Speedo Reading

[satancom]

Morning all..

I need to confirm my speedo reading. Tyre size is fine 225/45/17 on the fronts which is nearly the same rolling radius as standard. So how many miles per hour would I be doing in top gear per 1000rpm?

I have found some information which suggests in top I should be getting 26.070MPH per 1000 rpm, meaning im doing just under 105mph at 4000rpm.. Is this correct? If so my speedo is well off.. This is all on a 1984 2.5

Hard to judge speed as the two cars mi currently driving are a new VW passat that just wafts along and an old land rover that makes everything feel like 100mph [&o]

Any confirmation would be good, I can then see about sortign it out!

 

[James_G]

I doubt any 944 speedo is that accurate. Mine over reads by 10% at 70mph. Simplest way to find out is to use or borrow a sat nav and sit at a constant speed on a clear motorway. Trying to determine the theoretical speed is probably just confusing matters.

 

[944s2]

Use this

http://www.alloywheels.com/tyrecalc.asp

enter tyres you have and tyres standard car had and it will show difference

 

[satancom]

Well at 4000 RPM in top im readin gjust above 80mph. Will probably use sat nave but thats not even spot on due to the curvature of the earth I have always used engine revs and final drive ratio which should give an accurate indication of speed!

Anyhow I will let you know howI get on, cheers!

 

[PSH]

not sure what model you are driving but 80mph at 4000 in top sounds way off for all models , certainly is for the Turbo. Plus as a rule of thumb the true speed should be less than what the speedo says not more. I find sat nav to be very accurate and mine tells me that above 30mph my speedo reads a constant +6mph in top all the way to max which i have to admit surprised me that it didn't increase proportionately?

pete

 

[satancom]

not sure what model you are driving but 80mph at 4000 in top sounds way off for all models

Its a 1984 2.5 Lux.. Yes it would also explain the odd looks as I was traveling up the Dual carrigeway at 85mph (indicated)... which seem slike it may have been nearer to 110mph [:-]

Think i'll put my sat nav on tonight and find out as theres a lot of speed cameras around my area!

 

[tref]

ORIGINAL: PSH

not sure what model you are driving but 80mph at 4000 in top sounds way off for all models , certainly is for the Turbo. Plus as a rule of thumb the true speed should be less than what the speedo says not more. I find sat nav to be very accurate and mine tells me that above 30mph my speedo reads a constant +6mph in top all the way to max which i have to admit surprised me that it didn't increase proportionately?

pete

Surely that just means your speedo needle is positioned 6mph to the right of where it should be on the spindle.

 

[PSH]

hmmm i like your logic tref... guess knowing that no speedo gives a correct reading i just except it for what it is... mind you I don't think it's a good idea to try and move the needle...

 

[944s2]

Its a 1984 2.5 Lux.. Yes it would also explain the odd looks as I was traveling up the Dual carrigeway at 85mph (indicated)... which seem slike it may have been nearer to 110mph [:-]

Think i'll put my sat nav on tonight and find out as theres a lot of speed cameras around my area!

two things - if your gearing is that far out the car must be fairly slow accelerating and secondly does the fact that all the other cars are/should be going so much slower not give you a clue to your real speed?

 

[satancom]

two things - if your gearing is that far out the car must be fairly slow accelerating and secondly does the fact that all the other cars are/should be going so much slower not give you a clue to your real speed?

It does feel slow.. but I put that down to it being a 2.5 8v. The dual was relatively short and some people routinly do 50mph on it so its not a great way to judge speed to be fair! I have my sat nav with me today so I will check. I have now found another site that suggest the speed is more like 22mph per 1000rpm. If nobody can confirm this I guess sat nav is the only way!

 

[tref]

There are several ways in which a speedo can be out, and exasperated by a combination of them... If the tyre rolling radius is out, that will give a proportional change. If the speedo itself over or under reads, that will also give a propotional change - the difference between the two being that the odometer won't or will read correctly accordingly. Alternatively there can be an "offset", as PSH described, out by a fixed figure - commonly a problem where the gauges have had overlays stuck on them and the speedo needle hasn't been positioned on the shaft exactly where it came off. I've often meant to work out what the difference is just between a new set and worn out tyres, but I have never remembered to measure the depth of tread on a tyre when new - and I dare say it varies from brand to brand anyway.

 

[PSH]

according to a road test from May 1982 the 944 lux had a ratio in top 0.829:1 giving 22.8mph per 1000 rpm.

there does seem to be some difference in tests though and i'm not sure if this is due to gearboix ratio options.. I do recall reading at some time in the pass about a customer who ordered a closer ratio box giving quicker accelaration but lower top speed. The turbo in general seems to have same ratio but higher speed/rpm... 25.8 in fact but I also have a road test giving a final ratio of 0.730 for a 944 lux equating to 28.26 per 1000 rpm... so who knows what the true figue should be..lol

 

[vitesse]

ORIGINAL: PSH

not sure what model you are driving but 80mph at 4000 in top sounds way off for all models , certainly is for the Turbo. Plus as a rule of thumb the true speed should be less than what the speedo says not more. I find sat nav to be very accurate and mine tells me that above 30mph my speedo reads a constant +6mph in top all the way to max which i have to admit surprised me that it didn't increase proportionately?

pete

All my cars are around 5-6mph slower than the speedo from 30mph indicated ,so I always use the satnav to stay legal & it stops white van man being up one's whatnot in roadworks etc which happened pre-satnav when I relied on the speedo.

 

[satancom]

Cheers for that PSH.. AT least I know why different figures are quoted now

Cheers

 

[BlackMagic]

Just remenber that it is rear tyre size that determines speed (as the spedo is driven off the gearbox and not the front wheels).

My 944S2 underreads by 10% on worn tyres (ie 77mph indicated is actually 70mph)

 

[satancom]

Thought the early cars took speed from the near side front. ANyhow the rears are 255/40/17 so more or less the same rolling radius as the front anyhow so not that relevant!

 

[Mike_Pollock]

Early cars have speedo drive off the front n/s wheel.

My '83 2.5 has had 2 speedos one reads approx 69 with 70 on the satnav the other show 78!

I'm running 215/60 x 15 on cookies.

The tacho shows approx 3050 @ 70mph which works out at approx 22.9mph per 1000 rpm which agrees with the road test quoted.

Mike

 

[satancom]

I confirmed mine with sat nav.. Its reading is fairly good up to 70.. slightly under by 2-2 mph but nowt to get worried about

Cheers