Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

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Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by robinm on Tue Sep 14, 2010 12:52 pm

Hi all, excellent forum.

I have a Piaggio Porter 2006, RED! (It's a Hijet with different badges on.. no difference at all, every single bit on the car has the 'D' for Daihatsu on, Nippon Denso electrics etc. etc...)

I have two questions really.. is it just mine or are all the speedo's inaccurate.. I mean miles off.. I keep up with lorries (all limited to 56mph)
my speedo says 65mph.. I've checked with my GPS (very accurate).. it really is TEN mph out.. it's even bad at 30 (35mph on the speedo)
Are they all the same?

Secondly and most importantly, I never carry any heavy loads and find the gearing miles too short, I mean you don't even really need first gear, is there anyway of changing this simply?

I think bigger diameter back wheels wiould be the best option.. anyone done this? (it'd sort out the speedo too?)

Thanks in advance, your help much appreciated.

Rob.

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by difflock123 on Tue Sep 14, 2010 1:26 pm

the speedos arent very accurate...

But no vehicle speedo is really that accurate.

As for bigger wheels, some on here have fitted alloys (look in the pics area).
Its clearance issues you will struggle with, going too big with the wheels...

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by robinm on Tue Sep 14, 2010 2:16 pm

There seems to be plenty of space for bigger rears..? Not bothered about the fronts.. it'll end up looking like one of those custom vans from the 70's..?

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by difflock123 on Tue Sep 14, 2010 7:06 pm

It when the suspension moves, whether it will touch anything.

Wider wheels (which you will most likely need as you wont get bigger diameter tyres on the stock rims), are ok to a point.
If you go too wide, you wont be able to open your side doors.

Isnt the speedo driven off the front wheel?
If so, changing the rear wheels will make no difference to the speedo.

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by 8bit-ash on Tue Sep 14, 2010 8:50 pm

you might find that the speedo cable needs some lubing or replacing. Mine tends to stick sometimes but seems fairly accurate. As for the gearing mines a 1.3, I very rarely have a load and its fine. I'm sure I read somewhere that if anything they are best suited unloaded as they ain't commercial boxes on the 1.3.
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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by rich the mechanic on Tue Sep 14, 2010 9:25 pm

all but a few rare occassions are cars/vans speedos driven by any thing other than a drive out the gearbox usually around 5th.
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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by robinm on Wed Sep 15, 2010 8:38 am

Yes the drive for the speedo definitely comes from the gearbox so bigger rears will work..

Anyone fitted peugeot wheels of around 14'' with wobble bolts? thats how I am proceeding at the mo..


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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by difflock123 on Wed Sep 15, 2010 12:11 pm

Fair enough, I thought I heard the hijet's speedo was driven off the front wheel.

Bigger wheels will make the speedo read low.

The speedo drive will be set up for use on the standard wheels.

So the bigger rear wheels will be rotating slower at any given speed due to the extra diameter (causing a larger circumference)

Not sure of the hijet gearing, but I do know what Land Rover S3 gearing is (I worked it all out).
The speedo is driven off the gearbox still.

The overall ratio in 4th is 5.396:1 (diff is 4.7:1, gearbox is 1:1, and transfer box is 1.15:1)

On the stock tyres (205/80R16) the overall diameter of the wheel and tyre is 29 inches.
@ 3,500 engine rpm, the actual speed is 54mph

On the larger wheels I have fitted (235/85R16) the overall diameter of the wheel and tyre is 32 inches.
@ 3,500 engine rpm, the actual speed is 62mph.

But the speedo is setup for the original tyres.

So if you fit bigger wheels, your speedo is going to read lower.
Whether the wheels you are looking at will correct the speed, or infact make it worse, I cant tell you.

Just keep it in mind, so you dont get pulled for speeding....

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by robinm on Wed Sep 15, 2010 12:26 pm

Ha! That's a laugh.. Maybe it's because mine is a Piaggio Porter but it's M I L E S out.. most cars have about 5 to 7mph discrepancy at 70mph.. but TEN+ ?

I have found out this brilliant site all about tyre and wheel sizes and what differences they make, even tells you about ride height!.. I'm going for 185/70/ 14 inchers I think.. loads of cheap second hand Peugeot alloys about.. nobody wants 14 inch wheels.. except me!

http://www.net-comber.com/tirecalc.html

Brilliant!

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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

Post by difflock123 on Wed Sep 15, 2010 12:52 pm

Normal speedo error is between 5% and 15%

So at 70mph, that allows an error of between 3.5mph and 10.5mph.

It is a bit more that most, but its not that surprising really...

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inacurate speedo

Post by bones 851 on Thu Sep 16, 2010 7:09 pm

hi there, i,m the owner of bones, the alloy wheels on the van are original daihatsu/pagio , obtained them very very cheap buy pure luck, prior to fitting the speedo was out by a few miles per hour 3-5mph, when the wheels were fited it seemed to change to around 9-10 mph, mech stated something about tyre width, although i thought that would just change fuel consumption, very rairly take the van over 70mph, although have pushed it further. just have to remind myself that it is out..... oh 1st gear is a bit of a pain when unloaded, but handy with the bike/s in the back, part of the charm of driving the jet.... Bones851
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Re: Gearing too low and unbelievably innacurate speedo's.. bigger wheels?

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