SPEED BASED ON RPM

[PATTERNPART]

Hi

Does anyone know how many mph per 1,000rpm a 1.8 NA does in top (5th?)

If I knew that 5th was direct top and what the axle ratio was I could work it out I think.

Cheers!

[sbird]

http://www.miata.net/garage/KnowYourCar/S8_Gears.h...

[PATTERNPART]

Thanks for that. I've done some sums on the basis of a 0.814 top gear, a 4.1:1 differential and 1.809556mm tyre curcumference and 6,500rpm which makes 87mph.

6,500rpm in top seems to be faster than that! I must have a number wrong. I suspect the top gear figure. Do you think that means that the prop turns 814 times for 1000 engine revolutions?

[anonymous-user]

That page says that 1.8 NA cars were fitted with 195/55 R14 tyres. I've never heard this before, or seen an MX5 with this size on OEM rims and the ONLY tyre I can find in that size is a Toyo Proxes T1-R. Seems a bit strange.

[trashbat]

Try this: http://wahiduddin.net/calc/calc_speed_rpm.htm

Not sure about your tyre figure either.

[MX-5 Lazza]

PATTERNPART said:

1.809556mm tyre curcumference

They would be tichy-tiny-ickle wheels!

[Craikeybaby]

dme123 said:

That page says that 1.8 NA cars were fitted with 195/55 R14 tyres. I've never heard this before, or seen an MX5 with this size on OEM rims and the ONLY tyre I can find in that size is a Toyo Proxes T1-R. Seems a bit strange.

Mine was on 185/60 R14. FWIW I found that 4000rpm in 5th gear was a indicated 90mph.

[snotrag]

PATTERNPART said:

Thanks for that. I've done some sums on the basis of a 0.814 top gear, a 4.1:1 differential and 1.809556mm tyre curcumference and 6,500rpm which makes 87mph.

6,500rpm in top seems to be faster than that! I must have a number wrong. I suspect the top gear figure. Do you think that means that the prop turns 814 times for 1000 engine revolutions?

Have you got your multiplication the wrong way round for your 5th gear?

Remember 5th gear is an 'overdrive', with output turning faster than input. 4th gear is usually about 1:1, and 1-3 will be underdriven, IE engine turns faster than prop.

In 5th, 814 engine revolutions = 1000 prop revolutions.

The figure you got just looks me like its out by roughly the right kind of amount for you to have mixed up your under/over drive on the top gear.

Do the same calculation but replace '0.814' with 1.22 (I.E 1 over 0.814). That puts you more like 5000 odd RPM maybe which is more believable for 90mph.

[sbird]

dme123 said:

That page says that 1.8 NA cars were fitted with 195/55 R14 tyres. I've never heard this before, or seen an MX5 with this size on OEM rims and the ONLY tyre I can find in that size is a Toyo Proxes T1-R. Seems a bit strange.

It's either 195/50/15 or (I think) 185/60/14

It's worth knowing that the revcounter on the Mk1 is usually very inaccurate. The revlimiter is (from memory) 6750 on a Mk1, but you often see 7500 on the counter.

[MX-5 Lazza]

Rev limit on 1.8 is 7250