How many revs at 100 kph

[GLENN 70]

Do you know how many revs you are doing at 100 kph . My 66 does 2600 rpm .Same with my 70 ,both have 3.25 ratio diff . If you know what diff ratio and tyre size  you have i can tell you what revs at any speed you want .

[birdman]

Well my muzzy sits on 3,000 RPM at 100Kph,3 speed manual toploader,3.00 diff ratio,205/70-14's...Does this sound on par with your calculations?

[shaunp]

Well my muzzy sits on 3,000 RPM at 100Kph,3 speed manual toploader,3.00 diff ratio,205/70-14's...Does this sound on par with your calculations?

Doesn't sound right.

[GT350H]

2000 rpm, 5speed GForce with 0.63 OD, 4:1 ratio, 17x265x40.

[GLENN 70]

thebirdman, no wrong you are doing 2500 revs .Either your taco is out or you have 3.5 ratio diff .You can check your diff ratio sort of, by jacking up the rear end ,marking the tail shaft then turning the wheel or both 1 turn, counting the tail shaft turns ,now you have to do this very slow as the tail shaft will spin very fast. eg. 1 turn with a 3.00 will be 3 turns ,a 3.25 , 3 1/4 turns ,a 3.5 ,3 1/2 turns

[griff]

ive got same as birdman then glenn, 2500rpm instead of 3000rpm?

our bloody rally pacs are soooooo accurate then eh john, mine says similar at 100
either way at freeway speed im watchin the fuel gauge needle wanting to have a good lie down to the empty position
what the hey, you get that...
griff

[66 Hertz]

Your Tacho's may be right but you speedo's are most likely wrong.....be more likely

[GLENN 70]

66 Herz you are right the tacho s will be right ,the speedos will be out .Or there diff ratios are wrong . If you guys have a GPS  it will show your right speed .

[66 Hertz]

doesn't matter what engine or what you do to it, Revs are Revs.............everything else you do, gearbox, diff and wheel / tyre combo's will alter speed readings

[shaunp]

It is possible that the tachs are wrong, particularly if you have electronic ign in the car, they don't always trigger correctly, even more so if you are runing a CDI like a 6AL etc, the extra spark energy can false trigger them due to induction into the harness.

[GLENN 70]

A lot of our speedos will be out ,most dont run the same size wheel or tyre as factory ,i run 15s on my 66 and 17 s on my 70 ,so speedo is out on both . Most will be slow so dont get booked .

[StephenSLR]

A lot of our speedos will be out, most dont run the same size wheel or tyre as factory

Yep, any tyre or diff changes will affect the speedo.

I recall the old days going to the freeway and doing the 5km odometer test then working out what to divide my speed by to get the real figure. In the Torana it was 7.5:10 so doing 45 km/h on the speedo was actually doing 60 km/hr.

I now use the Nokia GPS in the Mustang and the delightful woman keeps telling me, 'Observe speed limit!' whenever I go over by too much and I can dial in how many km/hr over before she alerts me.

s

[birdman]

thebirdman, no wrong you are doing 2500 revs .Either your taco is out or you have 3.5 ratio diff .You can check your diff ratio sort of, by jacking up the rear end ,marking the tail shaft then turning the wheel or both 1 turn, counting the tail shaft turns ,now you have to do this very slow as the tail shaft will spin very fast. eg. 1 turn with a 3.00 will be 3 turns ,a 3.25 , 3 1/4 turns ,a 3.5 ,3 1/2 turns

I know my speedo isn't out because i have checked it with 2 GPS units...my diff looks untouched but saying that it's 40 odd years old who knows what the yanks put in there...i know the diff ratio check just havn't bothered checking it...I will check the ratio and the tacho reading...
Thanks Glenn

[birdman]

ive got same as birdman then glenn, 2500rpm instead of 3000rpm?

our bloody rally pacs are soooooo accurate then eh john, mine says similar at 100
either way at freeway speed im watchin the fuel gauge needle wanting to have a good lie down to the empty position
what the hey, you get that...
griff

I think it's either the tacho playing up or the ratio has been changed,cause as you know everything is all stock original...

[GLENN 70]

Over time the speedo gear may have been changed also .

[birdman]

66 Herz you are right the tacho s will be right ,the speedos will be out .Or there diff ratios are wrong . If you guys have a GPS  it will show your right speed .

Glenn and Dale,what makes you say the tacho would be correct?I will check tacho and diff ratio

[birdman]

Over time the speedo gear may have been changed also .

40 years,anything is possible,i know i have some GT350 options fitted so maybe the ratio is 3.50???

[birdman]

It is possible that the tachs are wrong, particularly if you have electronic ign in the car, they don't always trigger correctly, even more so if you are runing a CDI like a 6AL etc, the extra spark energy can false trigger them due to induction into the harness.

Standard single point dizzy...

[birdman]

doesn't matter what engine or what you do to it, Revs are Revs.............everything else you do, gearbox, diff and wheel / tyre combo's will alter speed readings

That's what i thought,wheel/tyre combo's original size,leaning towards diff centre or faulty tacho

[GLENN 70]

Here is an eg .100 kph with a 3 speed auto c4 ,a 3.00 diff ratio and 215/60x15 tyre .That will give you 2500 revs .at 100 kph .  OK another one ,a c4 with 3.5 ratio and 215/60x15 tyre gives you 2950 rpm . Now i am talking old school mustangs without 5 speed or overdrive g/boxs .

[GLENN 70]

What tyre size do you have 215/60 x15 etc .

[birdman]

Over time the speedo gear may have been changed also .

Hhhmmm,never crossed my mind...What colour would it need to be? 3 speed toploader,3.00 diff ratio,according to the tag

[birdman]

What tyre size do you have 215/60 x15 etc .

205/70 x 14's

[GLENN 70]

It will be ok because your speedo in correct ,its the revs that need sorting and maybe the diff ratio .Now if the diff ratio has been changed and the speedo is correct the gear has been changed also .

[GLENN 70]

ok with 3.00 ratio your revs will be 2480 at 100 kph ,with a 3.5 ratio will be 2950 revs at 100 kph .