7 replies to this topic

[rodandtom]

Hi,

 

I have a '94 Sprite with a 1275 in it, and I've changed it from 12" wheels to 10"s. 

 

It has a speedo going up to 110mph.

 

I want to fit a new speedo so if I put in something like this: http://www.minispare...|Back to search

 

would I have to do something to calibrate it to my car? Even if I don't put the new one in, do I need to recalibrate it because the car was set up with an expectation of 12" wheels on it?

 

Thanks,

 

Rod

[cal844]

Yes as your TPM will change!

[timmy850]

The rolling diameter of your tyres shouldn't change significantly. 12" wheels have thinner tyres.
E.g.

165/70R10 - diameter = 485mm

165/60R12 - diameter = 503mm (about 4% bigger). 

If you still have it calibrated to the 12" wheels it will read that you are going slightly faster than what you are actually doing. 

 

The speedo you have linked from minispares clearly states you can calibrate it yourself, using different methods. I'd suggest you contact them about how they actually work. I'm guessing it uses an electronic sensor and not a mechanical cable like the old factory ones. 

[dklawson]

IF you buy the electronic speedometer in the link, you will also need to buy a pulse generator sending unit that goes on the gearbox.  The pulse generator is connected to the electronic speedometer and you program the speedometer to count the pulses per mile from the sender.  So... you simply program the gauge to read correctly regardless of tire size or final drive changes.  Calibration is not hard in these days where almost everyone has a GPS to accurately measure speed and/or distance.

 

The electronic speedo is a very expensive solution though (figure close to 425 GPB once you add a sending unit and VAT).  You can buy mechanical speedometers that look similar to what was in the early cars.  I think most are calibrated for 1280 turns per mile.  That will require you to either have your brand new gauge calibrated or have a ratio box made.  The ratio box goes inline with the speedometer cable and converts the Turns-per-mile coming out of the gearbox to the Turns-per-mile needed by the input to the speedometer.  Re-calibration or the ratio box will both at least 100 GBP to the cost of your new mechanical speedo.

[rodandtom]

Wow that's a lot of cash!

 

What if I just put in a new speedo which uses the same cable drive that I already have. It's just that I couldn't see one for 110mph on the Minispares web site. They were minimum 130mph so the got me wondering about recalibrating and would it be easier to use the electronic one.

 

Rod

 

[dklawson]

With the mechanical gauges you can change the gearbox cable drive parts in some instances.  Then there is re-calibration of the gauge.  The analog speedometer part is not hard to change.  The odometer portion is where the expense will be.  MiniSpares even mentions in their speedometer link that you posted that you can expect to pay at least 100 GBP for re-calibration.

 

Google "Guess Works" and visit his web site.  There are calculators there that will tell you what turns-per-mile you will get from various component combinations and what gearbox drive gear changes may get you close.

 

When looking at the combinations, don't panic when you cannot get an exact match.  Get as close as possible and accept a small error.  Every car, even modern ones, will have some speedometer error.

[timmy850]

If you were going to buy a speedo like this one, http://www.minispare.../13H4442MG.aspx ask for the TPM of the gauge.

 

Go to the guessworks website as suggested above ( http://www.guess-wor.../Tech/ratio.htm )

, choose your tyre size, diff ratio and the TPM (turns per mile) of the new speedo. It then suggests a new set of worm and pinion gears that would make the speedo read correctly. The gears (around 30 pounds) are much cheaper than getting the speedo unit calibrated 

 

Worm gear http://www.guess-wor...play.htm?id=144

Pinion gear http://www.guess-wor...play.htm?id=321

 

The actual speed listed on the gauge doesn't necessarily relate to the TPM. You can possibly have 110mph and 130mph gauges with the same TPM so they would be a simple swap with no calibration needed.

Edited by timmy850, 07 March 2016 - 09:42 PM.

[maccers]

I've done this and used a new smiths centre speedo. I have a Garmin sat nav that's very accurate when you look at the MPH view. The difference between the Garmin and smiths speedo is tiny up to about 40, then goes a little "wavy" till about 70, then is fine to 90 and I leave it at that...! The smiths speedo is a 130mph unit, I don't think its ever seen 100 even on the rolling road. There are a few places that can make up new cables and have your speedo calibrated to match the wheels. I don't do many miles in mine so not really bothered about the odometer reading.