406oc.co.uk

Hi there guys!

I was in a bit of a hurry down the motorway the other day and I glanced at the speedo to see ~100MPH. I would have guessed it to be at about 4000RPM (2.0HDI 110) but it was just touching 3000RPM . Just wondering what you guys get and if my Pug revs low...

Thanks!

Benjamin

Hi Ben,

Should be about 31 mph/1000rpm

thanks...

so - the maths!

theoretical top speed = 31*5=155

there is no way I could get it to that speed - there just isn't enough power and torque?

or is there?

2.1TD, 3000RPM =90MPH, I have been led to believe.

Top speed for a 110 Hdi will be about 117, depending on who's road test you read.....

I'm normally a little over 3000 RPM at 70 MPH (mine's a 1.8 petrol)

At 3000 rpm it's more like 90mph from memory. Maybe your speed is reading high?

Yeah, 3000rpm is 90mph (not that I ever do such preposterous speeds)

Bugger, maybe my engine is revving high. It was only very recently - i.e. 3 weeks ago - given a full service (including brand new spark plugs, complete oil change, oil/fuel/air filter change, brake fluid top-up, new brake discs and pads, blah blah).

According to my TomTom, I'm actually only doing 63-65 when my speedo says I'm doing 70.

When I then accelerate until the TomTom says I'm doing 70, the speedo says I'm doing 75-ish. At that speed, the engine is revving at 3200RPM.

Without an accurate speedometer (I'm not trusting either the TomTom or the cars' speedo at this point), I'm guessing I'm actually doing around 70 because I've never been snapped by speedcams (never received a speeding ticket), and I am still being overtaken by blokes with German cars (that's all I'm ever overtaken by, is buggers in their Audis, BMWs and Mercedes-Benzes).

highlander wrote:According to my TomTom, I'm actually only doing 63-65 when my speedo says I'm doing 70.

At that speed, GPS devices are very accurate in terms of speed calculations as the large distance covered per time division substantially reduces the effects of GPS errors (due to atmospheric distortion, etc) on the calculations. Nearly all cars speedos over-read by 5-10% anyway.

Yep, I get pretty much the same, set my cruise at 76/77 on the speedo, TomTom shows 72/73, so I just ignore cameras etc. and haven't had a problem. This works out at about 2400 RPM
(I do reduce it a bit for the average speed jobbies though).

One question I have pondered. Do GPS systems give an accurate speed if you're not travelling in a straight line, or if you're on a steep hill?

dogslife wrote:Do GPS systems give an accurate speed if you're not travelling in a straight line, or if you're on a steep hill?

Depends, and probably not.

Tomtom does appear to base its speed calculations on the path followed (not telling you how I know this ), but I can well imagine other brands (especially the cheap ones) would tell you're going a lot slower than you are because they just do a single 2-dimensional sum because it's much less computationally expensive

No in-car satnav systems I know of have elevation information, however since GPS does work using three dimensional co-ordinates, it's not unthinkable that some systems may take historical elevation data into account. I wouldn't count on it though. Simple way to test is to take one aboard a jet fighter and pull into a vertical climb... or you could drop one off the side of a tall building with a wireless camera strapped to it

mjb wrote:

dogslife wrote: Simple way to test is to take one aboard a jet fighter and pull into a vertical climb... or you could drop one off the side of a tall building with a wireless camera strapped to it

Mythbusters?

totaleclipse wrote:

mjb wrote:Simple way to test is to take one aboard a jet fighter and pull into a vertical climb... or you could drop one off the side of a tall building with a wireless camera strapped to it

Mythbusters?

Did they do that? sweet!