Speedo calibration

[zigzag]

I hope there is an easy answer to this. We just bought our Bongo about a month ago and I suspect the speedo is reading under, e.g. doing 60-65mph on the motorway was more like 55mph actual. Is there any way of checking the speedometer, short of buying a satnav or trying to do a steady 30mph, for example, over a measured distance?

[dp bradford]

... Is there any way of checking the speedometer, short of buying a satnav or trying to do a steady 30mph, for example, over a measured distance?

That's how the police do it! Is there a Measured Mile anywhere near you?

My speedo under reads by about 7 - 8%, I believe it could be down to the way it's been chipped

[madmile]

The easy way if you have standard size wheels and tyres is to check the rev counter. As long as you have reached lock up on final drive then 1000rpm is 25mph, so at 50mph you should be doing 2000rpm and at 75 it should show 3000rpm.
This is accurate to within 1mph or so according to my sat nav and I have checked on several bongos.

[dreamwarrioruk]

i believe by law they have to read under, exactly but no overread no doubt someone will correct me if im wrong,
ours is out by 2mph, ie 60 on gps shows 62mph on dial

[Peg leg Pete]

I have standard size wheels and our speedo seems fine, unchipped speedo from 626, measured mile is fine too all thanks to cloggers fine work

[Bongoplod]

The easy way if you have standard size wheels and tyres is to check the rev counter. As long as you have reached lock up on final drive then 1000rpm is 25mph, so at 50mph you should be doing 2000rpm and at 75 it should show 3000rpm.
This is accurate to within 1mph or so according to my sat nav and I have checked on several bongos.

Ive heard this phrase before,can someone explain to me what "lock up on final drive" is?
Also,are SatNavs that accurate that speedos can actually be calibrated with them?

Brian

[Naughtydog]

i have a road angle, and a sat/nav both always read exactly the same mph,and are controlled by 2 different aerials, the only thing that makes them unreliable is the 1 second delay in converting the information recieved from the various satelites, in to mph. so its deadly accurate at a constant speed,

7-10% accuracy is standard on all motorcars, so the speedo reads under the true speed. it is a legal requirement on behalf of the manufacturers.
this allows for tyre wear, the closest your speedo will be to a "most trueist" reading is when you have brandnew tyres on.

phones on the motor way are 1 mile apart, travel at 60mph, and you should reach the next one in 1 minute.

if it takes longer then you are going slower than 60mph if its quicker then you are going faster than 60mph

[moonshine]

Lock-up is actually on the torque converter. When you start off from rest, the torque converter is designed to slip, which provides extra torque in each gear. When your gearbox is cold, the torque converter will continue in its controlled slip, even in 4th gear, to allow the engine to reach its working temperature quicker. The disadvantage of this is that this slip consumes slightly more fuel, and is the reason why older automatic vehicles were slightly more thirsty than their manual equivalents. The Bongo however has lock-up, which locks the torque converter, eliminating this controlled slip. When this occurs at about 40 MPH, the engine is driving the gearbox directly, as if the vehicle had an "extra gear". The rev-counter will stabilise with the road speed, and the needle will not waver about like it does before lock-up engages.

Well, the above is as I understand it anyway.

[Bongoplod]

phones on the motor way are 1 mile apart, travel at 60mph, and you should reach the next one in 1 minute.

if it takes longer then you are going slower than 60mph if its quicker then you are going faster than 60mph

What a cracking tip!
Never though of that one,im off for a blast down the M6

Brian

[zigzag]

Thanks for the replies. I've already done a motorway test and when the speedo was at 60mph my actual speed was about 55mph, but I don't think I'll be doing a steady 30mph for one mile on a motorway.

[Peg leg Pete]

Naughtydog what a great logical tip, will try it next time we venture onto the motorway-- think our speedo is not far out

[smartmonkey]

Unfortunately mine is deadly accurate. I always like a few spare mph to make me feel like I am going faster and give me a bit of leeway for speed limits. I assume it is because the needle was accidently put on in the most accurate position after the facia change.