gear ratios, how does it work?

well, satnav tells me i'm doing 185 km/h while the calculator says I should reach 210 easily.
And on the smaller tires i've used for a couple of weeks i'm only reaching 162 km/h, and calculator tells me 198 should be possible.

there is only a couple of gearsets available for my gearbox, and even the super rare close ratio gearset has a very similar 5th, so i'm pretty sure that ratio is fine. The final drive came out of a completely stock car, with the VIN tag still in place which stated that it should be a 4.3:1, if the speeds my satnav tells me are correct, it should be a 5.1:1 ratio, and i'm not even sure they made them in the size my diff is, let alone the chance of finding one in an original car! 5.1:1 is not a ratio i'd expect in anything else then a racing car.

Revcounter should be correct, i don't see it sticking or waving, and the speed and revs do go up in the same way in every gear. And when my rev counter sits at 7500 rpm, it really feels like that.

So that does indeed leave a dodgy GPS (which i didn't think could be possible for such a HUGE difference) or i've some super rare short ratio final drive, but i'll try to find out when i get the car back from the shop.

What? 185 kph isnt fast?

I cruise faster than that in my old volvo estate car on the motorways and i still have to move out of the way of faster cars.

I had a similar head scratching moment calculating S2000 gear ratios. turns out on the S2000 the gearbox ratios are not what goes in one end and out the other but direct ratios. in the calculation there also needs to be another ratio which is a reduction ratio from the input shaft of 1.16 to one. took me a while of googling to find this info out.

using simple maths will help find your problem

engine rpm - 7,500 (A)

5th gear ratio - 0.861 (B)

axle ratio - 4.3 to 1 (C)

rolling radius for 205/60x13 - 1.81 meters (D) (found online)

to find the rpm of the wheels

(A) divided by (B) is propshaft rpm (E)

(E) divided by (C) is wheel rpm (F)

(F) times (D) is distance travel'd (G) in one minute in meters

divide (G) by 1000 to get KM/minute then times 60 to get KM/hour


so (A) divided by (B) divided by (C) times (D) divided by 1000 times 60 = kmph

so simply put

7,500 divided by 0.861 divided by 4.3 times 1.81 divided by 1000 times 60 which equals 219 kph

to get mph divide by 8 then times by 5 = 137 mph



either there is another ratio to take into account or as you say you don't have the axle ratio you think you have

edit

this is the ratio calculator with the reduction ratio showing for the S2000. sadly only lists a few gearboxes

i've looked at pics from the gearbox at google and this is one i found:



so the extra ratio you're talking about, is the one going from the bottom shaft back up to the output shaft or the one on the input shaft?
Could be the case indeed, but strange that it's NOWHERE mentioned, and i think every gearbox works about the same, right?

going backwards on the maths 185kph is 6,306 rpm

what gearbox do you have .

That's how it works, yes Or in other words, the main shaft is a two-piece construction. With power input via the countershaft, it can be build as a single piece.

Yeah, it's confusing at first, but once you 'got into it' it's very simple mechanics

4.3 is still a very short axle ratio anyway. i have a 3.54 with slightly smaller rolling radius which is a good compromise between road and track use with a 7000 rpm red line. 3.77 would be slightly better for the track.

if there was a reduction ratio it would be on all gears. the reduction ratio is seperate from the gear ratios when a gearbox has one (the S2000 gearbox being a good example). the T50 doesn't have a reduction ratio so it's simple maths to work out the top speed in any gear.

the reduction ratio is 1.16 to one on the S2000. why they don't just give the input to output shaft ratios to stop a lot of confusion i don't know . would make things a lot simpler.