how fast am i going?

[Jamie-b]

hey,
i have bee having problems getting my speedo to work, and when it does the digital, and regular speedo show different speeds, so until i get it properly fixed i thought i would use my tacho to figure out how fast im going,
its a Series 1 manual v6 vs ute , could somebody please tell me what there taco sits on at popular speed limits (mostly 50 ), and in what gear,
thanks

[-ToM-]

I can see a fault with this solution.

You will be applying different amounts of throttle at different gradients.
Also have to take engine/gearbox wear/condition into effect

Youd be better off trying to wing it by matching the speed of other cars. Or getting a new cluster.

[Jamie-b]

Youd be better off trying to wing it by matching the speed of other cars. Or getting a new cluster.

Yer, was going to match the speed to a mates car but haven't had a chance yet...
I have tried a new cluster, but am still having problems..

[westbad]

you could always use a gps to show your speed, once at a constant speed my tomtom is quite accurate

[Maxxiz]

How different are the speeds? Mine is about 3k difference?

[Cheap6]

You might try this approach:

Copy and paste this:

=(((C6/100*27.777)/(B6*PI()))*3.08*A6*60)

into a spread sheet.

C6 is the road speed in km/h

B6 is the tyre diameter in metres, for a 205/65/15 this is nominally:

[(205 x 0.65 X 2) + (15 X 25.4)]/1000

this = 0.6475m

A6 is the ratio of the gear selected. For T5, V6 this should be:

First........ 3.25:1
Second..... 1.99:1
Third........ 1.29:1
Fourth...... 1.00:1
Fifth......... 0.72:1

An eg. calc. is that 50km/h in fourth gear rpm = 1262.

Note that there might be some error because the tyre size is nominal.

If you really wanted to be accurate, you could mark the tyre sidewall and an adjacent point on the ground with chalk, roll the car until the chalk mark on the tyre again touches the ground and mark the ground again. The distance between the chalk marks is the circumference.

Rolling the car so that the tyre rotates more than once and dividing the distance between the marks by the number of roataions will be more accurate.

[Tom_1569]

I can see a fault with this solution.

You will be applying different amounts of throttle at different gradients.
Also have to take engine/gearbox wear/condition into effect

Youd be better off trying to wing it by matching the speed of other cars. Or getting a new cluster.

He drives a manual, so unless he has a slipping clutch thats irrelevant.

I run 17" 235/45 Tyres, and I think Im just under 1500rpm at 50km/h in 4th.

PM me an email address and Ill send you a calculator that works out your RPM at speeds etc.