Speed v RPM

[tomtrout]

I bet this has come up before but would anybody out there like to have a stab at estimating the speed of a Vixen when in top gear and pulling 4k rpm on the rev counter?

I'm sure my speedo is over estimating my true speed and I'm keen to get a feel for how much it might be out before I add to my speeding points collection! I'm pretty sure my rev counter is accurate. The car is running a standard box and diff with 15 inch rims shod with 65% profile tyres. I know these will give me a slight increase in my actual speedo reading but my guess is that my error is considerably more than might be generated by the reduction in rolling wheel diameter over the standard wheel/tyre set up. Any thoughts?

[heightswitch]

http://www.car-videos.net/tools/speedrpm.asp

N.

[TVRdaydah]

Use your satnav on a level stretch of motorway to establish a speed v rpm datum. Get someone else to read the numbers so you don't run into anything.

[prideaux]

Running tvr fins with 185 70 tyres and a 5 speed box different set up to you Andy but gives you some idea dont worry about points i am sure your good lady has some spare
A

Edited by prideaux on Monday 14th July 19:01

[phillpot]

Follow a friend in a modern (accurate speedo) car on a straight clear stretch of road with a series of signals for set speeds, or walkie talkies, or passengers with mobile phones, or just use the online calculator

[griff 200]

I must agree with the above recommendation of using a sat nav ,always use mine if I'm in a strange vehicle as don't need any more points. And I do drive some strange vehicles !!!!! Richard

[Mr Tiger]

The Mintylamb site may help.

http://www.mintylamb.co.uk/gearspeed/

Although I think it's primarily for Triumphs there's still plenty of relevant info on there.

With the Vitesse 3.89 diff, 185/65 tyres it states 74.9 mph @ 4000 rpm and 73.3 mph with 175s.

Hope you find it useful and fun.

Chris

[RCK974X]

It's easier than you think, provided you have the base info.

The key is finding out the rolling circumference of the tyres, to give turns per mile, and that multiplied by the diff ratio, gives propshaft turns per mile. Then multiply by gear ratio (4th gear is one, 5th gear is less than one), and you have RPM.

For a Vixen 3.89 with 175/70 tyres....

Tyres are 838 turns per mile, 3.89 diff gives 18.4 mph per 1000 rpm in 4th gear and 22.6 mph in 5th gear (Ford Type 9 5 speed box).

4k in top, 4 speed = 73.6mph. 4k rpm in 5th = 90.4 mph.


For the more mathematic amongst us -

There is a formula to get turns per mile based on tyre sizes, which is pretty close.....

Turns per mile = 63360/ tyre circumference.
circumference = Tyre Width/aspect ratio + wheel size. 63360 is inches in a mile
which is diameter of wheel + two hieghts of tyre.

Mathematically - Rolling Diameter = 3.14159 x [(Width x ASR/1270) + WHSize] (1270 is from 25.4 mm per inch)

But tyres distort a little, so using 65261 works better, which is 3 percent lower.

If you have the gearbox ratios you can do every gear.

Shove this in a spreadsheet to make it dead simple !!

Edited by RCK974X on Monday 14th July 21:47

[madsvlund]

Slightly OT, but I happend to be in the same kind of troublewith my M. I have a GPS controlled speedo, so that part is quite simple. But the ECU on the car have double wheel sensor input to make some launch control and anti spin. the complicated matter is that i get the signal for the rear wheel speed from a VR sensor in the gearbox, and the front wheel speed from a abs sensor. To calibrate all of that did I do some 20 mph runs on the street in front of our house, with the laptop, the scope and a speed app on the mobile occuping the cabin with me. And for once will I agree with the old ladies walking their dogs.... youth nowadays..... :-)

[tomtrout]

Thanks a lot guys. Plenty to go at. Hey Mads, I've also got "launch control" when I'm in my car - it's called being scared of breaking my 45 year old diff!!

[RCK974X]

I had a very scary moment in my Marlin (kit car) which had a 3.27 GT6 diff... I was accelerating down the on-ramp to the motorway, doing about 75mph ish, and there was this almighty KERBANG!! and car started to weave around, and so I pulled over to hard shoulder. Obvious with a little push that diff had gone - nasty noises....

I realised afterwards I was very, very lucky that the diff didn't seize up..

After rescue....got home..took to bits... etc. Discovered diff had broken two teeth off crownwheel and it was a big mess inside, but case was intact.

Always been a bit wary of Spit diffs after that - I fitted my Vixen with a TR6/Saloon type diff instead.

Edited by RCK974X on Wednesday 16th July 00:32

[octanetorque]

tomtrout said:

Thanks a lot guys. Plenty to go at. Hey Mads, I've also got "launch control" when I'm in my car - it's called being scared of breaking my 45 year old diff!!

^^ ...why I had mine replaced...

but then you're into UJs, Uprights, crack testing quill shaft scare stories and even then you'll not have a LSD or an engine which is (really) capable of leaving 11s on every bend...

I think the key - as you've stated OP, is to just enjoy it for what it is and to not (overly) abuse it.
J

[madsvlund]

My diff is strong enough, but my Home made half shafts did require some aditional welds afer this. But then did the LSD show as a really usefull as a "limp home device"

[RCK974X]

Yeah, I guess it was my own fault by having a tuned 2500 engine in the front !!

As part of the Vixen rebuild, I decided to go for the bigger diff, knowing that Vix/M driveshafts could be found for it too.