speedo inaccurate

[photogdave]

Hi all,
First question of what may be many; may Speedo is very inaccurate. Is there a way to rectify it.
My readings at the moment are 4000 rpm; 80 mph indicated; 92 mph GPS.
I can only think that the rev 5 wheels (car is a rev3) are perhaps thowing it out.

[MRturboTom]

What model? What does the speedo read up to?

[chris]

If it's my old rev3 you're talking about, and I remember your introduction post stating that it is, then it was always out for me...
however I got used to it, although I was going to change the speedo clocks over to some 160mph UK clocks I have in my possession to try and rectify this. The problem with Jap imports is that the clocks are originally 180kph clocks with a mph facia either stuck over the top, or replaced. Once the needle has been removed it is very difficult to replace it accurately, I thought about taking it out on a drive with the clear plastic removed from in front of the dials, removing the needle and using a sat nav speed indicator to replace the needle in the correct position... I never did though unfortunately

...it's a rev3 G-Limited (JDM import) BTW

[fingers99]

Bigger wheels (more accurately, a bigger rolling circumference) will turn lower revs for a given road speed, so the speedo will under read.

Could be that later cars use a different ratio speed sensor.

Legally, speedos are supposed to under -read, but ..........

If all else fails, there are cheap GPS speedos out there.

And then, import cars..........

[chris]

rev5 wheels are the same size as rev3-4 wheels so that won't make any difference (unless the rolling radius of the tyres is different from recommended of course)

[Steevio]

i think my Rev2 tubby is also slightly under reading, like 27mph when doing 30 but further out when up at 70-80 etc. however the rev counter is my main issue, reading WAY too low and rises very leisurely even when giving it abit of right foot!
mine have been changed at import and i arent really fussed about the speedo but would like to sort the rev counter out if poss.

[gavsdavs]

What sizes are your rear tyres ?

Have a play with this:
http://mr2.gordaen.com/wt_calc.php

[photogdave]

Thanks for all the quick replies.
Chris, is the removing the needle and replacing it according to GPS a tried and tested method? And is it quite straight forward to get to the needle?

[peteV6R5]

Thanks for all the quick replies.
Chris, is the removing the needle and replacing it according to GPS a tried and tested method? And is it quite straight forward to get to the needle?

Dave, yes it is, can just take a bit of time.
Most people have to do this on imports or on 2's that have had their dials changed to lockwood or speedhut dial faces as examples.
remove the cluster
take off the perspex front cover (easy un clips)
ease the needle past the stop and then let it rest for about 30 seconds.
it will come to rest at about 6 o'clock (hope you understand that) pointing down .
put a mark on white back
If you know how much it is out then lever the needle off (easy just a push fit). let everything settle for 30 seconds.
put the needle back on without moving the shaft, if it was reading 10mph over then put it back on the distance over towards 5 oclock. if it was reading 10mph lower then put it back on the distance under towards 7 oclock.
install and take for a run and check the accuracy.
do above again if not.
It took me 2 re settings to get mine right after installing my speedhut dials.

The other method is as above but install without the perspex and have needle off completely. as you are actually running the car (not on public roads) get your passenger to put the needle on the speed you are running at. so if you keep constant 40mph they put the needle back on at 40mph.
Then do an accuracy check.
and repeat if required

[photogdave]

Pete, awsome how too. I think that will be the solution. Many thanks.

[Steevio]

Good stuff Pete, i might give that a go on my speedo too, but pretty sure it wont sort my RPM issue out will it as it just doesnt seem able to climb very well and last night noticed when i think revs were probably up at 6k it'd only climbed to about 3 and very slowly!!

[peteV6R5]

Good stuff Pete, i might give that a go on my speedo too, but pretty sure it wont sort my RPM issue out will it as it just doesnt seem able to climb very well and last night noticed when i think revs were probably up at 6k it'd only climbed to about 3 and very slowly!!

see you on a few threads with this issue - me I think its either sender or taco issues.
The easiest this to do is
1) check connections on back of dials - umplug and plug in a few times to clean up.
If sorted if not next:-
Change tacho with a known working one
if sorted if not then you know its the input side so start with sender unit feed from igniter on the diagram iI have BGB / systems ccts / combination dials