7 replies to this topic

[Dr Terry]

Hi hux454.

If you're doing 2500 rpm @ 100 km/h now, if nothing is changed except the diff ratio, then you will be doing 2917 rpm to achieve that same speed. That's over 3200 rpm @ 110 km/h, not very good on the freeway.

I personally wouldn't go any lower than 3.25:1 unless you had a gearbox with an overdrive ratio (Celica 5-sp or TH700 etc)

Dr Terry

[ls2lxhatch]

I just tried the link and the download works. The maths is simple but tedious to calculate, the gearcalc program is the way to go.

Edited by ls2lxhatch, 26 July 2007 - 05:29 PM.

[LS1LX]

Borrow some TALL tyres for the trip.
I run 3.5's but with 17's, drove it for 3 hours straight the other week, it was fine on the highway but my motor likes to be around 3,500rpm.

[Dr Terry]

Dr. Terry sir, do you have the formula for working that out, there was a link posted around here to calculate rpm and speed with what gearbox and diff ratio's but i couldn't get the file to work. The link was

http://locost7.info/gearcalc.php

When I downloaded the program, it came up as corrupt when I run it, damn it.

I have a T700 and wanted to see what the RPM at 100km/h and 110km/h using 3.5, 3.7 and 3.9 diff ratio's.

Thank you

Hi Guys.

I didn't use a formula. All I did was using the figure you gave me (2500 rpm @ 100 km/h) then added 1/6 (the change in diff ratio from 3.00 to 3.50) to get the 100 km/h revs & then added another 10% to get the 110 km/h revs.

You can work all this out by tyre size. You need to measure your actual rolling diameter accurately (in metres), then multiply it by pi (3.1416) that gives you the circumference. Then divide that into 1000 m (1 km). This gives you tyre rotations per km. Then just multiply that by the diff ratio, to give you tailshaft rotations per km. Divide this by 60 to get minutes. This gives you tailshaft rpm at 1 km/h. Then multiply that by any km/h figure you want.

This gives you the rpm at that speed in 1:1 'top' top. If you have an overdrive box (which a TH700 is) you need multiply this by the overdrive ratio.

Hope I've got that right.

Dr Terry.