Bongo Fury

missfixit70 wrote:And tyre sizes of course Can't remember exactly but I thought it was about 70 at 2500rpm.

My speedo is virtually spot on with the sat nav and I'm doing approx 2800rpm at 70, but mine is 4WD. In many of the old threads, they used to say 2500rpm was 65mph, which ties up with mine. Yours is 2WD and is going 5mph faster at 2500rpm - is there a different final drive ratio between the models

francophile1947 wrote:

missfixit70 wrote:And tyre sizes of course Can't remember exactly but I thought it was about 70 at 2500rpm.

My speedo is virtually spot on with the sat nav and I'm doing approx 2800rpm at 70, but mine is 4WD. In many of the old threads, they used to say 2500rpm was 65mph, which ties up with mine. Yours is 2WD and is going 5mph faster at 2500rpm - is there a different final drive ratio between the models

Mines a 2WD TinTop and it's 2600rpm at a true 70mph by GPS and 3000rpm is almost exactly 80mph.
It works out to just under 27mph/1000rpm in lock-up top.

According to my satnav, 65 comes up at a needles width over 2400RPM. The speedo says around 67MPH.

bigdaddycain wrote:According to my satnav, 65 comes up at a needles width over 2400RPM. The speedo says around 67MPH.

Is yours one 2 or 4WD Ste?

francophile1947 wrote:

bigdaddycain wrote:According to my satnav, 65 comes up at a needles width over 2400RPM. The speedo says around 67MPH.

Is yours one 2 or 4WD Ste?

....with 18" wheels of course. I've got 17" wheels and my figures are not far off BDCs

mikeonb4c wrote:

francophile1947 wrote:

bigdaddycain wrote:According to my satnav, 65 comes up at a needles width over 2400RPM. The speedo says around 67MPH.

Is yours one 2 or 4WD Ste?

....with 18" wheels of course. I've got 17" wheels and my figures are not far off BDCs

Wheel size makes no difference Mike, assuming that the correct size tyres are fitted

francophile1947 wrote: Wheel size makes no difference Mike, assuming that the correct size tyres are fitted

It did on mine John, no question about it. My speed used to over-read by about 7mph at 65mph with 15" wheels and now its spot on with 17" wheels (both readings done with SatNav).

mikeonb4c wrote:

francophile1947 wrote: Wheel size makes no difference Mike, assuming that the correct size tyres are fitted

It did on mine John, no question about it. My speed used to over-read by about 7mph at 65mph with 15" wheels and now its spot on with 17" wheels (both readings done with SatNav).

Hi Mike
What size tyres have you got and what did you have?

francophile1947 wrote:

mikeonb4c wrote:

francophile1947 wrote: Wheel size makes no difference Mike, assuming that the correct size tyres are fitted

It did on mine John, no question about it. My speed used to over-read by about 7mph at 65mph with 15" wheels and now its spot on with 17" wheels (both readings done with SatNav).

Hi Mike
What size tyres have you got and what did you have?

From dim meory, I think the 15" onse were 65's and (from less dim memory) the 17" ones are 55's. I think the rolling circumference has increased. It is weird to get your head around but I think that must mean that for a given shaft RPM, the speedo gives a certain speed (say 65rpm). But your true speed on 15" wheels is less (presumably it is calibrated for a speed limit safety margin). If you maintain the same shaft RMP but now increase the rolling circumference by putting on bigger wheels, the speedo still reads 65mph but your true speed has increased because your bigger wheel circumference is now pulling the car over more ground for each revolution. Since shaft RPM hasn't changed, this means you are covering more ground per unit time i.e. more miles per hour.

My head now aches and I need a cup of tea

mikeonb4c wrote: I think the rolling circumference has increased.

I think that's the point Franco was trying to make - you no longer have the "right" sized tyres as they are bigger than before.

If you had the "right" sized tyres to go on the 17" rims the circumference wouldn't have increased and the speedo error wouldn't have changed.

Are the Rev Counters telling the TRUTH

That's it precisely scanner
Mike, have a look here http://www.miata.net/garage/tirecalc.html - you'll be able to play and see what difference the tyre sizes make. I now have 225/55 x 17 tyres and my speedo is virtually spot on.

Thanks guys, yup I'd understood that but perhaps should have made it clearer. I don't have the calculator site to hand but it would be interesting to know how low a profile it would have to be to maintain the same rolling circumference as a 15" wheel with 65 profile tyre. Clearly, reducing it to 55 on a 17" wheel wasn't enough to do it. I didn't fancy lower profile than that I must admit.

mikeonb4c wrote:Thanks guys, yup I'd understood that but perhaps should have made it clearer. I don't have the calculator site to hand but it would be interesting to know how low a profile it would have to be to maintain the same rolling circumference as a 15" wheel with 65 profile tyre. Clearly, reducing it to 55 on a 17" wheel wasn't enough to do it. I didn't fancy lower profile than that I must admit.

It depends on the width of tyre Mike. A wider tyre will need a lower profile to be a good match, whereas a thinner tyre will need a higher profile - have a play with the link in my last post

Aha. Sorry Franco, should have paid more attention. Right, from that site we get:

215/65-15 5.5in 13.0in 26.0in 81.7in 776 0.0%
225/55-17 4.9in 13.4in 26.7in 84.0in 754 2.8%

So a difference of 2.8% on rolling circumference. Interesting that the speedo difference seems bigger than that. Maybe its a non-linear thing