engine and drivetrain Fun With Diff Ratios, Rpm, Tires, And Speed

[gliding_serpent2]

Here is some FYI info on ratios that may helped me get an idea of the effect of tire size and diff ratios, on speed and rpm.  I did these calculations to help me manage expectations at the track and to compare my current setup options, and help me plan future options.  After my first driving course with my Tii on our local tight track I was able to get an estimated 90-95mph on the back straight in fourth using the "street cruising setup" below.      

 

Classic stock:

1:1 transmission ratio (4 speed in 4th)

3.64:1 open diff

Michelin XAS 165/80/13 tires (22.4567 inches diameter, stock size)

6000rm

110.1mph

 

Street "cruising" Setup:

1:1 transmission ratio (4 speed in 4th)

3.64:1 open diff

Falken 615K 195/60/14 tires (22.28 inches diameter, 0.77% smaller)

6000rm

109.3mph

 

Track setup:

1:1 transmission ratio (4 speed in 4th)

e21 3.91:1 LSD

Falken 615K 195/60/14 tires (22.28 inches diameter, 0.77% smaller)

6000rm

101.8mph

 

Autocross/Track setup:

1:1 transmission ratio (4 speed in 4th)

e21 3.91:1 diff

Toyo R888 205/60/13 tires (21.78 inches diameter, 3% smaller) I do not have these tires. 

6000rm

99.47mph

 

Autocross setup:

1:1 transmission ratio (4 speed in 4th)

e21 3.91:1 diff

Toyo R888 185/60/13 tires (20.87 inches diameter, 7% smaller)

6000rm

95.2mph

 

useful links...

http://www.carbibles.com/tyre_bible_pg4.html

http://math.about.com/library/blcirclecalculator.htm

http://www.csgnetwork.com/speedcalc.html

Edited February 18, 2014 by gliding_serpent

[eurotrash]

Interestingly, I went to the drag strip this past fall with a tired original motor and a 32/36.  My 1975 (8:1 compression) with the e21 5 speed and 3.91 LSD had a best 60ft. time of 2.443 seconds on warmed up Yokahama SDrives. I went through the trap in third at about 6k rpm at 77 mph.

[seattle2002tii]

Isn't the stock tire diameter ~23.5"?

 

XAS tires listed here are 600mm diameter (~23.5") - http://www.vintagetyres.com/item/115/165hr13-michelin-xas-ff--car-tyre--race--road-legal-

[jireland2002]

Race car  -- 3.91 diff , 205x50x15 @8100 rpm - California Spreedway = about 135mph

[M3M3]

williamgruff posted a spreadsheet a while ago that had every transmission, all their ratios, all the possible diff ratios and the resultant speed in any gear at a range of rpm's. quite handy for reference.

for me steet/cruising is a 4.10 diff. race car is 4.44 diff...120mph at nowwhere near the 7500rpm redline on 205/50 15's.

[ray_]

williamgruff posted a spreadsheet a while ago that had every transmission, all their ratios, all the possible diff ratios and the resultant speed in any gear at a range of rpm's. quite handy for reference.

for me steet/cruising is a 4.10 diff. race car is 4.44 diff...120mph at nowwhere near the 7500rpm redline on 205/50 15's.

 

 

Seems like a good plan.

 

[ndog]

Race car  -- 3.91 diff , 205x50x15 @8100 rpm - California Spreedway = about 135mph

[Napes]

I agree about the OP's sizes being off.  Not sure what formula he uses (and there can be slight differences between manufacturers) but I've always used the formula of : width (in mm) x 2 x aspect ratio % divided by 25.4 (mm / inch) + the wheel diameter = tire diameter.  Thus I get 23.4" for the OE tires, and 195/60 x 14s should run about 23.2"  I run 185/60 x 14s which have a dia. of 22.74".  A little smaller than stock or 195/60 x 14s but I had North Hollywood Speedometer re-calibrate my speedo when they had in their shop for cleaning.

 

Bob Napier