I will try to give you a ballpark figure. I will also try to give you the formulas so if your car weights 3 metric tonnes and you like to drive at 90 mph between traffic lights, you can redo the calculations and come up with your number.
Assuming your are on flat ground
Your car has kinetic energy whilst in motion for which the formula is:
1/2 m*v^2
( v^2 is v to the power of 2, or v squared, or v*v).
where m is mass in kg, v is velocity in m/s. The result is in Joules.
When you stop, you transform all those joules to thermal energy. (Btw 1 Joule = 1Ws) {If you ever wanted to calculate how much you could re-charge your battery from that stop}.
1 kg of gasoline has 46 MJ of energy.
https://en.wikipedia.org/wiki/Energy_density
There are a lot of other variables at play here but these basic formulas will give you a ballpark number.
Lets crunch some numbers.
You state that you are traveling at 40 mph. -> approx 18 m/s.
Lets assume your car weights 1500 kg's.
So your car has (1500 * 18^2) /2 joules of energy. -> 243000J -> 243 kJ
Here is how you can do the calculations on google.
https://www.google.com.tr/search?q=40+mph+in+m%2Fs&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&ei=D8VkWe29OoLSXsLLivAG#q=(1500+*+18^2 )/2
If we assume that your engine is 1/3 efficient in converting chemical energy to mechanical energy. 1 kg of galsoline has 46/3 MJ. -> 15.3 MJ -> 15300kJ
243 / 15300 = 0.016 kg of gasoline. -> 16 grams.
Density of gasoline is 0.726g/cm3
16/0.726 -> 22 ml. (1 ml = 1cm3)
So in conclusion, if we assume that our simplistic model is semi accurate (with this level of calculation, once you are at 40mph you will continue at mph forever because we did not factor in friction and air drag) and we also assume that our car of 1500kg (3300lbs) traveling at 40mph coming to a full stop and then re-accelerating to 40 mph will waste 22 mililiters of gasoline. (between 11 ml and 44 ml {assuming %100 calculations error in both directions})
11 ml -> 0.003 US liquid gallon if we over estaimated by %100
22 ml -> 0.006 US liquid gallon
44 ml -> 0.012 US liquid gallon if we under estimated by %100
You can directly scale these results for your car's weight (i.e. if your car is 3000kg, just double everything). If you want to calculate for 20 mph (which is half of 40 mph) you have to take 1/4 of the value (20/40 = 1/2, (1/2)^2=1/4)
If it is 2000kg and you want to know for 30mph, you have to redo the calculations. But it is not too hard to do.
If you want to calculate for hills etc, the formula is m*g*h where m is mass in kg, g is gravity constant (approx 9.8) and h is height in meters, result also in Joules.