If someone has some stock figures for mph at various RPM in top gear, like maybe from a Road & Track spec sheet, please forward them.

I have a '74 145 with (I assume) a stock rear end, but since I swapped in the Ford T5 5-speed transmission, and hooked up the Volvo speedo to the T5 output, it reads about 10 mph fast. (Good for the gas mileage and ego, but makes me go a lot further than I intended. Maybe it's a good safety and economy feature.)

The 4th gear of the T5 is 1.00, so I can calculate the 5th (.79) from there.

This Website is usefull for everything related to wheel fitment(offset/diameter/width... the works) and gearing. Check it out, I'm sure you will find the answers you are looking for and more.
Later,
Kyle

The only thing I remember is when I picked up my car ('74 144 4-sp) my speedo showed 65 mph at 3800 rpm. But that was with 14" wheels on a car that's supposed to have 15", so nevermind. I'd suggest you get a hold of a GPS, they're very accurate and will show you speed in mph.

Kåre

The smaller wheels will not affect the speedometer readings. The thing that it changes, is your actual speed, your car will say it's going 65 when in reality it is going slower because of the shorter diameter tires. So your readings/rpm will not change along with different tire diameters. Only your actual speed. The reason I know so much about this topic is because I had the problem of buying a car from a jackass who didn't know anything about the car. So, It had the volvo 240GL 14" alloy rims on it, (which looked awesome by the way, just too small) with bald tires, and so I got some new 195/60/14 and my actual speed was much lower than my readings. For example, with those tires, I went an indicated 112mph at 6000rpm with 4.10 in fourth. In reality I was only going 102mph or something like that. I was pissed that I bought new tires and they were no where near the correct size. So I lost money doing that....

Yeah, so check out that site, it's really awesome.
Peace,
Kyle

http://gs.tolan-hoechst.com/tirecalc.htm

Note that your differential ratio may not be the 4.30 that the 142 had
in John's report.
Probably should check and see what speedo gears are available for your
tranny.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma

Thanks, George. I found this calculator that will help me get it all sorted out.
I'll have to see what drive gear I currently have, compare my speedo reading to the expected, and calc which drive gear to get.

http://www.kellnet.com/chambers/speedo_gears.htm

Here's what I calculated, which will tide me until I get a new speedo drive,

4th @5800 = 102 *(4.3/4.1) = 107 (my rear end is probably 4.1)
107 *(3000/5800) = 55.3 mph in 4th @ 3000 rpm
55.3/.79 = 70 mph in 5th @ 3000 rpm.
with variance for tires.

Usually, the M40 equipped cars came with the 4.10 rearend and the M41 cars came with the 4.30 gearing. But I highly recommend you checkout this site: http://gs.tolan-hoechst.com/tirecalc.htm . It should answer your questions and more...
Peace,
Kyle

I'm not sure what good they will do, but these are from a 1974 Rod & Truck (or is that Rodent Rack?) Road Test of a 142GL.

Speed In Gears
Gear / RPM / MPH
OD 4800 102
4th 5800 102
3rd 6500 79
2nd 6500 54
1st 6500 31

Ratios:
Rear axle = 4.30
OD - 0.80 - 3.43:1
4th - 1.00 - 4.30:1
3rd - 1.36 - 5.85:1
2nd - 1.99 - 8.56:1
1st - 3.41 - 14.66:1