Toyota Hiace 2004 Speedo

[Muszeo]

Hey all,

My wife and I recently bought a 2004 LWB Hiace to convert into a Campervan. It's a solid vehicle and a great platform to use. We've performed an initial conversion and took it away for two weeks just recently and enjoyed using it very much.

However, I have noticed that that speedo is registering upto about 8km/hr slower than what you're travelling at according to my GPS. At slow speeds the difference is hardly noticeable, but it gets worse as you get faster. At 100kph it's registering about 92kph. I've had vehicles with inaccurate speedos before but they have always overestimated speed rather than underestimated it. Does anyone have any suggestions as to how this could be fixed?

I have searched the web and understand that a few things could be at issue. It could be the speed sensor on the gearbox (I've located that I think -- it would be a quick and easy fix), the ECU (I'm not sure where this is located on the vehicle yet) or potentially the speedo itself, or the wiring.

Any advice as to what is more likely the issue would be appreciated. Also, if it is the ECU, is this something I can replace or will it require a specialist? The ECU on an old Volvo I had once couldn't be replaced without laptops and wizardry, so I am wondering if this is the same sort of thing?

Thanks,

Mart.

[FROSTYBALLS]

Has it had a change in wheels and/or tyres that could affect the speedo - ie. made it under read?

[Muszeo]

Hi, thanks for your reply!

Yes, I’ll check the wheels out as I think they are not originals. I will drop the spare down and see if the measurements are different.

[Muszeo]

Looks like the wheels sizes are different. The spare looks original and is an R14, whilst the fitted wheels are R15. They also look to have a larger tyre size. So this could be it!

[FROSTYBALLS]

Could use a tyre calculator to determine whether the different wheels/tyres are the culprit - eg 

http://www.autopartuk.com/tyre-calculator

[Muszeo]

Thanks mate, yes that seems to confirm it. Looks like the wheels (195/70/R15) are too large resulting in a 13.3% inaccuracy in the speedo!