tacho to speedo conversion

[scotty]

Hi , hopefully a quick question - speedo on 7 is U.S.  it is a standard 81 vintage dhc 2 litre with 5 speed box . Can someone tell me roughly what speeds would be at 3k revs , 3.5k revs and 4k revs cruising in 5th .

I'll maybe tell you why after

[scotty]

come on surely someone can tell me what speed they would be doing @ 4000 revs in 5th  

[andrewt]

With a 3.08 diff, 14" wheels & 195/60 tyres 4000 revs in 5th = 100mph on GPS

 

Proportionally then with a 3.9 diff it should be 79 mph......

 

Any help?

Cheers

Andy

[scotty]

Thanks Andy , I hope its 79 . I picked the car up and the speedo didnt work so have no knowledge of what the rev counter would be at various speeds . Unfortunately the miserable B%%%%%%%s in Lancashire with thier covert camera van was taking pictures on an overbridge .

Cheers Scotty

 

[Maxwell]

Fingers crossed you're OK.... Maxwell

[mikehardwick]

Scotty-

 

Checked it out last night on way home from York -

 

3000rpm in 5th.........................indicated 68mph

 

3500rpm in 5th.........................indicated 80mph

 

4000rpm in 5th.........................I would like to apologise, Your Worships! (O.K, 92 mph indicated!)

 

My car is an '81 built TR7 dhc on the standard diff, fitted with 15" rims and 205/50 rubber, giving near as dammit the same rolling radiuis as original wheel/tyre combo.

 

Good luck!

 

Mike

[scotty]

Ah well , I dare say some fan mail will be winging its way to me soon      

[malcolm]

I wouldn't bet on it Scotty.  An indicated speed of 92mph is likely to be more like 80mph if my experiences with a GPS are anything to go by.  This would also be more likely for 4000rpm in 5th on a standard car.

 

So I recon you are Ok   :cool:

 

Malcolm.

[jonlar]

I wouldn't bet on it Scotty.  An indicated speed of 92mph is likely to be more like 80mph if my experiences with a GPS are anything to go by.  This would also be more likely for 4000rpm in 5th on a standard car.

 

So I recon you are Ok   :cool:

 

Malcolm.

Ah, Malcolm, someone else who uses their GPS for that purpose, it's accurate as long as it's set to Statute Imperial miles not Nautical miles!!!, also most accurate in a reasonably straight line.

[martynharrison]

This is a fairly definitive answer:

 

A revs per min 4000 A

B eng rev / hour 240000 B = A * 60

C LT77 5th gear ratio 0.833

D prop rev / hour 288,115 D = B / C

E diff ratio 3.90

F axle rev / hour 73,876 F = D / E

G tyre designation 195/50.15

H tyre wall (cm) 19.5 tyre mm/10

J wall percentage 0.50 aspect 50%

K wheel diam (in) 15

L tyre diam (cm) 57.6 L = 2*H*J +  2.54*K

M circ (cm / axrv) 180 M = pi() * L

N trav cms / hour 13,297,680 N = M * F

P speed kms / hour 133 P = N / 100,000

Q actual spd mph 83 Q = N / 160,927

 

It makes more sense when pasted into a spreadsheet. Basically, if the tacho is true and the engine is turning at 4000 times per minute, the 0.833 overdrive fifth and the 3.90 diff ratio means the rear wheels are turning 73,876 times an hour.

 

Given the tyre size indicates a circumference of up to 180 cms, the vehicle will normally move forward up to 180 cms per axle rotation or 83 miles per 73,876 rotations in each hour.

 

If the tyres have a slightly smaller effective rolling radius, the car will be travelling a little slower than this, I would suggest a true 4000 rpm equates to an actual 80 mph, whatever the speedo may be saying.

 

The easy way to use this is to copy it into a spreadsheet with the formulas on the right to derive the figures in the table so that when you change a figure, the rest of the table updates to show how this affects things. E.g. pasting 3.45 or 3.08 into the diff box, ups the speed from 83 to 93 and then to 105 mph.

 

To complete this analysis (yeah ok) the rev counter is usually set to over read a bit, much like the speedo, so if you were actually seeing 4000 rpm, this is probably a maximum rpm rather than a low figure. So I'd say you are likely to escape any attention from the gendarmes. Still, this assumes a 3.90 rear diff and an engine speed no higher than 4000 rpm, plus the 10% +2 mph allowance on speeding, which brings the lower limit of a prosecution down to around 79 - 80 mph anyway.

 

P.S. I've tested this against GPS and it's pretty much spot on. Unlike the average speedometer on an old triumph.

[scotty]

Martyn , all I can say is WOW .

 

I have just been out , my wheels are 185/70 R13 which in my rusty maths equates to a circumference of 151.9 cm so I will live in hope although I went out with the wife following earlier and she thinks I was doing about 86

 

Will just have to wait and see.

 

Can I use your figures when I am up before the Beak?

[scotty]

Maths was out . circumference 185 ? == 85 mph (136Kmh) ?

 

[malcolm]

What was the wife driving though Jon?  Even if it's a modern motor it probably over reads too.  I'd think you've got away with it by the sound of things.  No doubt you will let us know.

 

Reminds me of a magazine article a few years back.  Some journos testing a BMW 520i (I think)  were followed by the police, clocked and then prosecuted.  The case was thrown out when the magazine proved that the evidence presented by the police could only have been correct if they were driving Thrust 1!

 

May be worth jotting down Martyn's figures just in case then....

 

Malcolm.