Road speed from RPMs - help?

Post by **27.86** » 12 Sep 2006 05:37 pm

Hi. I'm looking at getting a 360 with a duff speedo. I know it's possible to calculate road speed from a combination of revs & gear, but I'm wondering how it's done. The car's a 2 litre - don't know if they all had the same gear/diff ratios?

Any hints, guys?

Post by **sven360** » 12 Sep 2006 05:58 pm

most 360's come with 3.64 diffs
Wipe the crud off the diff and a sticker will reveal ratio.
If you got 3.64 let me know and will check what rpm apply to speed limits for ya.

Post by **Carl** » 12 Sep 2006 07:12 pm

I've found this quite helpful in the past if you know the values to work with:
http://www.fisc-racing.com/misc/ratios.html

Post by **27.86** » 12 Sep 2006 07:31 pm

Thanks, Sven & Carl. I'll get the information & see what I can do!

Post by **foggyjames** » 12 Sep 2006 09:17 pm

Wouldn't it be more to the point to fix the speedo? It'll either be a loose cable (easy), a snapped cable (also easy-ish), or a broken speedo head (also easy).

Carb 360s apart from really early ones (3.28) had a 3.36:1 diff, and injection ones had the 3.64 as the guys above have said.

cheers

James

Post by **Bilbo** » 12 Sep 2006 10:00 pm

u can work it out like me

rpm/ratio = gearbox output
gearbox output/diff ratio =wheel rpm
wheel rpm x 60 = wheel rph
wheel rph x rolling radius of tyres = mph

Post by **pettaw** » 12 Sep 2006 10:30 pm

the original g/box from my 360 GLEi had a 3.43ish diff, in between the carb one and the GLT one

Post by **d3k4y0** » 12 Sep 2006 10:40 pm

Bilbo wrote:u can work it out like me
rpm/ratio = gearbox output
gearbox output/diff ratio =wheel rpm
wheel rpm x 60 = wheel rph
wheel rph x rolling radius of tyres = mph

I wouldn't want to do that every time i passed a speed camera!

Post by **classicswede** » 12 Sep 2006 10:56 pm

Aprox for most volvos is
1st 1/2 x rpm 4000rpm = 20mph
2nd 1 xrpm 4000rpm = 40mph
3rd 1 1/2 x prm 4000rpm = 60mph
4th 2 x rpm 4000rpm = 80mph
5th 2 1/2 x rpm 4000rpm = 100mph

That is not 100% by any means but does provide a good guide to what speed you are doing and can be worked out very quickly

My old man used this in his Amazon for years

Post by **redline** » 12 Sep 2006 11:07 pm

foggyjames wrote:Wouldn't it be more to the point to fix the speedo? cheers James

last time I looked the redlines speedo worked just fine lol

Post by **27.86** » 13 Sep 2006 11:29 am

Thanks for all the good advice, guys. I will get the speedo looked at (if I get the car), but have to get it 100 miles home first!

Thanks, Mr classicswede. Will be up in your neck of the woods in a couple of weeks (hopefully by Volvo!) - going on holiday in the Lleyn Peninsula.

My (potential) car's a 1989 360 GLE on standard steels. Not sure of the rolling radius/gear ratios/diff ratio yet. Time to get the Haynes manual out!

Post by **classicswede** » 13 Sep 2006 08:00 pm

No problems.
The Llyn penisula is a very nice area (not far from me at all).
Imo it is the most Welsh area of Wales.
I had the spedo head out of my old 360 and the ratio on that is the same on the 340 with steels.

Post by **foggyjames** » 13 Sep 2006 09:05 pm

I can tell you that a 360 carb does 4000rpm at 60 in 3rd, and the same at 100 in 5th.

I'm sure someone will sell you a working speedo...Mr Farrell? I'll even adjust it to match your current mileage, if you want.

cheers

James

Post by **petefarrell360** » 13 Sep 2006 09:14 pm

Good point James, I hadn't thought of it being the actual speedo itself, however they normally still read speed, and just don't record the miles when they lcok up. I'd bet on it being the cable, connections or fitting in the hub as previously suggested.
Pete

Post by **27.86** » 14 Sep 2006 10:41 am

Thanks for the advice, everyone. I've got a spare 340, so could take the speedo out of that, if needed. Thanks for the offer to change the mileage on it for me, James.

Please could someone let me know what to tell my garage to check when it goes in for a service next week - it would be good to get the speedo fixed, if possible, before my 600+ mile journey!!