it felt like I could not go any faster....
Fastest ive been is 120 mph without the rims, and the rpms were not shooting up through the roof.....

My question is: With rims, do you have to go slower on the highway (50-60ish) to not destroy your tranny/car...??
Also, driving fast with rims is a bad thing ... correct??

Indio

iTrader: (1)

Driving fast period is a bad thing if you don't know what your'e doing but rims are rims as long as they are mounted, balanced and torqued properly, as for rpm's unless your shifter was in overdrive off mode I don't know but 20's won't cause your rpm's to jump like that.

Chpsk8

Seems excessive to me. I've been to 90 a few times with my 20s and the revs weren't all that high.

The issue at hand here is the circumference of the wheel has changed which will in turn effect the overall gearing ratio in relationship to 1 full rotation of the tire. In effect, one rotation of the wheel will take longer to do thus increasing your revs to move the same distance.

In this instance it has less to do with weight, and more to do with circumference .

rominl

iTrader: (4)

ok, first of all, when you say 80 or 90 i think you are rerferring to just the speedo, right? if so, i can tell you one thing for sure, having 20s will NOT change any of your speedo setup. it will affect your REAL speed, but the speedo only looks at the speed sensor at the wheels calculating the revolutions, it won't know the overall diameter change

and 5000 to 6000 rpm at 90mph, something is terribly wrong with your car (if you are not accelerating hard at that time). on a stock gs400 for example, at 80mph cruising, it's about 2800rpm. at 90, i would say at most 4000 (should be lower than that). my car with the rear super lsd, that's what changes the rpm. and even with my lsd, at 80mph (on speedo) it's only 3000rpm

and for the record, i have 20s too

Tranman

iTrader: (1)

sounds like its not shifting into fifth gear...

rominl

iTrader: (4)

that's what i was thinking too. but if he's already at 5000-6000 at 90mph, that somehow sounds like 3rd gear to me. redline is 6200, and supoosedly by then you should be able to hit 120 already?

JeffTsai

iTrader: (1)

Bigger rims will actually make your speedo read lower than your actual speed. Since the circumference of the whole wheel is bigger now....if the speedo says you're going 80, in reality you are probably going 83-85 or so.

D2-AUTOSPORT

correct, the larger wheels theoretically yield a taller final drive ratio, so the revs on the highway would decrease! Thats one of the reason the car feels so much quicker when you take 20's off and go back to stock 16's, the shorter final drive is "snappier".

Lexbox

Something similar happened to me one day although I dont recall the speed nor the exact RPMs. I only noticed that the RPMs were way too high. I just bumped the shift to the right a little like I was manually shifting from 4 to D and all was well. Hasnt happened since though.

rominl

iTrader: (4)

that's correct. overspec wheels make car go faster than what the speedo says. however keep in mind, speedo is still 100% in sync with the rpm, which both have no idea about the overall diameter of the wheel

for example, if you have stock wheels (or any wheels in spec):
speedo = 80mph
rpm = 2800rpm
actual speed (by radar) = 80mph

if you have 20s with overspec diameters:
speed = 80mph
rpm = 2800rpm
actual speed (by radar) = 83mph (for example)

the above case is true. however, to the car it still think it's going at 80mph.

AJ

^WoW , i didnt know that . always thought it was the other way around .

AZ-GS300

Good grief. The size of the wheel makes absolutely no difference. It's the overall outside diameter of the tire that creates the difference.

You can put on 20s all you want as long as you keep the outside diameter of the tire as close to the stock size as possible.

And yes. Your speedometer / tachometer relationship doesn't change no matter what size tire/wheel combo you throw on.

Calculate actual speed difference here:
http://350zx.com/tire-wheel.html

joshoowa

iTrader: (1)

gone 100+ on 20s no problem what so ever

bighitbike

Are you sure your shifter was in the right position? If it was, then take it to the dealer right away something could be seriously wrong.