Hey guys, when I purchased my zed, it had a tomei speedo and currently still has it, and i've been told by my friends that I drive too slow and my responses is I am doing the speed limit. I tested it with a roll with my friend and it seemed off since he was a car length infont when we were both doing 60km/h. I was wondering if anyone knew the ratios? since the tt have longer gears then the na.. like for example, in the TT.. if you're in 4th gear sitting at 2000rpm, what is the speed you guys are doing? etc.. Sorry if this post isn't clear! hopefully someone can understand what I am asking and help me out!
Gearbox ratios are the same. Diff ratios are different, with the NA being 4.083:1 & the TT at 3.692. Tyre size can affect speedo readings. If the tyre is smaller in diameter than OEM sizing, the speedo will read faster than the vehicle is actually travelling.
If you have oem size tyres.... or their equivalent (1983mm circumference), you should be travelling at 65km/hr @ 2000rpm in a manual ttz and 93km/hr in an auto ttz (assuming you have oem gearbox and diff)
Likely probability is your speedo is out and needs to be recalibrated or your speedo sensor is starting too play up. Go too a speedo shop there's one in Lidcombe Sydney if your in Sydney .
Use a smartphone app, some of them have a reverse display so you can sit the phone on your dash and reflect of the windscreen to give you a heads up display. Easy way to check speedo accuracy across the range. [yt]DY_I-b5_mvc[/yt]
Yes, just a Brisz has suggested. GPS can display speed by triangulation of your position to three satellites. The smartfone apps use the built-in GPS to do the same. If your speedo is over reporting your actual speed by a little bit, you might be able to correct it by putting wider tyres (with same profile) on your existing wheels. Use the tyresizecalculator website you'll work out what you need.
Here is a link http://www.tyresizecalculator.com/tyre-wheel-calculators/speedo-error-calculator As an example If you are running stock 16" wheels (no spacers) then you have plenty of clearance, enough for snow chains you could increase tyre profile by one size and tyre width by 2cm and still have clearance, I think there is plenty of room. Using the website, put the stock tyre size in as the baseline for comparison. Add tyre width but keep stock tyre profile. For every 2cm added width (10mm either side of rim) you will add +3.5% to your actual road speed. OR Add tyre profile but keep stock tyre width. For every size increase eg. 50-> 55 you will add +3.5% to your actual road speed. If you add 2cm in tyre width and one size bigger in profile, you will add approx 7% in actual road speed. Your speedo doesn't measure speed relative to the road, it measures speed from a combination of sensors on axial rotation and transmission. Changing the radius of the overal wheel allows you to tweak the % error between actual road speed and speedo reading. If you are ready to buy new tyres and are keeping the same wheels, then this is something to consider. If you are changing wheels and tyres, this is something to consider.
The speedometer..... ....receives a signal from a single sensor attached to the gearbox. The sensor has a gear wheel, that engages in a gear on the gearbox output shaft. There is a margin of error present in all the Z32 speedometers. As an example: A Twin Turbo Z32 built for the Japanese Domestic Market was fitted with 16 X 7.5JJ wheels, the specified tyre size is P225/50R/
Continuation of above post..... A TT built for the US market had different sized wheels & tyres. Being 16 X 8.5JJ with specified tyres of P245/45ZR 16. The circumference of the two tyre sizes is different, so the tyres on the 8.5" rims rotate 844 times over a one mile distance & the tyres on the 7.5" rims rotate 840 times over the same distance. The transmissions, speed sensors & differentials are identical, so there has to be speedo inaccuracy. GPS is a great way to determine speedo accuracy. HUD units are available on ebay & are excellent for determining speed when approaching speed cameras. On a side note, there is very little room between the sidewall of the tyre & the guard when running the stock 8.5" TT rear wheel. Therefore the fitment of snow chains could be an issue.
Yes, OK as a general comment, (and I started the snow chain reference and it is snowing in USA) but the author of this thread isn't driving a TT with USDM spec TT rear wheels. (probably not in snow country either) What is he driving? 16" JapSpec stock wheels (no spacers)? He should check the info in the tyre size calculator website against a GPS speedo and then he can work out the best way to improve his speedo accuracy. He can get the speedo recalibrated or make a different tyre/wheel choice to compensate for the inaccuracy. I suppose it depends on how much tread is left on his tyres
